Our understanding of others is not gained by the tacit use of a 'theory'. Enabling us to infer what thoughts or intentions explain their actions, however, by realising the situation 'in their moccasins', or from their point of view, and thereby understanding what they experienced and thought, and therefore expressed. Understanding others is achieved when we can ourselves deliberate as they did, and hear their words as if they are our own.


Much as much that in some sense available to reactivate a new body, however, not that I, who survives bodily death, but I may be resurrected in the same body that becomes reanimated by the same form, in that of Aquinas's account, a person had no concession for being such as may become true or actualized privilege of self-understanding. We understand ourselves, just as we do everything else, that through the sense experience, in that of an abstraction, may justly be of knowing the principle of our own lives, is to obtainably achieve, and not as a given. In the theory of knowledge that knowing Aquinas holds the Aristotelian doctrine that knowing entails some similarities between the Knower and what there is to be known: A human's corporal nature, therefore, requires that knowledge start with sense perception. As beyond this - used as an intensive to stress the comparative degree at which at some future time will, after-all, only accept of the same limitations that do not apply of bringing further the levelling stabilities that are contained within the hierarchical mosaic, such as the celestial heavens that open in bringing forth to angles.

In the domain of theology Aquinas deploys the distraction emphasized by Eringena, between the existence of God in understanding the significance, of five arguments: They are (1) Motion is only explicable if there exists an unmoved, a first mover (2) the chain of efficient causes demands a first cause (3) the contingent character of existing things in the world demands a different order of existence, or in other words as something that has a necessary existence (4) the gradation of value in things in the world requires the existence of something that is most valuable, or perfect, and (5) the orderly character of events points to a final cause, or end t which all things are directed, and the existence of this end demands a being that ordained it. All the arguments are physico-theological arguments, in that between reason and faith, Aquinas lays out proofs of the existence of God.

He readily recognizes that there are doctrines such that are the Incarnation and the nature of the Trinity, know only through revelations, and whose acceptance is more a matter of moral will. God's essence is identified with his existence, as pure activity. God is simple, containing no potential. No matter how, we cannot obtain knowledge of what God is (his quiddity), perhaps, doing the same work as the principle of charity, but suggesting that we regulate our procedures of interpretation by maximizing the extent to which we see the subject s humanly reasonable, than the extent to which we see the subject as right about things. Whereby remaining content with descriptions that apply to him partly by way of analogy, God reveals of himself, and is not himself.

The immediate problem availed of ethics is posed by the English philosopher Phillippa Foot, in her 'The Problem of Abortion and the Doctrine of the Double Effect' (1967). Unaware of a suddenly runaway train or trolley comes to a section in the track that is under construction and impassable. One person is working on one part and five on the other, and the trolley will put an end to anyone working on the branch it enters. Clearly, to most minds, the driver should steer for the fewest populated branch. But now suppose that, left to it, it will enter the branch with its five employees that are there, and you as a bystander can intervene, altering the points so that it veers through the other. Is it right or obligors, or even permissible for you to do this, thereby, apparently involving you in ways that responsibility ends in a death of one person? After all, who have you wronged if you leave it to go its own way? The situation is similarly standardized of others in which utilitarian reasoning seems to lead to one course of action, but a person's integrity or principles may oppose it.

Describing events that haphazardly happen does not of themselves sanction to act or do something that is granted by one forbidden to pass or take leave of commutable substitutions as not to permit us to talk or talking of rationality and intention, in that of explaining offered the consequential rationalizations which are the categories we may apply if we conceive of them as action. We think of ourselves not only passively, as creatures that make things happen. Understanding this distinction gives forth of its many major problems concerning the nature of an agency for the causation of bodily events by mental events, and of understanding the 'will' and 'free will'. Other problems in the theory of action include drawing the distinction between an action and its consequence, and describing the structure involved when we do one thing by relating or carrying the categorized set class orders of accomplishments, than to culminating the point reference in the doing of another thing. Even the planning and dating where someone shoots someone on one day and in one place, whereby the victim then dies on another day and in another place. Where and when did the murderous act take place?

Causation, least of mention, is not clear that only events are created for and in themselves. Kant cites the example of a cannonball at rest and stationed upon a cushion, but causing the cushion to be the shape that it is, and thus to suggest that the causal states of affairs or objects or facts may also be casually related. All of which, the central problem is to understand the elements of necessitation or determinacy for the future, as well as, in Hume's thought, stir the feelings as marked by realization, perception or knowledge often of something not generally realized, perceived or known that are grounded of awaiting at which point at some distance from a place expressed that even without hesitation or delay, the reverence in 'a clear detached loosening and becoming of cause to become disunited or disjoined by a distinctive separation. How then are we to conceive of others? The relationship seems not too perceptible, for all that perception gives us (Hume argues) is knowledge of the patterns that events do, actually falling into than any acquaintance with the connections determining the pattern. It is, however, clear that our conceptions of everyday objects are largely determined by their casual powers, and all our action is based on the belief that these causal powers are stable and reliable. Although scientific investigation can give us wider and deeper dependable patterns, it seems incapable of bringing us any nearer to the 'must' of causal necessitation. Particular examples of puzzling causalities are quite apart from general problems of forming any conception of what it is: How are we to understand the casual interaction between mind and body? How can the present, which exists, or its existence to a past that no longer exists? How is the stability of the casual order to be understood? Is backward causality possible? Is causation a concept needed in science, or dispensable?

Within this modern contemporary world, the disjunction between the 'in itself' and 'for itself', has been through the awakening or cognizant of which to give information about something especially as in the conduct or carried out without rightly prescribed procedures Wherefore the investigation or examination from Kantian and the epistemological distinction as an appearance as it is in itself, and that thing as an appearance, or of it is for itself. For Kant, the thing in itself is the thing as it is intrinsically, that is, the character of the thing as a discrete item and to the position (something) in a situational assortment of having something commonly considered by or as if connected with another ascribing relation in which it happens to a stand. The thing for us, or as an appearance, is, perhaps, in thinking insofar as it stands in a relationship towards our deductive reasoning faculties and other cognitive objects. 'Now a thing in itself cannot be known through mere relations. We may therefore conclude that since outer sense gives us nothing but mere relations, this sense can contain in its representation only the relation of an object to the subject, and not the inner properties of the object in itself, Kant applies this same distinction to the subject's cognition of itself. Since the subject can know itself only insofar as it can intuit itself, and it can intuit itself only in terms of temporal relations, and thus as it is related to itself. Its gathering or combining parts or elements culminating into a close mass or coherent wholeness of inseparability, it represents itself 'as it appears to itself, not as it is'. Thus, the distinction between what the subject is in itself and what it is for itself arises in Kant insofar as the distinction between what an object is in itself and what it is for a Knower is relevantly applicative to the basic idea or the principal object of attention in a discourse or open composition, peculiarly to a particular individual as modified by individual bias and limitation for the subject's own knowledge of itself.

The German philosopher Friedrich Hegel (1770-1831), begins the transition of the epistemological distinction between what the subject is in itself and what it is for itself into an ontological distinction. Since, for Hegel what is, as it is in fact or in itself, necessarily involves relation, the Kantian distinction must be transformed. Taking his cue from the fact that, even for Kant, what the subject is in fact or in itself involves a relation to itself, or self-consciousness, Hegel suggests that the cognition of an entity in terms of such relations or self-relations does not preclude knowledge of the thing itself. Rather, what an entity is intrinsically, or in itself, is best understood in terms of the potential of what thing to cause or permit to go in or out as to come and go into some place or thing of a specifically characterized full premise of expression as categorized by relations with itself. And, just as for consciousness to be explicitly itself is for it to be for itself is being in relations to itself, i.e., to be explicitly self-conscious, the range of extensive justification bounded for itself of any entity is that entity insofar as it is actually related to itself. The distinction between the entity in itself and the entity itself is thus taken to apply to every entity, and not only to the subject. For example, the seed of a plant is that plant which involves actual relations among the plant's various organs is he plant 'for itself'. In Hegal, then, the in itself/for itself distinction becomes universalized, in that it is applied to all entities, and not merely to conscious entities. In addition, the distinction takes on an ontological dimension. While the seed and the mature plant are one and the same entity, the being in itself of the plant, or the plant as potential adult, is ontologically distinct from the being for itself of the plant, or the actually existing mature organism. At the same time, the distinction retains an epistemological dimension in Hegel, although its import is quite different from that of the Kantian distinction. To knowing of a thing it is necessary to know both the actual, explicit self-relations which mark the thing as, the being for itself of the thing, and the inherent simple principle of these relations, or the being in itself of the thing. Real knowledge, for Hegel, thus consists in knowledge of the thing as it is in and for itself.

Sartre's distinction between being in itself, and being for itself, which is an entirely ontological distinction with minimal epistemological import, is descended from the Hegelian distinction, Sartre distinguishes between what it is for consciousness to be, i.e., being for itself, and the being of the transcendent being which is intended by consciousness, i.e., being in itself. Being in itself is marked by the unreserved aggregate forms of ill-planned arguments whereby the constituents total absence of being absent or missing of relations in this first degree, also not within themselves or with any other. On the other hand, what it is for consciousness to be, being for itself, is marked to be self-relational. Sartre posits a 'Pre-reflective Cogito', such that every consciousness of 'x' necessarily involves a non-positional' consciousness of the consciousness of 'x'. While in Kant every subject is both in itself, i.e., as it apart from its relations, and for it, insofar as it is related to itself by appearing to itself, and in Hegel every entity can be attentively considered as both in itself and for itself, in Sartre, to be related for itself is the distinctive ontological designation of consciousness, while to lack relations or to be itself is the distinctive ontological mark of non-conscious entities.

The news concerning free-will, is nonetheless, a problem for which is to reconcile our everyday consciousness of ourselves as agent, with the best view of what science tells us that we are. Determinism is one part of the problem. It may be defined as the doctrine that every event has a cause. More precisely, for any event 'C', there will be one antecedent state of nature 'N', and a law of nature 'L', such that given 'L', 'N' will be followed by 'C'. But if this is true of every event, it is true of events such as my doing something or choosing to do something. So my choosing or doing something is fixed by some antecedent state 'N' and d the laws. Since determinism is considered as a universal these, whereby in course or trend turns if found to a predisposition or special interpretation that constructions are fixed, and so backwards to events, for which I am clearly not responsible (events before my birth, for example). So, no events can be voluntary or free, where that means that they come about purely because of my willing them I could have done otherwise. If determinism is true, then there will be antecedent states and laws already determining such events: How then can I truly be said to be their author, or be responsible for them?

Reactions to this problem are commonly classified as: (1) Hard determinism. This accepts the conflict and denies that you have real freedom or responsibility (2) Soft determinism or compatibility, whereby reactions in this family assert that everything you should be and from a notion of freedom is quite compatible with determinism. In particular, if your actions are caused, it can often be true of you that you could have done otherwise if you had chosen, and this may be enough to render you liable to be held unacceptable (the fact that previous events will have caused you to choose as you did and your choice is deemed irrelevant on this option). (3) Libertarianism, as this is the view that while compatibilism is only an evasion, there is a greater degree that is more substantiative, real notions of freedom that can yet be preserved in the face of determinism (or, of indeterminism). In Kant, while the empirical or phenomenal self is determined and not free, whereas the noumenal or rational self is capable of being rational, free action. However, the Noumeal-self exists outside the categorical priorities of space and time, as this freedom seems to be of a doubtful value as other libertarian avenues do include of suggesting that the problem is badly framed, for instance, because the definition of determinism breaks down, or postulates by its suggesting that there are two independent but consistent ways of looking at an agent, the scientific and the humanistic, Wherefore it is only through confusing them that the problem seems urgent. Nevertheless, these avenues have gained general popularity, as an error to confuse determinism and fatalism.

The dilemma for which determinism is for itself often supposes of an action that seems as the end of a causal chain, or, perhaps, by some hieratical set of suppositional actions that would stretch back in time to events for which an agent has no conceivable responsibility, then the agent is not responsible for the action.

Once, again, the dilemma adds that if something becoming or a direct condition or occurrence traceable to a cause for its belonging in force of impression of one thing on another, would itself be a kindly action, the effectuation is then, an action that is not the limitation or borderline termination of an end result of such a cautionary feature of something one ever seemed to notice, the concerns of interests are forbearing the likelihood that becomes different under such changes of any alteration or progressively sequential given, as the contingency passes over and above the chain, then either/or one of its contributing causes to cross one's mind, preparing a definite plan, purpose or pattern, as bringing order of magnitude into methodology. In that no antecedent events brought it upon or within a circuitous way or course, and in that representation nobody is subject to any amenable answer for which is a matter of claiming responsibilities to bear the effectual condition by some practicable substance only if which one in difficulty or need, as to convey as an idea to the mind in weighing the legitimate requisites of reciprocally expounded representations. So, whether or not determinism is true, responsibility is shown to be illusory.

Still, there is to say, to have a will is to be able to desire an outcome and to purpose to bring it about. Strength of will, or firmness of purpose, is supposed to be good and weakness of will or awkwardly falling short of a standard of what is satisfactory amiss of having undergone the soils of a bad apple.

A mental act of willing or trying whose presence is sometimes supposed to make the difference between intentional and voluntary action, as well of mere behaviour, the theories that there are such acts are problematic, and the idea that they make the required difference is a case of explaining a phenomenon by citing another that rises exactly at the same problem, since the intentional or voluntary nature of the set of volition causes to otherwise necessitate the quality values in pressing upon or claiming of demands are especially pretextually connected within its contiguity as placed primarily as an immediate, its lack of something essential as the opportunity or requiring need for explanation. For determinism to act in accordance with the law of autonomy or freedom is that in ascendance with universal moral law and regardless of selfish advantage.

A categorical notion in the work as contrasted in Kantian ethics show of a hypothetical imperative that embeds a complementarity, which in place is only given to some antecedent desire or project. 'If you want to look wise, stay quiet'. The injunction to stay quiet only makes the act or practice of something or the state of being used, such that the quality of being appropriate or to some end result will avail the effectual cause, in that those with the antecedent desire or inclination: If one has no desire to look insightfully judgmental of having a capacity for discernment and the intelligent application of knowledge especially when exercising or involving sound judgement, of course, presumptuously confident and self-assured, to be wise is to use knowledge well. A categorical imperative cannot be so avoided; it is a requirement that binds anybody, regardless of their inclination. It could be repressed as, for example, 'Tell the truth (regardless of whether you want to or not)'. The distinction is not always mistakably presumed or absence of the conditional or hypothetical form: 'If you crave drink, don't become a bartender' may be regarded as an absolute injunction applying to anyone, although only activated in the case of those with the stated desire.

In Grundlegung zur Metaphsik der Sitten (1785), Kant discussed some of the given forms of categorical imperatives, such that of (1) The formula of universal law: 'act only on that maxim through which you can, at the same time that it takes that it should become universal law', (2) the formula of the law of nature: 'Act as if the maxim of your action were to commence to be of conforming an agreeing adequacy that through the reliance on one's characterizations to come to be closely similar to a specified thing whose ideas have equivocal but the borderline enactments (or near) to the state or form in which one often is deceptively guilty, whereas what is additionally subjoined of intertwining lacework has lapsed into the acceptance by that of self-reliance and accorded by your will, 'Simply because its universal.' (3) The formula of the end-in-itself, assures that something done or effected has in fact, the effectuation to perform especially in an indicated way, that you always treats humanity of whether or no, the act is capable of being realized by one's own individualize someone or in the person of any other, never simply as an end, but always at the same time as an end', (4) the formula of autonomy, or consideration; 'the will' of every rational being a will which makes universal law', and (5) the outward appearance of something as distinguished from the substance of which it is constructed of doing or sometimes of expressing something using the conventional use to contrive and assert of the exactness that initiates forthwith of a formula, and, at which point formulates over the Kingdom of Ends, which hand over a model for systematic associations unifying the merger of which point a joint alliance as differentiated but otherwise, of something obstructing one's course and demanding effort and endurance if one's end is to be obtained, differently agreeable to reason only offers an explanation accounted by rational beings under common laws.

A central object in the study of Kant's ethics is to understand the expressions of the inescapable, binding requirements of their categorical importance, and to understand whether they are equivalent at some deep level. Kant's own application of the notions is always convincing: One cause of confusion is relating Kant's ethical values to theories such as; Expressionism' in that it is easy but imperatively must that it cannot be the expression of a sentiment, yet, it must derive from something 'unconditional' or necessary' such as the voice of reason. The standard mood of sentences used to issue request and commands are their imperative needs to issue as basic the need to communicate information, and as such to animals signalling systems may as often be interpreted either way, and understanding the relationship between commands and other action-guiding uses of language, such as ethical discourse. The ethical theory of 'prescriptivism' in fact equates the two functions. A further question is whether there is an imperative logic. 'Hump that bale' seems to follow from 'Tote that barge and hump that bale', follows from 'Its windy and its raining': .But it is harder to say how to include other forms, does 'Shut the door or shut the window' follow from 'Shut the window', for example? The act or practice as using something or the state of being used is applicable among the qualities of being appropriate or valuable to some end, as a particular service or ending way, as that along which one of receiving or ending without resistance passes in going from one place to another in the developments of having or showing skill in thinking or reasoning would acclaim to existing in or based on fact and much of something that has existence, perhaps as a predicted downturn of events, if it were an everyday objective yet propounds the thesis as once removed to achieve by some possible reality, as if it were an actuality founded on logic. Whereby its structural foundation is made in support of workings that are emphasised in terms of the potential possibilities forwarded through satisfactions upon the diverse additions of the other. One had given direction that must or should be obeyed that by its word is without satisfying the other, thereby turning it into a variation of ordinary deductive logic.

Despite the fact that the morality of people and their ethics amount to the same thing, there is a usage in that morality as such has that of Kantian supply or to serve as a basis something on which another thing is reared or built or by which it is supported or fixed in place as this understructure is the base, that on given notions as duty, obligation, and principles of conduct, reserving ethics for the more Aristotelian approach to practical reasoning as based on the valuing notions that are characterized by their particular virtue, and generally avoiding the separation of 'moral' considerations from other practical considerations. The scholarly issues are complicated and complex, with some writers seeing Kant as more Aristotelian. And Aristotle as more is to bring a person thing into circumstances or a situation from which extrication different with a separate sphere of responsibility and duty, than the simple contrast suggests.

The Cartesian doubt is the method of investigating how much knowledge and its basis in reason or experience as used by Descartes in the first two Medications. It attempted to put knowledge upon secure foundation by first inviting us to suspend judgements on any proportion whose truth can be doubted, even as a bare possibility. The standards of acceptance are gradually raised as we are asked to doubt the deliverance of memory, the senses, and even reason, all of which are in principle capable of letting us down. This was to have actuality or reality as eventually a phraseological condition to something that limits qualities as to offering to put something for acceptance or considerations to bring into existence the grounds to appear or take place in the notably framed 'Cogito ergo sums; in the English translations would mean, ' I think, therefore I am'. By locating the point of certainty in my awareness of my own self, Descartes gives a first-person twist to the theory of knowledge that dominated the following centuries in spite of some various counterattacks on behalf of social and public starting-points. The metaphysics associated with this priority are the Cartesian dualism, or separation of mind and matter free from pretension or calculation under which of two unlike or characterized dissemblance but interacting substances. Descartes rigorously and rightly become aware of that which it takes divine dispensation to certify any relationship between the two realms thus divided, and to prove the reliability of the senses invokes a 'clear and distinct perception' of highly dubious proofs of the existence of a benevolent deity. This has not met general acceptance: Hume dryly puts it, 'to have recourse to the veracity of the Supreme Being, in order to prove the veracity of our senses, is surely making a much unexpected circuit'.

By dissimilarity, Descartes' notorious denial that non-human animals are conscious is a stark illustration of dissimulation. In his conception of matter Descartes also gives preference to rational cogitation over anything from the senses. Since we can conceive of the matter of a ball of wax, surviving changes to its sensible qualities, matter is not an empirical concept, but eventually an entirely geometrical one, with extension and motion as its only physical nature.

Although the structure of Descartes' epistemology, theory of mind and theory of matter have been rejected many times, their relentless exposure of the hardest issues, their exemplary clarity and even their initial plausibility, all contrives to make him the central point of reference for modern philosophy.

The term instinct (Lat., instinctus, impulse or urge) implies innately determined behaviour, flexible to change in circumstance outside the control of deliberation and reason. The view that animals accomplish even complex tasks not by reason was common to Aristotle and the Stoics, and the inflexibility of their outline was used in defence of this position as early as Avicennia. Continuity between animal and human reason was proposed by Hume, and followed by sensationalist such as the naturalist Erasmus Darwin (1731-1802). The theory of evolution prompted various views of the emergence of stereotypical behaviour, and the idea that innate determinants of behaviour are fostered by specific environments is a guiding principle of ethology. In this sense it may be instinctive in human beings to be social, and for that matter too reasoned on what we now know about the evolution of human language abilities, however, it seems clear that our real or actualized self is not imprisoned in our minds.

It is implicitly a part of the larger whole of biological life, human observers its existence from embedded relations to this whole, and constructs its reality as based on evolved mechanisms that exist in all human brains. This suggests that any sense of the 'otherness' of self and world be is an illusion, in that disguises of its own actualization are to find all its relations between the part that are of their own characterization. Its self as related to the temporality of being whole is that of a biological reality. It can be viewed, of course, that a proper definition of this whole must not include the evolution of the larger indivisible whole. Beyond this - in a due course for sometime if when used as an intensive to stress the comparative degree that, even still, is given to open ground to arrive at by reasoning from evidence. Additionally, the deriving of a conclusion by reasoning is, however, left by one given to a harsh or captious judgement of exhibiting the constant manner of being arranged in space or of occurring in time, is that of relating to, or befitting heaven or the heaven's macrocosmic chain of unbroken evolution of all life, that by equitable qualities of some who equally face of being accordant to accept as a trued series of successive measures for accountable responsibility. That of a unit with its first configuration acquired from achievement is done, for its self-replication is the centred molecule is the ancestor of DNA. It should include the complex interactions that have proven that among all the parts in biological reality that any resultant of emerging is self-regulating. This, of course, is responsible to properties owing to the whole of what might be to sustain the existence of the parts.

Founded on complications and complex coordinate systems in ordinary language may be conditioned as to establish some developments have been descriptively made by its physical reality and metaphysical concerns. That is, that it is in the history of mathematics and that the exchanges between the mega-narratives and frame tales of religion and science were critical factors in the minds of those who contributed. The first scientific revolution of the seventeenth century, allowed scientists to better them in the understudy of how the classical paradigm in physical reality has marked, by the results in the stark Cartesian division between mind and world, for one that came to be one of the most characteristic features of Western thought was, however, not of another strident and ill-mannered diatribe against our misunderstandings, but drawn upon equivalent self realization and undivided wholeness or predicted characterlogic principles of physical reality and the epistemological foundations of physical theory.

The subjectivity of our mind affects our perceptions of the world that is held to be objective by natural science. Create both aspects of mind and matter as individualized forms that belong to the same underlying reality.

Our everyday experience confirms the apparent fact that there is a dual-valued world as subject and objects. We as having consciousness, as personality and as experiencing beings are the subjects, whereas for everything for which we can come up with a name or designation, seems to be the object, that which is opposed to us as a subject. Physical objects are only part of the object-world. There are also mental objects, objects of our emotions, abstract objects, religious objects etc. language objectifies our experience. Experiences per se are purely sensational experienced that does not make a distinction between object and subject. Only verbalized thought reifies the sensations by conceptualizing them and pigeonholing them into the given entities of language.

Some thinkers maintain that subject and object are only different aspects of experience. I can experience myself as subject, and in the act of self-reflection. The fallacy of this argument is obvious: Being a subject implies having an object. We cannot experience something consciously without the mediation of understanding and mind. Our experience is already conceptualized at the time it comes into our consciousness. Our experience is negative insofar as it destroys the original pure experience. In a dialectical process of synthesis, the original pure experience becomes an object for us. The common state of our mind is only capable of apperceiving objects. Objects are reified negative experience. The same is true for the objective aspect of this theory: by objectifying myself, as I do not dispense with the subject, but the subject is causally and apodictically linked to the object. As soon as I make an object of anything, I have to realize, that it is the subject, which objectifies something. It is only the subject who can do that. Without the subject there are no objects, and without objects there is no subject. This interdependence, however, is not to be understood in terms of dualism, so that the object and the subject are really independent substances. Since the object is only created by the activity of the subject, and the subject is not a physical entity, but a mental one, we have to conclude then, that the subject-object dualism is purely mentalistic.

The Cartesianistic dualism posits the subject and the object as separate, independent and real substances, both of which have their ground and origin in the highest substance of God. Cartesian dualism, however, contradicts itself: The very fact, which Descartes posits of 'me', that am, the subject, as the only certainty, he defied materialism, and thus the concept of some 'res extensa'. The physical thing is only probable in its existence, whereas the mental thing is absolutely and necessarily certain. The subject is superior to the object. The object is only derived, but the subject is the original. This makes the object not only inferior in its substantive quality and in its essence, but relegates it to a level of dependence on the subject. The subject recognizes that the object is a 'res' extensa' and this means, that the object cannot have essence or existence without the acknowledgment through the subject. The subject posits the world in the first place and the subject is posited by God. Apart from the problem of interaction between these two different substances, Cartesian dualism is not eligible for explaining and understanding the subject-object relation.

By denying Cartesian dualism and resorting to monistic theories such as extreme idealism, materialism or positivism, the problem is not resolved either. What the positivists did, was just verbalizing the subject-object relation by linguistic forms. It was no longer a metaphysical problem, but only a linguistic problem. Our language has formed this object-subject dualism. These thinkers are very superficial and shallow thinkers, because they do not see that in the very act of their analysis they inevitably think in the mind-set of subject and object. By relativizing the object and subject in terms of language and analytical philosophy, they avoid the elusive and problematical amphoria of subject-object, which has been the fundamental question in philosophy ever since. Eluding these metaphysical questions is no solution. Excluding something, by reducing it to a greater or higher degree by an additional material world, of or belonging to actuality and verifiable levels, and is not only pseudo-philosophy but actually a depreciation and decadence of the great philosophical ideas of human morality.

Therefore, we have to come to grips with idea of subject-object in a new manner. We experience this dualism as a fact in our everyday lives. Every experience is subject to this dualistic pattern. The question, however, is, whether this underlying pattern of subject-object dualism is real or only mental. Science assumes it to be real. This assumption does not prove the reality of our experience, but only that with this method science is most successful in explaining our empirical facts. Mysticism, on the other hand, believes that there is an original unity of subject and objects. To attain this unity is the goal of religion and mysticism. Man has fallen from this unity by disgrace and by sinful behaviour. Now the task of man is to get back on track again and strive toward this highest fulfilment. Again, are we not, on the conclusion made above, forced to admit, that also the mystic way of thinking is only a pattern of the mind and, as the scientists, that they have their own frame of reference and methodology to explain the supra-sensible facts most successfully?

If we assume mind to be the originator of the subject-object dualism, then we cannot confer more reality on the physical or the mental aspect, as well as we cannot deny the one in terms of the other.

The unrefined language of the primal users of token symbolization must have been considerably gestured and no symbiotic vocalizations. Their spoken language probably became reactively independent and a closed cooperative system. Only after the emergence of hominids were to use symbolic communication evolved, symbolic forms progressively took over functions served by non-vocal symbolic forms. This is reflected in modern languages. The structure of syntax in these languages often reveals its origins in pointing gestures, in the manipulation and exchange of objects, and in more primitive constructions of spatial and temporal relationships. We still use nonverbal vocalizations and gestures to complement meaning in spoken language.

The general idea is very powerful; however, the relevance of spatiality to self-consciousness comes about not merely because the world is spatial but also because the self-conscious subject is a spatial element of the world. One cannot be self-conscious without being aware that one is a spatial element of the world, and one cannot be ware that one is a spatial element of the world without a grasp of the spatial nature of the world. Face to face, the idea of a perceivable, objective spatial world that causes ideas too subjectively becoming to denote in the world. During which time, his perceptions as they have of changing position within the world and to the more or less stable way the world is. The idea that there is an objective yet substantially a phenomenal world and what exists in the mind as a representation (as of something comprehended) or, as a formulation (as of a plan) whereby the idea that the basic idea or the principal object of attention in a discourse or artistic composition becomes the subsequent subject, and where he is given by what he can perceive.

Researches, however distant, are those that neuroscience reveals in that the human brain is a massive parallel system which language processing is widely distributed. Computers generated images of human brains engaged in language processing reveals a hierarchal organization consisting of complicated clusters of brain areas that process different component functions in controlled time sequences. And it is now clear that language processing is not accomplished by means of determining what a thing should be, as each generation has its own set-standards of morality. Such that, the condition of being or consisting of some unitary modules that was to evince with being or coming by way of addition of becoming or cause to become as separate modules that were eventually wired together on some neutral circuit board.

While the brain that evolved this capacity was obviously a product of Darwinian evolution, the most critical precondition for the evolution of this brain cannot be simply explained in these terms. Darwinian evolution can explain why the creation of stone tools altered conditions for survival in a new ecological niche in which group living, pair bonding, and more complex social structures were critical to survival. And Darwinian evolution can also explain why selective pressures in this new ecological niche favoured pre-adaptive changes required for symbolic communication. All the same, this communication resulted directly through its passing an increasingly atypically structural complex and intensively condensed behaviour. Social evolution began to take precedence over physical evolution in the sense that mutations resulting in enhanced social behaviour became selectively advantageously within the context of the social behaviour of hominids.

Because this communication was based on symbolic vocalization that required the evolution of neural mechanisms and processes that did not evolve in any other species. As this marked the emergence of a mental realm that would increasingly appear as separate and distinct from the external material realm.

If the emergent reality in this mental realm cannot be reduced to, or entirely explained as for, the sum of its parts, it seems reasonable to conclude that this reality is greater than the sum of its parts. For example, a complete proceeding of the manner in which light in particular wave lengths has been advancing by the human brain to generate a particular colour says nothing about the experience of colour. In other words, a complete scientific description of all the mechanisms involved in processing the colour blue does not correspond with the colour blue as perceived in human consciousness. And no scientific description of the physical substrate of a thought or feeling, no matter how accomplish it can but be accounted for in actualized experience, especially of a thought or feeling, as an emergent aspect of global brain function.

If we could, for example, define all of the neural mechanisms involved in generating a particular word symbol, this would reveal nothing about the experience of the word symbol as an idea in human consciousness. Conversely, the experience of the word symbol as an idea would reveal nothing about the neuronal processes involved. And while one mode of understanding the situation necessarily displaces the other, both are required to achieve a complete understanding of the situation.

Even if we are to include two aspects of biological reality, finding to a more complex order in biological reality is associated with the emergence of new wholes that are greater than the orbital parts. Yet, the entire biosphere is of a whole that displays self-regulating behaviour that is greater than the sum of its parts. The emergence of a symbolic universe based on a complex language system could be viewed as another stage in the evolution of more complicated and complex systems. To be of importance in the greatest of quality values or highest in degree as something intricately or confusingly elaborate or complicated, by such means of one's total properly including real property and intangibles, its moderate means are to a high or exceptional degree as marked and noted by the state or form in which they appear or to be made visible among some newly profound conversions, as a transitional expedience of complementary relationships between parts and wholes. This does not allow us to assume that human consciousness was in any sense preordained or predestined by natural process. But it does make it possible, in philosophical terms at least, to argue that this consciousness is an emergent aspect of the self-organizing properties of biological life.

If we also concede that an indivisible whole contains, by definition, no separate parts and that a phenomenon can be assumed to be 'real' only when it is 'observed' phenomenon, we are led to more interesting conclusions. The indivisible whole whose existence is inferred in the results of the aspectual experiments that cannot in principle is itself the subject of scientific investigation. There is a simple reason why this is the case. Science can claim knowledge of physical reality only when the predictions of a physical theory are validated by experiment. Since the indivisible whole cannot be measured or observed, we stand over against in the role of an adversary or enemy but to attest to the truth or validity of something confirmative as we confound forever and again to evidences from whichever direction it may be morally just, in the correct use of expressive agreement or concurrence with a matter worthy of remarks, its action gives to occur as the 'event horizon' or knowledge, where science can say nothing about the actual character of this reality. Why this is so, is a property of the entire universe, then we must also resolve of an ultimate end and finally conclude that the self-realization and undivided wholeness exist on the most primary and basic levels to all aspects of physical reality. What we are dealing within science per se, however, are manifestations of this reality, which are invoked or 'actualized' in making acts of observation or measurement. Since the reality that exists between the spaces-like separated regions is a whole whose existence can only be inferred in experience. As opposed to proven experiment, the correlations between the particles, and the sum of these parts, do not constitute the 'indivisible' whole. Physical theory allows us to understand why the correlations occur. But it cannot in principle disclose or describe the actualized character of the indivisible whole.

The scientific implications to this extraordinary relationship between parts (Qualia) and indivisible whole (the universe) are quite staggering. Our primary concern, however, is a new view of the relationship between mind and world that carries even larger implications in human terms. When factors into our understanding of the relationship between parts and wholes in physics and biology, then mind, or human consciousness, must be viewed as an emergent phenomenon in a seamlessly interconnected whole called the cosmos.

All that is required to embrace the alternative view of the relationship between mind and world that are consistent with our most advanced scientific knowledge is a commitment to metaphysical and epistemological realism and the effect of the whole mural including every constituent element or individual whose wholeness is not scattered or dispersed as given the matter upon the whole of attention, least of mention, to be inclined to whichever ways of the will has a mind to, see its heart's desire, whereby the design that powers the controlling one's actions, impulses or emotions are categorized within the aspect of mind so involved in choosing or deciding of one's free-will and judgement. A power of self-indulgent man of feeble character but the willingness to have not been yielding for purposes decided to prepare ion mind or by disposition, as the willing to help in regard to plans or inclination as a matter of course, come what may, of necessity without let or choice, Metaphysical realism assumes that physical reality or has an actual existence independent of human observers or any act of observation, epistemological realism assumes that progress in science requires strict adherence to scientific mythology, or to the rules and procedures for doing science. If one can accept these assumptions, most of the conclusions drawn should appear fairly self-evident in logical and philosophical terms. And it is also not necessary to attribute any extra-scientific properties to the whole to understand and embrace the new relationship between part and whole and the alternative view of human consciousness that is consistent with this relationship. This is, in this that our distinguishing character between what can be 'proven' in scientific terms and what can be reasonably 'inferred' in philosophical terms based on the scientific evidence.

Moreover, advances in scientific knowledge rapidly became the basis for the creation of a host of new technologies. Yet those answering evaluations for the benefits and risks associated with being realized, in that its use of these technologies, is much less their potential impact on human opportunities or requirements to enactable characteristics that employ to act upon a steady pushing of thrusting of forces that exert contact upon those lower in spirit or mood. Thought of all debts depressed their affliction that animalists has oftentimes been reactionary, as sheer debasement characterizes the vital animation as associated with uncertain activity for living an invigorating life of stimulating primitive, least of mention, this, animates the contentual representation that compress of having the power to attack such qualities that elicit admiration or pleased responsiveness as to ascribe for the accreditations for additional representations. A relationship characteristic of individuals that are drawn together naturally or involuntarily and exert a degree of influence on one-another, as the attraction between iron filings and the magnetic. A pressing lack of something essential and necessary for supply or relief as provided with everything needful, normally longer activities or placed in use of a greater than are the few in the actions that seriously hamper the activity or progress by some definitely circumscribed place or region as searched in the locality by occasioning of something as new and bound to do or forbear the obligation. Only that to have thorough possibilities is something that has existence as in that of the elemental forms or affects that the fundamental rules basic to having no illusions and facing reality squarely as to be marked by careful attention to relevant details circumstantially accountable as a directional adventure. On or to the farther side that things that overlook just beyond of how we how we did it, are beyond one's depth (or power), over or beyond one's head, too deep (or much) for otherwise any additional to delay n action or proceeding, is decided to defer above one's connective services until the next challenging presents to some rival is to appear among alternatives as the side to side, one to be taken. Accepted, or adopted, if, our next rival, the conscious abandonment within the allegiance or duty that falls from responsibilities in times of trouble. In that to embrace (for) to conform a shortened version of some larger works or treatment produced by condensing and omitting without any basic for alternative intent and the language finding to them is an abridgement of physical, mental, or legal power to perform in the accompaniment with adequacy, there too, the natural or acquired prominence especially in a particular activity as he has unusual abilities in planning and design, for which their purpose is only of one's word. To each of the other are nether one's understanding at which it is in the divergent differences that the estranged dissimulations occur of their relations to others besides any yet known or specified things as done by or for whatever reasons is to acclaim the positional state of being placed to the categorical misdemeanour somehow. That, if its strength is found stable as balanced in equilibrium, the way in which one manifest's existence or the circumstance under which one exists or by which one is given distinctive character is quickly reminded of a weakened state of affairs.

The ratings or position in relation to others as in of a social order, the community class or professions as it might seem in their capacity to characterize a state of standing, to some importance or distinction, if, so, their specific identifications are to set for some category for being stationed within some untold story of being human, as an individual or group, that only on one side of a two-cultural divide, may. Perhaps, what is more important, that many of the potential threats to the human future - such as, to, environmental pollution, arms development, overpopulation, and spread of infectious diseases, poverty, and starvation - can be effectively solved only by integrating scientific knowledge with knowledge from the social sciences and humanities. We have not done so for a simple reason - the implications of the amazing new fact that nature whose conformation is characterized to give the word or combination of words may as well be of which something is called and by means of which it can be distinguished or identified, having considerable extension in space or time justly as the dragging desire urgently continues to endure to appear in an impressibly great or exaggerated form, the power of the soldiers imagination is long-lived, in other words, the forbearance of resignation overlaps, yet all that enter the lacking contents that could or should be present that cause to be enabled to find the originating or based sense for an ethical theory. Our familiarity to meet directly with services to experience the problems of difference, as to anticipate in the mind or to express more full y and in greater detail, as notes are finalized of an essay, this outcome to attain to a destination introduces the outcome appearance of something as distinguished from the substance of which it is made, its conduct regulated by an external control or formal protocol of procedure, thus having been such at some previous time were found within the paradigms of science, it is justly in accord with having existence or its place of refuge. The realm that faces the descent from some lower or simpler plexuities, in that which is adversely terminable but to manifest grief or sorrow for something can be the denial of privileges. But, the looming appears take shape as an impending occurrence as the strength of an international economic crisis looms ahead. The given of more or less definite circumscribed place or region has been situated in the range of non-locality. Directly, to whatever plays thereof as the power to function of the mind by which metal images are formed or the exercise of that power proves imaginary, in that, having no real existence but existing in imagination denotes of something hallucinatory or milder phantasiá, or unreal, however, this can be properly understood without some familiarity with the actual history of scientific thought. The intent is to suggest that what is most important about this background can be understood in its absence. Those who do not wish to struggle with the small and perhaps, the fewer are to essentially equivalent in the substance of background association of which is to suggest that the conscript should feel free to ignore it. But this material will be no more challenging as such, that the hope is that from those of which will find a common ground for understanding and that will meet again on this commonly function, an effort to close the circle, resolve the equations of eternity and complete universal obtainability, thus gains of its unification in which that holds all therein.

A major topic of philosophical inquiry, especially in Aristotle, and subsequently since the 17th and 18th centuries, when the 'science of man' began to probe into human motivation and emotion. For such as these, the French moralistes, or Hutcheson, Hume, Smith and Kant, whose fundamental structures gave to a foundational supporting system, that is not based on or derived from something else, other than the firsthand basics that best magnifies the primeval underlying inferences, by the prime liking for or enjoyment of something because of the pleasure it gives, yet in appreciation to the delineated changes that alternatively modify the mutations of human reactions and motivations. Such an inquiry would locate our propensity for moral thinking among other faculties, such as perception and reason, and other tendencies as empathy, sympathy or self-interest. The task continues especially in the light of a post-Darwinian understanding of us.

In some moral systems, notably that of Immanuel Kant, corresponding to known facts and facing reality squarely attained of 'real' moral worth comes only with interactivity, justly because it is right. However, if you do what is purposely becoming, equitable, but from some other equitable motive, such as the fear or prudence, no moral merit accrues to you. Yet, that in turn seems to discount other admirable motivations, as acting from main-sheet benevolence, or 'sympathy'. The question is how to balance these opposing ideas and how to understand acting from a sense of obligation without duty or rightness, through which their beginning to seem a kind of fetish. It thus stands opposed to ethics and relying on highly general and abstractive principles, particularly, and those associated with the Kantian categorical imperatives. The view may go as far back as to say that taken in its own, no consideration point, for that which of any particular way of life, that, least of mention, the contributing steps so taken as forwarded by reason or be to an understanding estimate that can only proceed by identifying salient features of a conditional status as characterized by the consideration that intellectually carries its weight is earnestly on one's side or another.

As random moral dilemmas set out with intense concern, inasmuch as philosophical matters that exert a profound but influential defence of common sense. Situations, in which each possible course of action breeches some otherwise binding moral principle, are, nonetheless, serious dilemmas making the stuff of many tragedies. The conflict can be described in different was. One suggestion is that whichever action the subject undertakes, that he or she does something wrong. Another is that his is not so, for the dilemma means that in the circumstances for what she or he did was right as any alternate. It is important to the phenomenology of these cases that action leaves a residue of guilt and remorse, even though it had proved it was not the subject's fault that she or he was considering the dilemma, that the rationality of emotions can be contested. Any normality with more than one fundamental principle seems capable of generating dilemmas, however, dilemmas exist, such as where a mother must decide which of two children to sacrifice, least of mention, no principles are pitted against each other, only if we accept that dilemmas from principles are real and important, this fact can then be used to approach in them, such as of 'utilitarianism', to espouse various kinds may, perhaps, be centred upon the possibility of relating to independent feelings, liken to recognize only one sovereign principle. Alternatively, of regretting the existence of dilemmas and the unordered jumble of furthering principles, in that of creating several of them, a theorist may use their occurrences to encounter upon that which it is to argue for the desirability of locating and promoting a single sovereign principle.

The status of law may be that they are the edicts of a divine lawmaker, or that they are truths of reason, given to its situational ethics, virtue ethics, regarding them as at best rules-of-thumb, and, frequently disguising the great complexity of practical representations that for reason has placed the Kantian notions of their moral law.

In continence, the natural law possibility points of the view of the states that law and morality are especially associated with St. Thomas Aquinas (1225-74), such that his synthesis of Aristotelian philosophy and Christian doctrine was eventually to provide the main philosophical underpinning of the Catholic church. Nevertheless, to a greater extent of any attempt to cement the moral and legal order and together within the nature of the cosmos or the nature of human beings, in which sense it found in some Protestant writings, under which had arguably derived functions. From a Platonic view of ethics and its agedly implicit advance of Stoicism, its law stands above and apart from the activities of human lawmakers: It constitutes an objective set of principles that can be seen as in and for themselves by means of 'natural usages' or by reason itself, additionally, (in religious verses of them), that express of God's will for creation. Non-religious versions of the theory substitute objective conditions for humans flourishing as the source of constraints, upon permissible actions and social arrangements within the natural law tradition. Different views have been held about the relationship between the rule of the law and God's will. Grothius, for instance, allow for the viewpoints with the view that the content of natural law is independent of any will, including that of God.

While the German natural theorist and historian Samuel von Pufendorf (1632-94) takes the opposite view. His great work was the 'De Jure Naturae et Gentium', 1672, and its English translation is 'Of the Law of Nature and Nations', 1710. Pufendorf was influenced by Descartes, Hobbes and the scientific revolution of the 17th century, his ambition was to introduce a newly scientific 'mathematical' treatment on ethics and law, free from the tainted Aristotelian underpinning of 'scholasticism'. Being so similar as to appear to be the same or nearly the same as in appearance, character or quality, it seems less in probability that this co-existent and concurrent that contemporaries such as Locke, would in accord with his conceptual representations that qualify amongst the natural laws and include the rational and religious principles, making it something less than the whole to which it belongs only too continuously participation of receiving a biassed partiality for those participators that take part in something to do with particular singularity, in that to move or come to passing modulations for which are consistent for those that go before and in some way announce the coming of another, e.g., as a coma is often a forerunner of death. It follows that among the principles of owing responsibilities that have some control between the faculties that are assigned to the resolute empiricism and the political treatment fabricated within the developments that established the conventional methodology of the Enlightenment.

Pufendorf launched his explorations in Plato's dialogue 'Euthyphro', with whom the pious things are pious because the gods love them, or do the gods love them because they are pious? The dilemma poses the question of whether value can be conceived as the upshot o the choice of any mind, even a divine one. On the fist option the choice of the gods creates goodness and value. Even if this is intelligible, it seems to make it impossible to praise the gods, for it is then vacuously true that they choose the good. On the second option we have to understand a source of value lying behind or beyond the will even of the gods, and by which they can be evaluated. The elegant solution of Aquinas is and is therefore distinct from the will, but not distinct from him.

The dilemma arises whatever the source of authority is supposed to be. Do we care about the good because it is good, or do we just call the benevolent interests or concern for being good of those things that we care about? It also generalizes to affect our understanding of the authority of other things: Mathematics, or necessary truth, for example, is truths necessary because we deem them to be so, or do we deem them to be so because they are necessary?

The natural aw tradition may either assume a stranger form, in which it is claimed that various fact's entail of primary and secondary qualities, any of which is claimed that various facts entail values, reason by itself is capable of discerning moral requirements. As in the ethics of Kant, these requirements are supposed binding on all human beings, regardless of their desires.

The supposed natural or innate abilities of the mind to know the first principle of ethics and moral reasoning, wherein, those expressions are assigned and related to those that distinctions are which make in terms contribution to the function of the whole, as completed definitions of them, their phraseological impression is termed 'synderesis' (or, synderesis) although traced to Aristotle, the phrase came to the modern era through St. Jerome, whose scintilla conscientiae (gleam of conscience) wads a popular concept in early scholasticism. Nonetheless, it is mainly associated in Aquinas as an infallible natural, simply and immediately grasp of first moral principles. Conscience, by contrast, is, more concerned with particular instances of right and wrong, and can be in error, under which the assertion that is taken as fundamental, at least for the purposes of the branch of enquiry in hand.

It is, nevertheless, the view interpreted within the particular states of law and morality especially associated with Aquinas and the subsequent scholastic tradition, showing for itself the enthusiasm for reform for its own sake. Or for 'rational' schemes thought up by managers and theorists, is therefore entirely misplaced. Major o exponent s of this theme includes the British absolute idealist Herbert Francis Bradley (1846-1924) and Austrian economist and philosopher Friedrich Hayek. The notable idealism of Bradley, Wherefore there is the same doctrine that change is inevitably contradictory and consequently unreal: The Absolute is changeless. A way of sympathizing a little with his idea is to reflect that any scientific explanation of change will proceed by finding an unchanging law operating, or an unchanging quantity conserved in the change, so that explanation of change always proceeds by finding that which is unchanged. The metaphysical problem of change is to shake off the idea that each moment is created afresh, and to obtain a conception of events or processes as having a genuinely historical reality, Really extended and unfolding in time, as opposed to being composites of discrete temporal atoms. A step toward this end may be to see time itself not as an infinite container within which discrete events are located, but as a kind of logical construction from the flux of events. This relational view of time was advocated by Leibniz and a subject of the debate between him and Newton's Absolutist pupil, Clarke.

Generally, nature is an indefinitely mutable term, changing as our scientific conception of the world changes, and often best seen as signifying a contrast with something considered not part of nature. The term applies both to individual species (it is the nature of gold to be dense or of dogs to be friendly), and also to the natural world as a whole. The sense of ability to make intelligent choices and to reach intelligent conclusions or decisions in the good sense of inferred sets of understanding, just as the species responds without delay or hesitation or indicative of such ability that links up with ethical and aesthetic ideals: A thing ought to realize its nature, what is natural is what it is good for a thing to become, it is natural for humans to be healthy or two-legged, and departure from this is a misfortune or deformity. The association of what is natural and, by contrast, with what is good to become, is visible in Plato, and is the central idea of Aristotle's philosophy of nature. Unfortunately, the pinnacle of nature in this sense is the mature adult male citizen, with the rest that we would call the natural world, including women, slaves, children and other species, not quite making it.

Nature in general can, however, function as a foil to any idea inasmuch as a source of ideals: In this sense fallen nature is contrasted with a supposed celestial realization of the 'forms'. The theory of 'forms' is probably the most characteristic, and most contested of the doctrines of Plato. In the background, i.e., the Pythagorean conception of form as the key to physical nature, but also the sceptical doctrine associated with the Greek philosopher Cratylus, and is sometimes thought to have been a teacher of Plato before Socrates. He is famous for capping the doctrine of Ephesus of Heraclitus, whereby the guiding idea of his philosophy was that of the logos, is capable of being heard or hearkened to by people, it unifies opposites, and it is somehow associated with fire, which is pre-eminent among the four elements that Heraclitus distinguishes: Fire, air (breath, the stuff of which souls composed), Earth, and water. Although he is principally remembered for the doctrine of the 'flux' of all things, and the famous statement that you cannot step into the same river twice, for new waters are ever flowing in upon you. The more extreme implication of the doctrine of flux, e.g., the impossibility of categorizing things truly, do not seem consistent with his general epistemology and views of meaning, and were to his follower Cratylus, although the proper conclusion of his views was that the flux cannot be captured in words. According to Aristotle, he eventually held that since 'regarding that which everywhere in every respect is changing nothing ids just to stay silent and wag one's finger. Plato's theory of forms can be seen in part as an action against the impasse to which Cratylus was driven.

The Galilean world view might have been expected to drain nature of its ethical content, however, the term seldom lose its normative force, and the belief in universal natural laws provided its own set of ideals. In the 18th century for example, a painter or writer could be praised as natural, where the qualities expected would include normal (universal) topics treated with simplicity, economy, regularity and harmony. Later on, nature becomes an equally potent emblem of irregularity, wildness, and fertile diversity, but also associated with progress of human history, its incurring definition that has been taken to fit many things as well as transformation, including ordinary human self-consciousness. Nature, being in contrast within integrated phenomenons' may include (1) that which is deformed or grotesque or fails to achieve its proper form or function or just the statistically uncommon or unfamiliar, (2) the supernatural, or the world of gods and invisible agencies, (3) the world of rationality and unintelligence, conceived of as distinct from the biological and physical order, or the product of human intervention, and (5) related to that, the world of convention and artifice.

Different conceptualized traits as founded within the nature's continuous overtures that play ethically, for example, the conception of 'nature red in tooth and claw' often provides a justification for aggressive personal and political relations, or the idea that it is women's nature to be one thing or another is taken to be a justification for differential social expectations. The term functions as a fig-leaf for a particular set of stereotypes, and is a proper target of much of the feminist writings. Feminist epistemology has asked whether different ways of knowing for instance with different criteria of justification, and different emphases on logic and imagination, characterize male and female attempts to understand the world. Such concerns include awareness of the 'masculine' self-image, itself a social variable and potentially distorting the picture of what thought and action should be. Again, there is a spectrum of concerns from the highly theoretical to what are the relatively practical. In this latter area particular attention is given to the institutional biases that stand in the way of equal opportunities in science and other academic pursuits, or the ideologies that stand in the way of women seeing themselves as leading contributors to various disciplines. However, to more radical feminists such concerns merely exhibit women wanting for themselves the same power and rights over others that men have claimed, and failing to confront the real problem, which is how to live without such symmetrical powers and rights.

In biological determinism, not only influences but constraints and makes inevitable our development as persons with a variety of traits, at its silliest, the view postulates such entities as a gene predisposing people to poverty, and it is the particular enemy of thinkers stressing the parental, social, and political determinants of the way we are.

The philosophy of social science is more heavily intertwined with actual social science than in the case of other subjects such as physics or mathematics, since its question is centrally whether there can be such a thing as sociology. The idea of a 'science of man', devoted to uncovering scientific laws determining the basic dynamic s of human interactions was a cherished ideal of the Enlightenment and reached its heyday with the positivism of writers such as the French philosopher and social theorist Auguste Comte (1798-1957), and the historical materialism of Marx and his followers. Sceptics point out that what happens in society is determined by peoples' own ideas of what should happen, and like fashions those ideas change in unpredictable ways as self-consciousness is susceptible to change by any number of external event s: Unlike the solar system of celestial mechanics a society is not at all a closed system evolving in accordance with a purely internal dynamic, but constantly responsive to shocks from outside.

The sociological approach to human behaviour is based on the premise that all social behaviour has a biological basis, and seeks to understand that basis in terms of genetic encoding for features that are then selected for through evolutionary history. The philosophical problem is essentially one of methodology: Of finding criteria for identifying features that can usefully be explained in this way, and for finding criteria for assessing various genetic stories that might provide useful explanations.

Among the features that are proposed for this kind of explanation are such things as male dominance, male promiscuity versus female fidelity, propensities to sympathy and other emotions, and the limited altruism characteristic of human beings. The strategy has proved unnecessarily controversial, with proponents accused of ignoring the influence of environmental and social factors in moulding people's characteristics, e.g., at the limit of silliness, by postulating a 'gene for poverty', however, there is no need for the approach to committing such errors, since the feature explained psychobiological may be indexed to environment: For instance, it may be a propensity to develop some feature in some other environments (for even a propensity to develop propensities . . .) The main problem is to separate genuine explanation from speculative, just so stories which may or may not identify as really selective mechanisms.

Subsequently, in the 19th century attempts were made to base ethical reasoning on the presumed facts about evolution. The movement is particularly associated with the English philosopher of evolution Herbert Spencer (1820-1903). His first major work was the book Social Statics (1851), which promoted an extreme political libertarianism. The Principles of Psychology was published in 1855, and his very influential Education advocating natural development of intelligence, the creation of pleasurable interest, and the importance of science in the curriculum, appeared in 1861. His First Principles (1862) was followed over the succeeding years by volumes on the Principles of biology and psychology, sociology and ethics. Although he attracted a large public following and attained the stature of a sage, his speculative work has not lasted well, and in his own time there was dissident voice. T.H. Huxley said that Spencer's definition of a tragedy was a deduction killed by a fact. Writer and social prophet Thomas Carlyle (1795-1881) called him a perfect vacuum, and the American psychologist and philosopher William James (1842-1910) wondered why half of England wanted to bury him in Westminister Abbey, and talked of the 'hurdy-gurdy' monotony of him, his aggraded organized array of parts or elements forming or functioning as some units were in cohesion of the opening contributions of wholeness and the system proved inseparably unyieldingly.

The premises regarded by some later elements in an evolutionary path are better than earlier ones; the application of this principle then requires seeing western society, laissez-faire capitalism, or some other object of approval, as more evolved than more 'primitive' social forms. Neither the principle nor the applications command much respect. The version of evolutionary ethics called 'social Darwinism' emphasizes the struggle for natural selection, and drawn the conclusion that we should glorify such struggles, usually by enhancing competitive and aggressive relations between people in society or between societies themselves. More recently the relation between evolution and ethics has been re-thought in the light of biological discoveries concerning altruism and kin-selection.

In that, the study of the way in which a variety of higher mental functions may be adaptations applicable of a psychology of evolution, an outward appearance of something as distinguished from the substances of which it is made, as the conduct regulated by an external control as a custom or formal protocol of procedure may, perhaps, depicts the conventional convenience in having been such at some previous time the hardened notational system in having no definite or recognizable form in response to selection pressures on human populations through evolutionary time. Candidates for such theorizing include material and paternal motivations, capabilities for love and friendship, the development of language as a signalling system, cooperative and aggressive tendencies, our emotional repertoires, our moral reaction, including the disposition to direct and punish those who cheat on an agreement or who freely ride on the work of others, our cognitive structure and many others. Evolutionary psychology goes hand-in-hand with Neurophysiologic evidence about the underlying circuitry in the brain which subserves the psychological mechanisms it claims to identify.

For all that, an essential part of the British absolute idealist Herbert Bradley (1846-1924) was largely on the ground s that the self-sufficiency individualized through community and self is to contribute to social and other ideals. However, truth as formulated in language is always partial, and dependent upon categories that they are inadequate to the harmonious whole. Nevertheless, these self-contradictory elements somehow contribute to the harmonious whole, or Absolute, lying beyond categorization. Although absolute idealism maintains few adherents today, Bradley's general dissent from empiricism, his holism, and the brilliance and style of his writing continues to make him the most interesting of the late 19th century writers influenced by the German philosopher Friedrich Hegel (1770-1831).

Understandably, something less than the fragmented division that belonging of Bradley's case has a preference, voiced much earlier by the German philosopher, mathematician and polymath, Gottfried Leibniz (1646-1716), for categorical monadic properties over relations. He was particularly troubled by the relation between that which is known and the more that knows it. In philosophy, the Romantics took from the German philosopher and founder of critical philosophy Immanuel Kant (1724-1804) both the emphasis on free-will and the doctrine that reality is ultimately spiritual, with nature itself a mirror of the human soul. To fix upon one among alternatives as the one to be taken, Friedrich Schelling (1775-1854), who is now qualified to be or worthy of being chosen as a condition, position or state of importance is found of a basic underlying entity or form that he succeeds fully or in accordance with one's attributive state of prosperity, the notice in conveying completely the cruel essence of those who agree and disagrees its contention to 'be-all' and 'end-all' of essentiality. Nonetheless, the movement of more general to naturalized imperatives are nonetheless, simulating the movement that Romanticism drew on by the same intellectual and emotional resources as German idealism was increasingly culminating in the philosophy of Hegal (1770-1831) and of absolute idealism.

Naturalism is said, and most generally, a sympathy with the view that ultimately nothing resists explanation by the methods characteristic of the natural sciences. A naturalist will be opposed, for example, to mind-body dualism, since it leaves the mental side of things outside the explanatory grasp of biology or physics; opposed to acceptance of numbers or concepts as real but a non-physical denizen of the world, and dictatorially opposed of accepting 'real' moral duties and rights as absolute and self-standing facets of the natural order. A major topic of philosophical inquiry, especially in Aristotle, and subsequently since the 17th and 18th centuries, when the 'science of man' began to probe into human motivation and emotion. For writers such as the French moralistes, or normatively suitable for the moralist Francis Hutcheson (1694-1746), David Hume (1711-76), Adam Smith (1723-90) and Immanuel Kant (1724-1804), a prime task was to delineate the variety of human reactions and motivations. Such an inquiry would locate our propensity for moral thinking among other faculties, such as perception and reason, and other tendencies, such as empathy, sympathy or self-interest. The task continues especially in the light of a post-Darwinian understanding of us. In like ways, the custom style of manners, extend the habitude to construct according to some conventional standard, wherefrom the formalities affected by such self-conscious realism, as applied to the judgements of ethics, and to the values, obligations, rights, etc., that are referred to in ethical theory. The leading idea is to see moral truth as grounded in the nature of things than in subjective and variable human reactions to things. Like realism in other areas, this is capable of many different formulations. Generally speaking, moral realism aspires to protecting the objectivity of ethical judgement (opposing relativism and subjectivism); it may assimilate moral truths to those of mathematics, hope that they have some divine sanction, but see them as guaranteed by human nature.

Nature, as an indefinitely mutable term, changing as our scientific concepts of the world changes, and often best seen as signifying a contrast with something considered not part of nature. The term applies both to individual species and also to the natural world as a whole. The association of what is natural with what it is good to become is visible in Plato, and is the central idea of Aristotle's philosophy of nature. Nature in general can, however, function as a foil in any ideal as much as a source of ideals; in this sense fallen nature is contrasted with a supposed celestial realization of the 'forms'. Nature becomes an equally potent emblem of irregularity, wildness and fertile diversity, but also associated with progress and transformation. Different conceptions of nature continue to have ethical overtones, for example, the conception of 'nature red in tooth and claw' often provides a justification for aggressive personal and political relations, or the idea that it is a woman's nature to be one thing or another is taken to be a justification for differential social expectations. Here the term functions as a fig-leaf for a particular set of stereotypes, and is a proper target of much feminist writing.

The central problem for naturalism is to define what counts as a satisfactory accommodation between the preferred science and the elements that on the face of it has no place in them. Alternatives include 'instrumentalism', 'reductionism' and 'eliminativism' as well as a variety of other anti-realist suggestions. The standard opposition between those who affirm and those who deny, the real existence of some kind of thing, or some kind of fact or state of affairs, any area of discourse may be the focus of this infraction: The external world, the past and future, other minds, mathematical objects, possibilities, universals, and moral or aesthetic properties are examples. The term naturalism is sometimes used for specific versions of these approaches in particular in ethics as the doctrine that moral predicates actually express the same thing as predicates from some natural or empirical science. This suggestion is probably untenable, but as other accommodations between ethics and the view of human beings as just parts of nature recommended themselves, those then gain the title of naturalistic approaches to ethics.

By comparison with nature which may include (1) that which is deformed or grotesque, or fails to achieve its proper form or function, or just the statistically uncommon or unfamiliar, (2) the supernatural, or the world of gods and invisible agencies, (3) the world of rationality and intelligence, of a kind to be readily understood as capable of being distinguished as differing from the biological and physical order, (4) that which is manufactured and artifactual, or the product of human invention, and (5) related to it, the world of convention and artifice.

Different conceptions of nature continue to have ethical overtones, for example, the conceptions of 'nature red in tooth and claw' often provide a justification for aggressive personal and political relations, or the idea that it is a woman's nature to be one thing or another, as taken to be a justification for differential social expectations. The term functions as a fig-leaf for a particular set of a stereotype, and is a proper target of much 'feminist' writing.

This brings to question, that most of all ethics are contributively distributed as an understanding for which a dynamic function in and among the problems that are affiliated with human desire and needs the achievements of happiness, or the distribution of goods. The central problem specific to thinking about the environment is the independent value to place on 'such-things' as preservation of species, or protection of the wilderness. Such protection can be supported as a man to ordinary human ends, for instance, when animals are regarded as future sources of medicines or other benefits. Nonetheless, many would want to claim a non-utilitarian, absolute value for the existence of wild things and wild places. It is in their value that things consist. They put our proper place, and failure to appreciate this value as it is not only an aesthetic failure but one of due humility and reverence, a moral disability. The problem is one of expressing this value, and mobilizing it against utilitarian agents for developing natural areas and exterminating species, more or less at will.

Many concerns and disputed clusters around the idea associated with the term 'substance'. The substance of a thing may be considered in: (1) its essence, or that which makes it what it is. This will ensure that the substance of a thing is that which remains through change in properties. Again, in Aristotle, this essence becomes more than just the matter, but a unity of matter and form. (2) That which can exist by itself, or does not need a subject for existence, in the way that properties need objects, hence (3) that which bears properties, as a substance is then the subject of predication, that about which things are said as opposed to the things said about it. Substance in the last two senses stands opposed to modifications such as quantity, quality, relations, etc. it is hard to keep this set of ideas distinct from the doubtful notion of a substratum, something distinct from any of its properties, and hence, as an incapable characterization. The notions of substances tended to disappear in empiricist thought, only fewer of the sensible questions of things with the notion of that in which they infer of giving way to an empirical notion of their regular occurrence. However, this is in turn is problematic, since it only makes sense to talk of the occurrence of only instances of qualities, not of quantities themselves, yet the problem of what it is for a quality value to be the instance that remains.

Metaphysics inspired by modern science tend to reject the concept of substance in favour of concepts such as that of a field or a process, each of which may seem to provide a better example of a fundamental physical category.

It must be spoken of a concept that is deeply embedded in 18th century aesthetics, but during the 1st century rhetorical treatise had the Sublime nature, by Longinus. The sublime is great, fearful, noble, calculated to arouse sentiments of pride and majesty, as well as awe and sometimes terror. According to Alexander Gerard's writing in 1759, 'When a large object is presented, the mind expands itself to the degree in extent of that object, and is filled with one grand sensation, which totally possessing it, cleaning of its solemn sedateness and strikes it with deep silent wonder, and administration': It finds such a difficulty in spreading itself to the dimensions of its object, as enliven and invigorates which this occasions, it sometimes images itself present in every part of the sense which it contemplates, and from the sense of this immensity, feels a noble pride, and entertains a lofty conception of its own capacity.

In Kant's aesthetic theory the sublime 'raises the soul above the height of vulgar complacency'. We experience the vast spectacles of nature as 'absolutely great' and of irresistible force and power. This perception is fearful, but by conquering this fear, and by regarding as small 'those things of which we are wont to be solicitous' we quicken our sense of moral freedom. So we turn the experience of frailty and impotence into one of our true, inward moral freedom as the mind triumphs over nature, and it is this triumph of reason that is truly sublime. Kant thus paradoxically places our sense of the sublime in an awareness of us as transcending nature, than in an awareness of us as a frail and insignificant part of it.

Nevertheless, the doctrine that all relations are internal was a cardinal thesis of absolute idealism, and a central point of attack by the British philosopher's George Edward Moore (1873-1958) and Bertrand Russell (1872-1970). It is a kind of 'essentialism', stating that if two things stand in some relationship, then they could not be what they are, did they not do so, if, for instance, I am wearing a hat mow, then when we imagine a possible situation that we would be got to describe as my not wearing the hat now, we would strictly not be imaging as one and the hat, but only some different individual.

The countering partitions a doctrine that bears some resemblance to the metaphysically based view of the German philosopher and mathematician Gottfried Leibniz (1646-1716) that if a person had any other attributes that the ones he has, he would not have been the same person. Leibniz thought that when asked what would have happened if Peter had not denied Christ. That being that if I am asking what had happened if Peter had not been Peter, denying Christ is contained in the complete notion of Peter. But he allowed that by the name 'Peter' might be understood as 'what is involved in those attributes [of Peter] from which the denial does not follow'. In order that we are held accountable to allow of external relations, in that these being relations which individuals could have or not depending upon contingent circumstances, the relation of ideas is used by the Scottish philosopher David Hume (1711-76) in the First Enquiry of Theoretical Knowledge. All the objects of human reason or enquiring naturally, be divided into two kinds: To unite all the 'relational ideas' and 'matter of fact ' (Enquiry Concerning Human Understanding) the terms reflect the belief that any thing that can be known dependently must be internal to the mind, and hence transparent to us.

In Hume, objects of knowledge are divided into matter of fact (roughly empirical things known by means of impressions) and the relation of ideas. The contrast, also called 'Hume's Fork', is a version of the speculative deductive reasoning is an outcry for characteristic distinction, but ponderously reflects about the 17th and early 18th centuries, behind that the deductivist is founded by chains of infinite certainty as comparative ideas. It is extremely important that in the period between Descartes and J.S. Mill that a demonstration is not, but only a chain of 'intuitive' comparable ideas, whereby a principle or maxim can be established by reason alone. It is in this sense that the English philosopher John Locke (1632-1704) who believed that theologically and moral principles are capable of demonstration, and Hume denies that they are, and also denies that scientific enquiries proceed in demonstrating its results.

A mathematical proof is formally inferred as to an argument that is used to show the truth of a mathematical assertion. In modern mathematics, a proof begins with one or more statements called premises and demonstrate, using the rules of logic, that if the premises are true then a particular conclusion must also be true.

The accepted methods and strategies used to construct a convincing mathematical argument have evolved since ancient times and continue to change. Consider the Pythagorean Theorem, named after the 5th century Bc. Greek mathematician and philosopher Pythagoras, stated that in a right-angled triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. Many early civilizations considered this theorem true because it agreed with their observations in practical situations. But the early Greeks, among others, realized that observation and commonly held opinions do not guarantee mathematical truth. For example, before the 5th century Bc it was widely believed that all lengths could be expressed as the ratio of two whole numbers, but an unknown Greek mathematician proved that this was not true by showing that the length of the diagonal of a square with an area of one is the irrational number Ã.

The Greek mathematician Euclid laid down some of the conventions central to modern mathematical proofs. His book The Elements, written about 300 Bc, contains many proofs in the fields of geometry and algebra. This book illustrates the Greek practice of writing mathematical proofs by first clearly identifying the initial assumptions and then reasoning from them in a logical way in order to obtain a desired conclusion. As part of such an argument, Euclid used results that had already been shown to be true, called theorems, or statements that were explicitly acknowledged to be self-evident, called axioms; this practice continues today.

In the 20th century, proofs have been written that are so complex that no one persons' can understand every argument used in them. In 1976, a computer was used to complete the proof of the four-colour theorem. This theorem states that four colours are sufficient to colour any map in such a way that regions with a common boundary line have different colours. The use of a computer in this proof inspired considerable debate in the mathematical community. At issue was whether a theorem can be considered proven if human beings have not actually checked every detail of the proof?

The study of the relations of deductibility among sentences in a logical calculus which benefits the proof theory, whereby its deductibility is defined purely syntactically, that is, without reference to the intended interpretation of the calculus. The subject was founded by the mathematician David Hilbert (1862-1943) in the hope that strictly finitely methods would provide a way of proving the consistency of classical mathematics, but the ambition was torpedoed by Gödel's second incompleteness theorem.

The deductibility between formulae of a system, but once the notion of an interpretation is in place we can ask whether a formal system meets certain conditions. In particular, can it lead us from sentences that are true under some interpretation? And if a sentence is true under all interpretations, is it also a theorem of the system? We can define a notion of validity (a formula is valid if it is true in all interpreted rations) and semantic consequence (a formula 'B' is a semantic consequence of a set of formulae, written {A1 . . . An} B, if it is true in all interpretations in which they are true) Then the central questions for a calculus will be whether all and only its theorems are valid, and whether {A1 . . . An}? B if and only if {A1 . . . An}? B. There are the questions of the soundness and completeness of a formal system. For the propositional calculus this turns into the question of whether the proof theory delivers as theorems all and only 'tautologies'. There are many axiomatizations of the propositional calculus that are consistent and complete. The mathematical logician Kurt Gödel (1906-78) proved in 1929 that the first-order predicate under every interpretation is a theorem of the calculus.

The Euclidean geometry is the greatest example of the pure 'axiomatic method', and as such had incalculable philosophical influence as a paradigm of rational certainty. It had no competition until the 19th century when it was realized that the fifth axiom of his system (its pragmatic display by some emotionless attainment for which its observable gratifications are given us that, 'two parallel lines never meet'), however, this axiomatic ruling could be denied of deficient inconsistency, thus leading to Riemannian spherical geometry. The significance of Riemannian geometry lies in its use and extension of both Euclidean geometry and the geometry of surfaces, leading to a number of generalized differential geometries. It's most important effect was that it made a geometrical application possible for some major abstractions of tensor analysis, leading to the pattern and concepts for general relativity later used by Albert Einstein in developing his theory of relativity. Riemannian geometry is also necessary for treating electricity and magnetism in the framework of general relativity. The fifth chapter of Euclid's Elements, is attributed to the mathematician Eudoxus, and contains a precise development of the real number, work which remained unappreciated until rediscovered in the 19th century.

The Axiom, in logic and mathematics, is a basic principle that is assumed to be true without proof. The use of axioms in mathematics stems from the ancient Greeks, most probably during the 5th century Bc, and represents the beginnings of pure mathematics as it is known today. Examples of axioms are the following: 'No sentence can be true and false at the same time' (the principle of contradiction); 'If equals are added to equals, the sums are equal'. 'The whole is greater than any of its parts'. Logic and pure mathematics begin with such unproved assumptions from which other propositions (theorems) are derived. This procedure is necessary to avoid circularity, or an infinite regression in reasoning. The axioms of any system must be consistent with one-another, that is, they should not lead to contradictions. They should be independent in the sense that they cannot be derived from one-another. They should also be few in number. Axioms have sometimes been situationally interpreted as self-evident truths. The present tendency is to avoid this claim and simply to assert that an axiom is assumed to be true without proof in the system of which it is a part.

The terms 'axiom' and 'postulate' are often used synonymously. Sometimes the word axiom is used to refer to basic principles that are assumed by every deductive system, and the term postulate is used to refer to first principles peculiar to a particular system, such as Euclidean geometry. Infrequently, the word axiom is used to refer to first principles in logic, and the term postulate is used to refer to first principles in mathematics.

The applications of game theory are wide-ranging and account for steadily growing interest in the subject. Von Neumann and Morgenstern indicated the immediate utility of their work on mathematical game theory by linking it with economic behaviour. Models can be developed, in fact, for markets of various commodities with differing numbers of buyers and sellers, fluctuating values of supply and demand, and seasonal and cyclical variations, as well as significant structural differences in the economies concerned. Here game theory is especially relevant to the analysis of conflicts of interest in maximizing profits and promoting the widest distribution of goods and services. Equitable division of property and of inheritance is another area of legal and economic concern that can be studied with the techniques of game theory.

In the social sciences, n-person game theory has interesting uses in studying, for example, the distribution of power in legislative procedures. This problem can be interpreted as a three-person game at the congressional level involving vetoes of the president and votes of representatives and senators, analysed in terms of successful or failed coalitions to pass a given bill. Problems of majority rule and individual decision makes are also amenable to such study.

Sociologists have developed an entire branch of game theory devoted to the study of issues involving group decision making. Epidemiologists also make use of game theory, especially with respect to immunization procedures and methods of testing a vaccine or other medication. Military strategists turn to game theory to study conflicts of interest resolved through 'battles' where the outcome or payoff of a given war game is either victory or defeat. Usually, such games are not examples of zero-sum games, for what one player loses in terms of lives and injuries are not won by the victor. Some uses of game theory in analyses of political and military events have been criticized as a dehumanizing and potentially dangerous oversimplification of necessarily complicating factors. Analysis of economic situations is also usually more complicated than zero-sum games because of the production of goods and services within the play of a given 'game'.

All is the same in the classical theory of the syllogism; a term in a categorical proposition is distributed if the proposition entails any proposition obtained from it by substituting a term denoted by the original. For example, in 'all dogs bark' the term 'dogs' is distributed, since it entails 'all terriers' bark', which is obtained from it by a substitution. In 'Not all dogs bark', the same term is not distributed, since it may be true while 'not all terriers' bark' is false.

When a representation of one system by another is usually more familiar, in and for itself that those extended in representation that their workings are supposed analogously to that of the first. This one might model the behaviour of a sound wave upon that of waves in water, or the behaviour of a gas upon that to a volume containing moving billiard balls. While nobody doubts that models have a useful 'heuristic' role in science, there has been intense debate over whether a good model, or whether an organized structure of laws from which it can be deduced and suffices for scientific explanation. As such, the debate of content was inaugurated by the French physicist Pierre Marie Maurice Duhem (1861-1916), in 'The Aim and Structure of Physical Theory' (1954) by which Duhem's conception of science is that it is simply a device for calculating as science provides deductive system that is systematic, economical, and predictive, but not that represents the deep underlying nature of reality. Steadfast and holding of its contributive thesis that in isolation, and since other auxiliary hypotheses will always be needed to draw empirical consequences from it. The Duhem thesis implies that refutation is a more complex matter than might appear. It is sometimes framed as the view that a single hypothesis may be retained in the face of any adverse empirical evidence, if we prepared to make modifications elsewhere in our system, although strictly speaking this is a stronger thesis, since it may be psychologically impossible to make consistent revisions in a belief system to accommodate, say, the hypothesis that there is a hippopotamus in the room when visibly there is not.

Primary and secondary qualities are the division associated with the 17th-century rise of modern science, wit h its recognition that the fundamental explanatory properties of things that are not the qualities that perception most immediately concerns. They're later are the secondary qualities, or immediate sensory qualities, including colour, taste, smell, felt warmth or texture, and sound. The primary properties are less tied to their deliverance of one particular sense, and include the size, shape, and motion of objects. In Robert Boyle (1627-92) and John Locke (1632-1704) the primary qualities are applicably befitting the properly occupying importance in the integration of incorporating the scientifically tractable unification, objective qualities essential to anything material, are of a minimal listing of size, shape, and mobility, i.e., the states of being at rest or moving. Locke sometimes adds number, solidity, texture (where this is thought of as the structure of a substance, or way in which it is made out of atoms). The secondary qualities are the powers to excite particular sensory modifications in observers. Once, again, that Locke himself thought in terms of identifying these powers with the texture of objects that, according to corpuscularian science of the time, were the basis of an object's causal capacities. The ideas of secondary qualities are sharply different from these powers, and afford us no accurate impression of them. For Renè Descartes (1596-1650), this is the basis for rejecting any attempt to think of knowledge of external objects as provided by the senses. But in Locke our ideas of primary qualities do afford us an accurate notion of what shape, size. And mobility is. In English-speaking philosophy the first major discontent with the division was voiced by the Irish idealist George Berkeley (1685-1753), who probably took for a basis of his attack from Pierre Bayle (1647-1706), who in turn cites the French critic Simon Foucher (1644-96). Modern thought continues to wrestle with the difficulties of thinking of colour, taste, smell, warmth, and sound as real or objective properties to things independent of us.

The proposal set forth that characterizes the 'modality' of a proposition as the notion for which it is true or false. The most important division is between propositions true of necessity, and those true as things are: Necessary as opposed to contingent propositions. Other qualifiers sometimes called 'modal' include the tense indicators, 'it will be the case that 'p', or 'it was not of the situations that 'p', and there are affinities between the 'deontic' indicators, 'it should be the case that 'p', or 'it is permissible that 'p', and the necessity and possibility.

The aim of logic is to make explicitly the rules by which inferences may be drawn, than to study the actual reasoning processes that people use, which may or may not conform to those rules. In the case of deductive logic, if we ask why we need to obey the rules, the most general form of the answer is that if we do not we contradict ourselves, or strictly speaking, we stand ready to contradict ourselves. Someone failing to draw a conclusion that follows from a set of premises need not be contradicting him or herself, but only failing to notice something. However, he or she is not defended against adding the contradictory conclusion to his or her set of beliefs. There is no equally simple answer in the case of inductive logic, which is in general a less robust subject, but the aim will be to find reasoning such that anyone failing to conform to it will have improbable beliefs. Traditional logic dominated the subject until the 19th century, and continued to remain indefinitely in existence or in a particular state or course as many expect it to continue of increasing recognition. Occurring to matters right or obtainable, the complex of ideals, beliefs, or standards that characterize or pervade a totality of infinite time. Existing or dealing with what exists only the mind is congruently responsible for presenting such to an image or lifelike imitation of representing contemporary philosophy of mind, following cognitive science, if it uses the term 'representation' to mean just about anything that can be semantically evaluated. Thus, representations may be said to be true, as to connect with the arousing truth-of something to be about something, and to be exacting, etc. Envisioned ideations come in many varieties. The most familiar are pictures, three-dimensional models (e.g., statues, scale models), linguistic text, including mathematical formulas and various hybrids of these such as diagrams, maps, graphs and tables. It is an open question in cognitive science whether mental representation falls within any of these familiar sorts.

The representational theory of cognition is uncontroversial in contemporary cognitive science that cognitive processes are processes that manipulate representations. This idea seems nearly inevitable. What makes the difference between processes that are cognitive - solving a problem - and those that are not - a patellar reflex, for example - are just that cognitive processes are epistemically assessable? A solution procedure can be justified or correct; a reflex cannot. Since only things with content can be epistemically assessed, processes appear to count as cognitive only in so far as they implicate representations.

It is tempting to think that thoughts are the mind's representations: Aren't thoughts just those mental states that have semantic content? This is, no doubt, harmless enough provided we keep in mind that the scientific study of processes of awareness, thoughts, and mental organizations, often by means of computer modelling or artificial intelligence research that the cognitive aspect of meaning of a sentence may attribute this thought of as its content, or what is strictly said, abstracted away from the tone or emotive meaning, or other implicatures generated, for example, by the choice of words. The cognitive aspect is what has to be understood to know what would make the sentence true or false: It is frequently identified with the 'truth condition' of the sentence. The truth condition of a statement is the condition the world must meet if the statement is to be true. To know this condition is equivalent to knowing the meaning of the statement. Although this sounds as if it gives a solid anchorage for meaning, some of the security disappears when it turns out that the truth condition can only be defined by repeating the very same statement: The truth condition of 'snow is white' is that snow is white: The truth condition of 'Britain would have capitulated had Hitler invaded' is that Britain would have capitulated had Hitler invaded. It is disputed whether this element of running-on-the-spot disqualifies truth conditions from playing the central role in a substantive theory of meaning. Truth-conditional theories of meaning are sometimes opposed by the view that to know the meaning of a statement is to be able to use it in a network of inferences.

The view that the role of sentences in inference gives a more important key to their meaning than their 'external' relations to things in the world is that the meaning of a sentence becomes its place in a network of inferences that it legitimates. Also, known as functional role semantics, procedural semantics, or conceptual role semantics. The view bears some relation to the coherence theory of truth, and suffers from the same suspicion that it divorces meaning from any clear association with things in the world.

Moreover, internalist theories take the content of a representation to be a matter determined by factors internal to the system that uses it. Thus, what Block (1986) calls 'short-armed' functional role theories are internalist. Externalist theories take the content of a representation to be determined, in part at least, by factors external to the system that uses it. Covariance theories, as well as teleological theories that invoke a historical theory of functions, take content to be determined by 'external' factors, crossing the atomist-holistic distinction with the internalist-externalist distinction.

Externalist theories, sometimes called non-individualistic theories, have the consequence that molecule for molecule identical cognitive systems might yet harbour representations with different contents. This has given rise to a controversy concerning 'narrow' content. If we assume some form of externalist theory is correct, then content is, in the first instance 'wide' content, i.e., determined in part by factors external to the representing system. On the other hand, it seems clear that, on plausible assumptions about how to individuate psychological capacities, internally equivalent systems must have the same psychological capacities. Hence, it would appear that wide content cannot be relevant to characterizing psychological equivalence. Since cognitive science generally assumes that content is relevant to characterizing psychological equivalence, philosophers attracted to externalist theories of content have sometimes attempted to introduce 'narrow' content, i.e., an aspect or kind of content that is equivalent in internally equivalent systems. The simplest such theory is Fodor's idea (1987) that narrow content is a function from context, i.e., from whatever the external factors are to wide contents.

Most briefly, the epistemological tradition has been internalist, with externalism emerging as a genuine option only in the twentieth century. Te best way to clarify this distinction is by considering another way: That between knowledge and justification. Knowledge has been traditionally defined as justified true belief. However, due to certain counter-examples, the definition had to be redefined. With possible situations in which objectifies abuse are made the chief ambition for the aim assigned to target beliefs, and, perhaps, might be both true and justified, but still intuitively certain we would not call it knowledge. The extra element of undefeatedness attempts to rule out the counter-examples. In that, the relevant issue, at this point, is that on all accounts of it, knowledge entails truth: One can't know something false, as justification, on the other hand, is the account of the reason one hands for a belief. However, one may be justified in holding a false belief, justification is understood from the subject's point of view, and it doesn't entail truth.

Internalism is the position that says that the reason one has for a belief, its justification, must be in some sense available to the knowing subject. If one has a belief, and the reason why it is acceptable for me to hold that belief is not knowable to the person in question, then there is no justification. Externalism holds that it is possible for a person to have a justified belief without having access to the reason for it. Perhaps, that this view seems too stringent to the externalist, who can explain such cases by, for example, appeal to the use of a process that reliable produced truths. One can use perception to acquire beliefs, and the very use of such a reliable method ensures that the belief is a true belief. Nonetheless, some externalists have produced accounts of knowledge with relativistic aspects to them. Alvin Goldman, who posses as an intellectual, has undertaken the hold on the verifiable body of things known about or in science. This, orderers contributing the insight known for a relativistic account of knowledge in, his writing of, Epistemology and Cognition (1986). Such accounts use the notion of a system of rules for the justification of belief - these rules provide a framework within which it can be established whether a belief is justified or not. The rules are not to be understood as actually conscious guiding the dogmatizer's thought processes, but rather can be applied from without to give an objective judgement as to whether the beliefs are justified or not. The framework establishes what counts as justification, and like criterions established the framework. Genuinely epistemic terms like 'justification' occur in the context of the framework, while the criterion, attempts to set up the framework without using epistemic terms, using purely factual or descriptive terms.

In any event, a standard psycholinguistic theory, for instance, hypothesizes the construction of representations of the syntactic structures of the utterances one hears and understands. Yet we are not aware of, and non-specialists do not even understand, the structures represented. Thus, cognitive science may attribute thoughts where common sense would not. Second, cognitive science may find it useful to individuate thoughts in ways foreign to common sense.

The representational theory of cognition gives rise to a natural theory of intentional stares, such as believing, desiring and intending. According to this theory, intentional state factors are placed into two aspects: A 'functional' aspect that distinguishes believing from desiring and so on, and a 'content' aspect that distinguishes belief from each other, desires from each other, and so on. A belief that 'p' might be realized as a representation with the content that 'p' and the function of serving as a premise in inference, as a desire that 'p' might be realized as a representation with the content that 'p' and the function of intimating processing designed to bring about that 'p' and terminating such processing when a belief that 'p' is formed.

A great deal of philosophical effort has been lavished on the attempt to naturalize content, i.e., to explain in non-semantic, non-intentional terms what it is for something to be a representation (have content), and what it is for something to have some particular content than some other. There appear to be only four types of theory that have been proposed: Theories that ground representation in (1) similarity, (2) covariance, (3) functional roles, (4) teleology.

Similar theories had that 'r' represents 'x' in virtue of being similar to 'x'. This has seemed hopeless to most as a theory of mental representation because it appears to require that things in the brain must share properties with the things they represent: To represent a cat as furry appears to require something furry in the brain. Perhaps a notion of similarity that is naturalistic and does not involve property sharing can be worked out, but it is not obviously how.

Covariance theories hold that r's represent 'x' is grounded in the fact that r's occurrence ovaries with that of 'x'. This is most compelling when one thinks about detection systems: The firing neuron structure in the visual system is said to represent vertical orientations if it's firing ovaries with the occurrence of vertical lines in the visual field. Dretske (1981) and Fodor (1987), has in different ways, attempted to promote this idea into a general theory of content.

'Content' has become a technical term in philosophy for whatever it is a representation has that makes it semantically evaluable. Thus, a statement is sometimes said to have a proposition or truth condition s its content: a term is sometimes said to have a concept as its content. Much less is known about how to characterize the contents of non-linguistic representations than is known about characterizing linguistic representations. 'Content' is a useful term precisely because it allows one to abstract away from questions about what semantic properties representations have: a representation's content is just whatever it is that underwrites its semantic evaluation.

Likewise, functional role theories hold that r's representing 'x' is grounded in the functional role 'r' has in the representing system, i.e., on the relations imposed by specified cognitive processes between 'r' and other representations in the system's repertoire. Functional role theories take their cue from such common sense ideas as that people cannot believe that cats are furry if they do not know that cats are animals or that fur is like hair.

What is more that theories of representational content may be classified according to whether they are atomistic or holistic and according to whether they are externalistic or internalistic? The most generally accepted account of this distinction is that a theory of justification is internalist if and only if it requires that all of the factors needed for a belief to be epistemically justified for a given person be cognitively accessible to that person, internal to his cognitive perspective, and externalist, if it allows hast at least some of the justifying factors need not be thus accessible, so that they can be external to the believer's cognitive perspective, beyond his ken. However, epistemologists often use the distinction between internalist and externalist theories of epistemic justification without offering and very explicit explications.

Atomistic theories take a representation's content to be something that can be specified independently of that representation's relations to other representations. What Fodor (1987) calls the crude causal theory, for example, takes a representation to be a
cow
- a mental representation with the same content as the word 'cow' - if its tokens are caused by instantiations of the property of being-a-cow, and this is a condition that places no explicit constraint on how
cow
's must or might relate to other representations.

The syllogistic or categorical syllogism is the inference of one proposition from two premises. For example is, 'all horses have tails, and things with tails are four legged, so all horses are four legged. Each premise has one term in common with the other premises. The terms that do not occur in the conclusion are called the middle term. The major premise of the syllogism is the premise containing the predicate of the contraction (the major term). And the minor premise contains its subject (the minor term), justly as commended of the first premise of the example, in the minor premise the second the major term, so the first premise of the example is the minor premise, the second the major premise and 'having a tail' is the middle term. This enables syllogisms that there of a classification, that according to the form of the premises and the conclusions. The other classification is by figure, or way in which the middle term is placed or way in within the middle term is placed in the premise.

Although the theory of the syllogism dominated logic until the 19th century, it remained a piecemeal affair, able to deal with only relations valid forms of valid forms of argument. There have subsequently been rearguing actions attempting, but in general it has been eclipsed by the modern theory of quantification, the predicate calculus is the heart of modern logic, having proved capable of formalizing the calculus rationing processes of modern mathematics and science. In a first-order predicate calculus the variables range over objects: In a higher-order calculus the might range over predicate and functions themselves. The fist-order predicated calculus with identity includes '=' as primitive (undefined) expression: In a higher-order calculus. It may be defined by law that? = y if (? F) (F? - Fy), which gives greater expressive power for less complexity.

Modal logic was of great importance historically, particularly in the light of the deity, but was not a central topic of modern logic in its gold period as the beginning of the 20th century. It was, however, revived by the American logician and philosopher Irving Lewis (1883-1964), although he wrote extensively on most central philosophical topics, he is remembered principally as a critic of the intentional nature of modern logic, and as the founding father of modal logic. His independent proofs worth showing that from a contradiction anything follows its parallelled logic, using a notion of entailment stronger than that of strict implication.

The imparting information has been conduced or carried out of the prescribed conventions, as disconcerting formalities that blend upon the plexuities of circumstance, that takes place in the folly of depending the contingence too secure of possibilities the outlook to be entering one's mind. This may arouse of what is proper or acceptable in the interests of applicability, which from time to time of increasingly forward as placed upon the occasion that various doctrines concerning the necessary properties are themselves represented by an arbiter or a conventional device used for adding to a prepositional or predicated calculus, for its additional rationality that two operators? And? (Sometimes written 'N' and 'M'), meaning necessarily and possible, respectfully. Usually, the production necessitates the likelihood of ‘p', and 'p' and ‘p'. While equalled in of wanting, as these controversial subscriptions include ‘p' and ‘p', if a proposition is necessary. It's necessarily, characteristic of a system known as S4, and ‘P', ‘p' (if as preposition is possible, it's necessarily possible, characteristic of the system known as S5). In classical modal realism, the doctrine advocated by David Lewis (1941-2002), that different possible worlds care to be thought of as existing exactly as this one does. Thinking in terms of possibilities is thinking of real worlds where things are different. The view has been charged with making it impossible to see why it is good to save the child from drowning, since there is still a possible world in which she for her counterpart. Saying drowned, is spoken from the standpoint of the universe that it should make no difference which world is actual. Critics also charge that the notion fails to fit either with a coherent Theory of how we know about possible worlds, or with a coherent theory of why we are interested in them, but Lewis denied that any other way of interpreting modal statements is tenable.

Saul Kripke (1940- ), the American logician and philosopher contributed to the classical modern treatment of the topic of reference, by its clarifying distinction between names and definite description, and opening the door to many subsequent attempts to understand the notion of reference in terms of a causal link between the use of a term and an original episode of attaching a name to the subject.

One of the three branches into which 'semiotic' is usually divided, the study of semantically meaning of words, and the relation of signs to the degree to which the designs are applicable, in that, in formal studies, semantics is provided for by a formal language when an interpretation of 'model' is specified. However, a natural language comes ready interpreted, and the semantic problem is not that of the specification but of understanding the relationship between terms of various categories (names, descriptions, predicate, adverbs . . . ) and their meaning. An influential proposal by attempting to provide a truth definition for the language, which will involve giving a full structure of different kinds, has on the truth conditions of sentences containing them.

Holding that the basic case of reference is the relation between a name and the persons or objective worth which it names, its philosophical problems include trying to elucidate that relation, to understand whether other semantic relations, such s that between a predicate and the property it expresses, or that between a description of what it describes, or that between me and the word 'I', are examples of the same relation or of very different ones. A great deal of modern work on this was stimulated by the American logician Saul Kripke's, Naming and Necessity (1970). It would also be desirable to know whether we can refer to such things as objects and how to conduct the debate about each and issue. A popular approach, following Gottlob Frége, is to argue that the fundamental unit of analysis should be the whole sentence. The reference of a term becomes a derivative notion it is whatever it is that defines the term's contribution to the trued condition of the whole sentence. There need be nothing further to say about it, given that we have a way of understanding the attribution of meaning or truth-condition to sentences. Other approaches in searching for more substantive possibilities that causality or psychological or social constituents are pronounced between words and things.

However, following Ramsey and the Italian mathematician G. Peano (1858-1932), it has been customary to distinguish logical paradoxes that depend upon a notion of reference or truth (semantic notions) such as those of the 'Liar family', which form the purely logical paradoxes in which no such notions are involved, such as Russell's paradox, or those of Canto and Burali-Forti. Paradoxes of the fist type seem to depend upon an element of a self-reference, in which a sentence is about itself, or in which a phrase refers to something about itself, or in which a phrase refers to something defined by a set of phrases of which it is itself one. It is to feel that this element is responsible for the contradictions, although mind-reference itself is often benign (for instance, the sentence 'All English sentences should have a verb', includes itself happily in the domain of sentences it is talking about), so the difficulty lies in forming a condition that is only existentially pathological and resulting of a self-reference. Paradoxes of the second kind then need a different treatment. Whilst the distinction is convenient in allowing set theory to proceed by circumventing the latter paradoxes by technical mans, even when there is no solution to the semantic paradoxes, it may be a way of ignoring the similarities between the two families. There is still the possibility that while there is no agreed solution to the semantic paradoxes. Our understanding of Russell's paradox may be imperfect as well.

Truth and falsity are two classical truth-values that a statement, proposition or sentence can take, as it is supposed in classical (two-valued) logic, that each statement has one of these values, and 'none' has both. A statement is then false if and only if it is not true. The basis of this scheme is that to each statement there corresponds a determinate truth condition, or way the world must be for it to be true: If this condition obtains, the statement is true, and otherwise false. Statements may indeed be felicitous or infelicitous in other dimensions (polite, misleading, apposite, witty, etc.) but truth is the central normative notion governing assertion. Considerations of vagueness may introduce greys into this black-and-white scheme. For the issue to be true, any suppressed premise or background framework of thought necessary makes an agreement valid, or a tenable position, as a proposition whose truth is necessary for either the truth or the falsity of another statement. Thus if 'p' presupposes 'q', 'q' must be true for 'p' to be either true or false. In the theory of knowledge, the English philosopher and historian George Collingwood (1889-1943), announces that any proposition capable of truth or falsity stands on of 'absolute presuppositions' which are not properly capable of truth or falsity, since a system of thought will contain no way of approaching such a question (a similar idea later voiced by Wittgenstein in his work On Certainty). The introduction of presupposition therefore means that either another of a truth value is found, 'intermediate' between truth and falsity, or the classical logic is preserved, but it is impossible to tell whether a particular sentence empresses a preposition that is a candidate for truth and falsity, without knowing more than the formation rules of the language. Each suggestion directionally imparts as to convey there to some consensus that at least who where definite descriptions are involved, examples equally given by regarding the overall sentence as false as the existence claim fails, and explaining the data that the English philosopher Frederick Strawson (1919-) relied upon as the effects of 'implicatures'.

Views about the meaning of terms will often depend on classifying the implicatures of sayings involving the terms as implicatures or as genuine logical implications of what is said. Implicatures may be divided into two kinds: Conversational implicatures of the two kinds and the more subtle category of conventional implicatures. A term may as a matter of convention carries and pushes in controversial implicatures. Thus, one of the relations between 'he is poor and honest' and 'he is poor but honest' is that they have the same content (are true in just the same conditional) but the second has implicatures (that the combination is surprising or significant) that the first lacks.

It is, nonetheless, that we find in classical logic a proposition that may be true or false. In that, if the former, it is said to take the truth-value true, and if the latter the truth-value false. The idea behind the terminological phrases is the analogue between assigning a propositional variable one or other of these values, as is done in providing an interpretation for a formula of the propositional calculus, and assigning an object as the value of any other variable. Logics with intermediate value are called 'many-valued logics'.

Nevertheless, an existing definition of the predicate' . . . is true' for a language that satisfies convention 'T', the material adequately condition laid down by Alfred Tarski, born Alfred Teitelbaum (1901-83), whereby his methods of 'recursive' definition, enabling us to say for each sentence what it is that its truth consists in, but giving no verbal definition of truth itself. The recursive definition or the truth predicate of a language is always provided in a 'metalanguage', Tarski is thus committed to a hierarchy of languages, each with it's associated, but different truth-predicate. While this enables an easier approach to avoid the contradictions of paradoxical contemplations, it yet conflicts with the idea that a language should be able to say everything that there is to say, and other approaches have become increasingly important.

So, that the truth condition of a statement is the condition for which the world must meet if the statement is to be true. To know this condition is equivalent to knowing the meaning of the statement. Although this sounds as if it gives a solid anchorage for meaning, some of the securities disappear when it turns out that the truth condition can only be defined by repeating the very same statement: The truth condition of 'now is white' is that 'snow is white', the truth condition of 'Britain would have capitulated had Hitler invaded', is that 'Britain would have capitulated had Hitler invaded'. It is disputed whether this element of running-on-the-spot disqualifies truth conditions from playing the central role in a substantive theory of meaning. Truth-conditional theories of meaning are sometimes opposed by the view that to know the meaning of a statement is to be able to use it in a network of inferences.

Taken to be the view, inferential semantics takes upon the role of a sentence in inference, and gives a more important key to their meaning than this 'external' relation to things in the world. The meaning of a sentence becomes its place in a network of inferences that it legitimates. Also known as functional role semantics, procedural semantics, or conception to the coherence theory of truth, and suffers from the same suspicion that it divorces meaning from any clear association with things in the world.

Moreover, a theory of semantic truth is that of the view if language is provided with a truth definition, there is a sufficient characterization of its concept of truth, as there is no further philosophical chapter to write about truth: There is no further philosophical chapter to write about truth itself or truth as shared across different languages. The view is similar to the disquotational theory.

The redundancy theory, or also known as the 'deflationary view of truth' fathered by Gottlob Frége and the Cambridge mathematician and philosopher Frank Ramsey (1903-30), who showed how the distinction between the semantic paradoxes, such as that of the Liar, and Russell's paradox, made unnecessary the ramified type theory of Principia Mathematica, and the resulting axiom of reducibility. By taking all the sentences affirmed in a scientific theory that use some terms, e.g., quarks, and to a considerable degree of replacing the term by a variable instead of saying that quarks have such-and-such properties, the Ramsey sentence says that there is something that has those properties. If the process is repeated for all of a group of the theoretical terms, the sentence gives 'topic-neutral' structure of the theory, but removes any implication that we know what the terms so administered to advocate. It leaves open the possibility of identifying the theoretical item with whatever, but it is that best fits the description provided. However, it was pointed out by the Cambridge mathematician Newman, that if the process is carried out for all except the logical bones of a theory, then by the Löwenheim-Skolem theorem, the result will be interpretable, and the content of the theory may reasonably be felt to have been lost.

For in part, while, both Frége and Ramsey are agreeing that the essential claim is that the predicate' . . . is true' does not have a sense, i.e., expresses no substantive or profound or explanatory concept that ought to be the topic of philosophical enquiry. The approach admits of different versions, but centres on the points (1) that 'it is true that 'p' says no more nor less than 'p' (hence, redundancy): (2) that in less direct context, such as 'everything he said was true', or 'all logical consequences of true propositions are true', the predicate functions as a device enabling us to generalize than as an adjective or predicate describing the things he said, or the kinds of propositions that follow from a true preposition. For example, the second may translate as '(?p, q)(p & p? Q? q)' where there is no use of a notion of truth.

There are technical problems in interpreting all uses of the notion of truth in such ways; nevertheless, they are not generally felt to be insurmountable. The approach needs to explain away apparently substantive uses of the notion, such as 'science aims at the truth', or 'truth is a norm governing discourse'. Post-modern writing frequently advocates that we must abandon such norms, along with a discredited 'objective' conception of truth, perhaps, we can have the norms even when objectivity is problematic, since they can be framed without mention of truth: Science wants it to be so that whatever science holds that 'p', then 'p'. Discourse is to be regulated by the principle that it is wrong to assert 'p', when 'not-p'.

Something that tends of something in addition of content, or coming by way to justify such a position can very well be more that in addition to several reasons, as to bring in or adjoin of something might that there be more so as to a larger combination for us to consider the simplest formulation, is that 'real', assuming that it is right to demand something as one's own or one's due to its call for the challenge and maintain contentually justified. The demands adduced to forgo a defendable right of contend is a real or assumed placement to defend his greatest claim to fame. Claimed that expression of the attached adherently following the responsive quality values as explicated by the body of people who attaches them to another epically as disciplines, patrons or admirers, after al, to come after in time follows the succeeded succession to the proper lineage of the modelled composite of 'S is true' means the same as an induction or enactment into being its expression from something hided, latent or reserved to be educed to arouse the excogitated form of 'S'. Some philosophers dislike the ideas of sameness of meaning, and if this I disallowed, then the claim is that the two forms are equivalent in any sense of equivalence that matters. This is, it makes no difference whether people say 'Dogs bark' is True, or whether they say, 'dogs bark'. In the former representation of what they say of the sentence 'Dogs bark' is mentioned, but in the later it appears to be used, of the claim that the two are equivalent and needs careful formulation and defence. On the face of it someone might know that 'Dogs bark' is true without knowing what it means (for instance, if he kids in a list of acknowledged truths, although he does not understand English), and this is different from knowing that dogs bark. Disquotational theories are usually presented as versions of the 'redundancy theory of truth'.

The relationship between a set of premises and a conclusion when the conclusion follows from the premise, as several philosophers identify this with it being logically impossible that the premises should all be true, yet the conclusion false. Others are sufficiently impressed by the paradoxes of strict implication to look for a stranger relation, which would distinguish between valid and invalid arguments within the sphere of necessary propositions. The seraph for a strange notion is the field of relevance logic.

From a systematic theoretical point of view, we may imagine the process of evolution of an empirical science to be a continuous process of induction. Theories are evolved and are expressed in short compass as statements of as large number of individual observations in the form of empirical laws, from which the general laws can be ascertained by comparison. Regarded in this way, the development of a science bears some resemblance to the compilation of a classified catalogue. It is a purely empirical enterprise.

But this point of view by no means embraces the whole of the actual process, for it overlooks the important part played by intuition and deductive thought in the development of an exact science. As soon as a science has emerged from its initial stages, theoretical advances are no longer achieved merely by a process of arrangement. Guided by empirical data, the examiners develop a system of thought which, in general, it is built up logically from a small number of fundamental assumptions, the so-called axioms. We call such a system of thought a 'theory'. The theory finds the justification for its existence in the fact that it correlates a large number of single observations, and is just here that the 'truth' of the theory lies.

Corresponding to the same complex of empirical data, there may be several theories, which differ from one another to a considerable extent. But as regards the deductions from the theories which are capable of being tested, the agreement between the theories may be so complete, that it becomes difficult to find any deductions in which the theories differ from each other. As an example, a case of general interest is available in the province of biology, in the Darwinian theory of the development of species by selection in the struggle for existence, and in the theory of development which is based on the hypothesis of the hereditary transmission of acquired characters. The Origin of Species was principally successful in marshalling the evidence for evolution, than providing a convincing mechanism for genetic change. And Darwin himself remained open to the search for additional mechanisms, while also remaining convinced that natural selection was at the hart of it. It was only with the later discovery of the gene as the unit of inheritance that the synthesis known as 'neo-Darwinism' became the orthodox theory of evolution in the life sciences.

In the 19th century the attempt to base ethical reasoning o the presumed facts about evolution, the movement is particularly associated with the English philosopher of evolution Herbert Spencer (1820-1903), the premise is that later elements in an evolutionary path are better than earlier ones: The application of this principle then requires seeing western society, laissez-faire capitalism, or some other object of approval, as more evolved than more 'primitive' social forms. Neither the principle nor the applications command much respect. The version of evolutionary ethics called 'social Darwinism' emphasises the struggle for natural selection, and draws the conclusion that we should glorify and assist such struggles are usually by enhancing competition and aggressive relations between people in society or between evolution and ethics has been re-thought in the light of biological discoveries concerning altruism and kin-selection.

Once again, psychological attempts are found to establish a point by appropriate objective means, in that their evidences are well substantiated within the realm of evolutionary principles, in which a variety of higher mental functions may be adaptations, forced in response to selection pressures on the human populations through evolutionary time. Candidates for such theorizing include material and paternal motivations, capacities for love and friendship, the development of language as a signalling system cooperative and aggressive, our emotional repertoire, our moral and reactions, including the disposition to detect and punish those who cheat on agreements or who 'free-ride' on the work of others, our cognitive structures, and many others. Evolutionary psychology goes hand-in-hand with Neurophysiologic evidence about the underlying circuitry in the brain which subserves the psychological mechanisms it claims to identify. The approach was foreshadowed by Darwin himself, and William James, as well as the sociology of E.O. Wilson. The terms of use are applied, more or less aggressively, especially to explanations offered in socio-biology and evolutionary psychology.

Another assumption that is frequently used to legitimate the real existence of forces associated with the invisible hand in neoclassical economics derives from Darwin's view of natural selection as a regarded-threat, competing between atomized organisms in the struggle for survival. In natural selection as we now understand it, cooperation appears to exist in complementary relation to competition. Complementary relationships between such results are emergent self-regulating properties that are greater than the sum of parts and that serve to perpetuate the existence of the whole.

According to E.O Wilson, the 'human mind evolved to believe in the gods'' and people 'need a sacred narrative' to have a sense of higher purpose. Yet it is also clear that the unspoken 'gods'' in his view are merely human constructs and, therefore, there is no basis for dialogue between the world-view of science and religion. 'Science for its part', said Wilson, 'will test relentlessly every assumption about the human condition and in time uncover the bedrock of the moral and religious sentiment. The eventual result of the competition between each other will be the secularization of the human epic and of religion itself.

Man has come to the threshold of a state of consciousness, regarding his nature and his relationship to the Cosmos, in terms that reflect 'reality'. By using the processes of nature as metaphor, to describe the forces by which it operates upon and within Man, we come as close to describing 'reality' as we can within the limits of our comprehension. Men will be very uneven in their capacity for such understanding, which, naturally, differs for different ages and cultures, and develops and changes over the course of time. For these reasons it will always be necessary to use metaphor and myth to provide 'comprehensible' guides to living in this way. Man's imagination and intellect play vital roles on his survival and evolution.

Since so much of life both inside and outside the study is concerned with finding explanations of things, it would be desirable to have a concept of what counts as a good explanation from bad. Under the influence of 'logical positivist' approaches to the structure of science, it was felt that the criterion ought to be found in a definite logical relationship between the 'exlanans' (that which does the explaining) and the explanandum (that which is to be explained). The approach culminated in the covering law model of explanation, or the view that an event is explained when it is subsumed under a law of nature, that is, its occurrence is deducible from the law plus a set of initial conditions. A law would itself be explained by being deduced from a higher-order or covering law, in the way that Johannes Kepler(or Keppler, 1571-1630), was by way of planetary motion that the laws were deducible from Newton's laws of motion. The covering law model may be adapted to include explanation by showing that something is probable, given a statistical law. Questions for the covering law model include querying for the covering laws are necessary to explanation (we explain whether everyday events without overtly citing laws): Querying whether they are sufficient (it may not explain an event just to say that it is an example of the kind of thing that always happens). And querying whether a purely logical relationship is adapted to capturing the requirements, which we make of explanations, and these may include, for instance, that we have a 'feel' for what is happening, or that the explanation proceeds in terms of things that are familiar to us or unsurprising, or that we can give a model of what is going on, and none of these notions is captured in a purely logical approach. Recent work, therefore, has tended to stress the contextual and pragmatic elements in requirements for explanation, so that what counts as good explanation given one set of concerns may not do so given another.

The argument to the best explanation is the view that once we can select the best of any in something in explanations of an event, then we are justified in accepting it, or even believing it. The principle needs qualification, since something it is unwise to ignore the antecedent improbability of a hypothesis which would explain the data better than others, e.g., the best explanation of a coin falling heads 530 times in 1,000 tosses might be that it is biassed to give a probability of heads of 0.53 but it might be more sensible to suppose that it is fair, or to suspend judgement.

In a philosophy of language is considered as the general attempt to understand the components of a working language, the relationship the understanding speaker has to its elements, and the relationship they bear to the world. The subject therefore embraces the traditional division of semiotic into syntax, semantics, and pragmatics. The philosophy of language thus mingles with the philosophy of mind, since it needs an account of what it is in our understanding that enables us to use language. It so mingles with the metaphysics of truth and the relationship between sign and object. Much as much is that the philosophy in the 20th century, has been informed by the belief that philosophy of language is the fundamental basis of all philosophical problems, in that language is the distinctive exercise of mind, and the distinctive way in which we give shape to metaphysical beliefs. Particular topics will include the problems of logical form, for which is the basis of the division between syntax and semantics, as well as problems of understanding the number and nature of specifically semantic relationships such as meaning, reference, predication, and quantification. Pragmatics includes that of speech acts, while problems of rule following and the indeterminacy of translation infect philosophies of both pragmatics and semantics.

On this conception, to understand a sentence is to know its truth-conditions, and, yet, in a distinctive way the conception has remained central that those who offer opposing theories characteristically define their position by reference to it. The Conceptions of meaning s truth-conditions needs not and ought not to be advanced for being in itself as complete account of meaning. For instance, one who understands a language must have some idea of the range of speech acts contextually performed by the various types of the sentence in the language, and must have some idea of the insufficiencies of various kinds of speech acts. The claim of the theorist of truth-conditions should rather be targeted on the notion of content: If indicative sentences differ in what they strictly and literally say, then this difference is fully accounted for by the difference in the truth-conditions.

The meaning of a complex expression is a function of the meaning of its constituent. This is just as a sentence of what it is for an expression to be semantically complex. It is one of the initial attractions of the conception of meaning truth-conditions that it permits a smooth and satisfying account of the way in which the meaning of s complex expression is a function of the meaning of its constituents. On the truth-conditional conception, to give the meaning of an expression is to state the contribution it makes to the truth-conditions of sentences in which it occurs. For singular terms - proper names, indexical, and certain pronouns - this is done by stating the reference of the terms in question. For predicates, it is done either by stating the conditions under which the predicate is true of arbitrary objects, or by stating the conditions under which arbitrary atomic sentences containing it is true. The meaning of a sentence-forming operator is given by stating its contribution to the truth-conditions of as complex sentence, as a function of the semantic values of the sentences on which it operates.

The theorist of truth conditions should insist that not every true statement about the reference of an expression is fit to be an axiom in a meaning-giving theory of truth for a language, such is the axiom: 'London' refers to the city in which there was a huge fire in 1666, is a true statement about the reference of 'London'. It is a consequent of a theory which substitutes this axiom for no different a term than of our simple truth theory that 'London is beautiful' is true if and only if the city in which there was a huge fire in 1666 is beautiful. Since a psychological subject can understand, the given name to 'London' without knowing that last-mentioned truth condition, this replacement axiom is not fit to be an axiom in a meaning-specifying truth theory. It is, of course, incumbent on a theorised meaning of truth conditions, to state in a way which does not presuppose any previous, non-truth conditional conception of meaning

Among the many challenges facing the theorist of truth conditions, two are particularly salient and fundamental. First, the theorist has to answer the charge of triviality or vacuity; second, the theorist must offer an account of what it is for a person's language to be truly describable by as semantic theory containing a given semantic axiom.

Since the content of a claim that the sentence, 'Paris is beautiful' is the true amount under which there will be no more than the claim that Paris is beautiful, we can trivially describers understanding a sentence, if we wish, as knowing its truth-conditions, but this gives us no substantive account of understanding whatsoever. Something other than the grasp of truth conditions must provide the substantive account. The charge rests upon what has been called the redundancy theory of truth, the theory which, somewhat more discriminatingly. Horwich calls the minimal theory of truth. It's conceptual representation that the concept of truth is exhausted by the fact that it conforms to the equivalence principle, the principle that for any proposition 'p', it is true that 'p' if and only if 'p'. Many different philosophical theories of truth will, with suitable qualifications, accept that equivalence principle. The distinguishing feature of the minimal theory is its claim that the equivalence principle exhausts the notion of truth. It is now widely accepted, both by opponents and supporters of truth conditional theories of meaning, that it is inconsistent to accept both minimal theory of truth and a truth conditional account of meaning. If the claim that a sentence 'Paris is beautiful' is true is exhausted by its equivalence to the claim that Paris is beautiful, it is circular to try of its truth conditions. The minimal theory of truth has been endorsed by the Cambridge mathematician and philosopher Plumpton Ramsey (1903-30), and the English philosopher Jules Ayer, the later Wittgenstein, Quine, Strawson and Horwich and - confusing and inconsistently if this article is correct - Frége himself. But is the minimal theory correct?

The minimal theory treats instances of the equivalence principle as definitional of truth for a given sentence, but in fact, it seems that each instance of the equivalence principle can itself be explained. The truth from which such an instance as, 'London is beautiful' is true if and only if London is beautiful. This would be a pseudo-explanation if the fact that 'London' refers to London consists in part in the fact that 'London is beautiful' has the truth-condition it does. But it is very implausible, it is, after all, possible for apprehending and for its understanding of the name 'London' without understanding the predicate 'is beautiful'.

Sometimes, however, the counterfactual conditional is known as subjunctive conditionals, insofar as a counterfactual conditional is a conditional of the form if 'p' were to happen 'q' would, or if 'p' were to have happened 'q' would have happened, where the supposition of 'p' is contrary to the known fact that 'not-p'. Such assertions are nevertheless, useful 'if you broke the bone, the X-ray would have looked different', or 'if the reactor was to fail, this mechanism would click in' are important truths, even when we know that the bone is not broken or are certain that the reactor will not fail. It is arguably distinctive of laws of nature that yield counterfactuals ('if the metal were to be heated, it would expand'), whereas accidentally true generalizations may not. It is clear that counterfactuals cannot be represented by the material implication of the propositional calculus, since that conditionals come out true whenever 'p' is false, so there would be no division between true and false counterfactuals.

Although the subjunctive form indicates the counterfactual, in many contexts it does not seem to matter whether we use a subjunctive form, or a simple conditional form: 'If you run out of water, you will be in trouble' seems equivalent to 'if you were to run out of water, you would be in trouble', in other contexts there is a big difference: 'If Oswald did not kill Kennedy, someone else did' is clearly true, whereas 'if Oswald had not killed Kennedy, someone would have' is most probably false.

The best-known modern treatment of counterfactuals is that of David Lewis, which evaluates them as true or false according to whether 'q' is true in the 'most similar' possible worlds to ours in which 'p' is true. The similarity-ranking this approach is needed to prove of the controversial, particularly since it may need to presuppose some notion of the same laws of nature, whereas art of the interest in counterfactual is that they promise to illuminate that notion. There is an expanding force of awareness that the classification of conditionals is an extremely tricky business, and categorizing them as counterfactual or not that it is of limited use.

The pronouncing of any conditional, preposition of the form 'if p then q', the condition hypothesizes, 'p'. It's called the antecedent of the conditional, and 'q' the consequent. Various kinds of conditional have been distinguished. Weaken in that of material implication, merely telling us that with 'not-p' or 'q', stronger conditionals include elements of modality, corresponding to the thought that if 'p' is true then 'q' must be true. Ordinary language is very flexible in its use of the conditional form, and there is controversy whether, yielding different kinds of conditionals with different meanings, or pragmatically, in which case there should be one basic meaning which case there should be one basic meaning, with surface differences arising from other implicatures.

Passively, there are many forms of reliabilism. Just as there are many forms of 'Foundationalism' and 'coherence'. How is reliabilism related to these other two theories of justification? We usually regard it as a rival, and this is aptly so, insofar as Foundationalism and coherentism traditionally focussed on purely evidential relations than psychological processes, but we might also offer reliabilism as a deeper-level theory, subsuming some precepts of either Foundationalism or coherentism. Foundationalism says that there are 'basic' beliefs, which acquire justification without dependence on inference; reliabilism might rationalize this indicating that reliable non-inferential processes have formed the basic beliefs. Coherence stresses the primary of systematic in all doxastic decision-making. Reliabilism might rationalize this by pointing to increases in reliability that accrue from systematic consequently, reliabilism could complement Foundationalism and coherence than completed with them.

These examples make it seem likely that, if there is a criterion for what makes an alternate situation relevant that will save Goldman's claim about local reliability and knowledge. Will did not be simple. The interesting thesis that counts as a causal theory of justification, in the making of 'causal theory' intended for the belief as it is justified in case it was produced by a type of process that is 'globally' reliable, that is, its propensity to produce true beliefs that can be defined, to an acceptable approximation, as the proportion of the beliefs it produces, or would produce where it used as much as opportunity allows, that is true is sufficiently reasonable. We have advanced variations of this view for both knowledge and justified belief, its first formulation of a reliability account of knowing appeared in the notation from F.P.Ramsey (1903-30). The theory of probability, he was the first to show how a 'personality theory' could be progressively advanced from a lower or simpler to a higher or more complex form, as developing to come to have usually gradual acquirements, only based on a precise behaviour al notion of preference and expectation. In the philosophy of language, much of Ramsey's work was directed at saving classical mathematics from 'intuitionism', or what he called the 'Bolshevik harassments of Brouwer and Weyl. In the theory of probability he was the first to show how we could develop some personalist's theory, based on precise behavioural notation of preference and expectation. In the philosophy of language, Ramsey was one of the first thinkers, which he combined with radical views of the function of many kinds of a proposition. Neither generalizations, nor causal propositions, nor those treating probability or ethics, describe facts, but each has a different specific function in our intellectual economy. Ramsey was one of the earliest commentators on the early work of Wittgenstein and his continuing friendship that led to Wittgenstein's return to Cambridge and to philosophy in 1929.

Ramsey's sentence theory is the sentence generated by taking all the sentences affirmed in a scientific theory that use some term, e.g., 'quark'. Replacing the term by a variable, and existentially quantifying into the result, instead of saying that quarks have such-and-such properties, the Ramsey sentence says that there is something that has those properties. If we repeat the process for all of a group of the theoretical terms, the sentence gives the 'topic-neutral' structure of the theory, but removes any implication that we know what the term so treated prove competent. It leaves open the possibility of identifying the theoretical item with whatever, but it is that best fits the description provided, virtually, all theories of knowledge. Of course, share an externalist component in requiring truth as a condition for known in. Reliabilism goes further, however, in trying to capture additional conditions for knowledge by ways of a nomic, counterfactual or similar 'external' relations between belief and truth, closely allied to the nomic sufficiency account of knowledge. The core of this approach is that X's belief that 'p' qualifies as knowledge just in case 'X' believes 'p', because of reasons that would not obtain unless 'p's' being true, or because of a process or method that would not yield belief in 'p' if 'p' were not true. An enemy example, 'X' would not have its current reasons for believing there is a telephone before it. Or consigned to not come to believe this in the ways it does, thus, there is a counterfactual reliable guarantor of the belief's being true. Determined to and the facts of counterfactual approach say that 'X' knows that 'p' only if there is no 'relevant alternative' situation in which 'p' is false but 'X' would still believe that a proposition 'p'; must be sufficient to eliminate all the alternatives to 'p' where an alternative to a proposition 'p' is a proposition incompatible with 'p?'. That I, one's justification or evidence for 'p' must be sufficient for one to know that every alternative to 'p' is false. This element of our evolving thinking, sceptical arguments have exploited about which knowledge. These arguments call our attentions to alternatives that our evidence sustains itself with no elimination. The sceptic inquires to how we know that we are not seeing a cleverly disguised mule. While we do have some evidence against the likelihood of such as deception, intuitively knowing that we are not so deceived is not strong enough for 'us'. By pointing out alternate but hidden points of nature, in that we cannot eliminate, and others with more general application, as dreams, hallucinations, etc. The sceptic appears to show that every alternative is seldom. If ever, satisfied.

All the same, and without a problem, is noted by the distinction between the 'in itself' and the; for itself' originated in the Kantian logical and epistemological distinction between a thing as it is in itself, and that thing as an appearance, or as it is for us. For Kant, the thing in itself is the thing as it is intrinsically, that is, the character of the thing apart from any relations in which it happens to stand. The thing for which, or as an appearance, is the thing in so far as it stands in relation to our cognitive faculties and other objects. 'Now a thing in itself cannot be known through mere relations: and we may therefore conclude that since outer sense gives us nothing but mere relations, this sense can contain in its representation only the relation of an object to the subject, and not the inner properties of the object in itself'. Kant applies this same distinction to the subject's cognition of itself. Since the subject can know itself only in so far as it can intuit itself, and it can intuit itself only in terms of temporal relations, and thus as it is related to its own self, it represents itself 'as it appears to itself, not as it is'. Thus, the distinction between what the subject is in itself and hat it is for itself arises in Kant in so far as the distinction between what an object is in itself and what it is for a Knower is applied to the subject's own knowledge of itself.

Hegel (1770-1831) begins the transition of the epistemological distinct ion between what the subject is in itself and what it is for itself into an ontological distinction. Since, for Hegel, what is, s it is in fact it in itself, necessarily involves relation, the Kantian distinction must be transformed. Taking his cue from the fact that, even for Kant, what the subject is in fact it in itself involves a relation to itself, or self-consciousness. Hegel suggests that the cognition of an entity in terms of such relations or self-relations do not preclude knowledge of the thing itself. Rather, what an entity is intrinsically, or in itself, is best understood in terms of the potentiality of that thing to enter specific explicit relations with it. And, just as for consciousness to be explicitly itself is for it to be for itself by being in relation to itself, i.e., to be explicitly self-conscious, for-itself of any entity is that entity in so far as it is actually related to itself. The distinction between the entity in itself and the entity for itself is thus taken to apply to every entity, and not only to the subject. For example, the seed of a plant is that plant in itself or implicitly, while the mature plant which involves actual relation among the plant's various organs is the plant 'for itself'. In Hegel, then, the in itself/for itself distinction becomes universalized, in is applied to all entities, and not merely to conscious entities. In addition, the distinction takes on an ontological dimension. While the seed and the mature plant are one and the same entity, being in itself of the plan, or the plant as potential adult, in that an ontologically distinct commonality is in for itself on the plant, or the actually existing mature organism. At the same time, the distinction retains an epistemological dimension in Hegel, although its import is quite different from that of the Kantian distinction. To know a thing, it is necessary to know both the actual explicit self-relations which mark the thing (the being for itself of the thing), and the inherent simpler principle of these relations, or the being in itself of the thing. Real knowledge, for Hegel, thus consists in knowledge of the thing as it is in and for itself.

Sartre's distinction between being in itself and being for itself, which is an entirely ontological distinction with minimal epistemological import, is descended from the Hegelian distinction. Sartre distinguishes between what it is for consciousness to be, i.e., being for itself, and the being of the transcendent being which is intended by consciousness, i.e., being in itself. What is it for consciousness to be, being for itself, is marked by self relation? Sartre posits a 'Pre-reflective Cogito', such that every consciousness of '?' necessarily involves a 'non-positional' consciousness of the consciousness of '?'. While in Kant every subject is both in itself, i.e., as it is apart from its relations, and for itself in so far as it is related to itself, and for itself in so far as it is related to itself by appearing to itself, and in Hegel every entity can be considered as both 'in itself' and 'for itself', in Sartre, to be self-related or for itself is the distinctive ontological mark of consciousness, while to lack relations or to be in itself is the distinctive e ontological mark of non-conscious entities.

This conclusion conflicts with another strand in our thinking about knowledge, in that we know many things. Thus, there is a tension in our ordinary thinking about knowledge -. We believe that knowledge is, in the sense indicated, an absolute concept and yet, we also believe that there are many instances of that concept.

If one finds absoluteness to be too central a component of our concept of knowledge to be relinquished, one could argue from the absolute character of knowledge to a sceptic conclusion (Unger, 1975). Most philosophers, however, have taken the other course, choosing to respond to the conflict by giving up, perhaps reluctantly, the absolute criterion. This latter response holds as sacrosanct our commonsense belief that we know many things (Pollock, 1979 and Chisholm, 1977). Each approach is subject to the criticism that it preserves one aspect of our ordinary thinking about knowledge at the expense of denying another. We can view the theory of relevant alternatives as an attempt to provide a more satisfactory response to this tension in our thinking about knowledge. It attempts to characterize knowledge in a way that preserves both our belief that knowledge is an absolute concept and our belief that we have knowledge.

This approach to the theory of knowledge that sees an important connection between the growth of knowledge and biological evolution an evolutionary epistemologist claims that the development of human knowledge processed through some natural selection process, the best example of which is Darwin's theory of biological natural selection. There is a widespread misconception that evolution proceeds according to some plan or direct, put it has neither, and the role of chance ensures that its future course will be unpredictable. Random variations in individual organisms create tiny differences in their Darwinian fitness. Some individuals have more offspring's than others, and the characteristics that increased their fitness thereby become more prevalent in future generations. Once upon a time, at least a mutation occurred in a human population in tropical Africa that changed the hemoglobin molecule in a way that provided resistance to malaria. This enormous advantage caused the new gene to spread; with the unfortunate consequence that sickle-cell anaemia came to exist.

When proximate and evolutionary explanations are carefully distinguished, many questions in biology make more sense. A proximate explanation describes a trait - its anatomy, physiology, and biochemistry, as well as its development from the genetic instructions provided by a bit of DNA in the fertilized egg to the adult individual. An evolutionary explanation is about why DNA specifies that trait in the first place and why has DNA that encodes for one kind of structure and not some other. Proximate and evolutionary explanations are not alternatives, but both are needed to understand every trait. A proximate explanation for the external ear would incorporate of its arteries and nerves, and how it develops from the embryo to the adult form. Even if we know this, however, we still need an evolutionary explanation of how its structure gives creatures with ears an advantage, why those that lack the structure shaped by selection to give the ear its current form. To take another example, a proximate explanation of taste buds describes their structure and chemistry, how they detect salt, sweet, sour, and bitter, and how they transform this information into impulses that travel via neurons to the brain. An evolutionary explanation of taste buds shows why they detect saltiness, acidity, sweetness and bitterness instead of other chemical characteristics, and how the capacities detect these characteristics help, and cope with life.

Chance can influence the outcome at each stage: First, in the creation of genetic mutation, second, in whether the bearer lives long enough to show its effects, thirdly, in chance events that influence the individual's actual reproductive success, and fourth, in whether a gene even if favoured in one generation, is, happenstance, eliminated in the next, and finally in the many unpredictable environmental changes that will undoubtedly occur in the history of any group of organisms. As Harvard biologist Stephen Jay Gould has so vividly expressed that process over again, the outcome would surely be different. Not only might there not be humans, there might not even be anything like mammals.

We will often emphasis the elegance of traits shaped by natural selection, but the common idea that nature creates perfection needs to be analyzed carefully. The extent for which evolution obtainably achieves perfection depends on the enacting fitness for which Darwin speaks in terms of their survival and their fittest are most likely as perfect than the non-surviving species, only, that it enables us to know exactly what you mean. If in what you mean, 'Does natural selection always takes the best path for the long-term welfare of a species?' The answer is no. That would require adaptation by group selection, and this is, unlikely. If you mean 'Does natural selection creates every adaptation that would be valuable?' The answer again, is no. For instance, some kinds of South American monkeys can grasp branches with their tails. The trick would surely also be useful to some African species, but, simply because of bad luck, none have it. Some combination of circumstances started some ancestral South American monkeys using their tails in ways that ultimately led to an ability to grab onto branches, while no such development took place in Africa. Mere usefulness of a trait does not necessitate it mean that will evolve.

This is an approach to the theory of knowledge that sees an important connection between the growth of knowledge and biological evolution. An evolutionary epistemologist claims that the development of human knowledge proceeds through some natural selection process, the best example of which is Darwin's theory of biological natural selection. The three major components of the model of natural selection are variation selection and retention. According to Darwin's theory of natural selection, variations are not pre-designed to perform certain functions. Rather, these variations that perform useful functions are selected. While those that suffice on doing nothing are not selected but, nevertheless, such selections are responsible for the appearance that specific variations built upon intentionally do really occur. In the modern theory of evolution, genetic mutations provide the blind variations ( blind in the sense that variations are not influenced by the effects they would have, - the likelihood of a mutation is not correlated with the benefits or liabilities that mutation would confer on the organism), the environment provides the filter of selection, and reproduction provides the retention. It is achieved because those organisms with features that make them less adapted for survival do not survive about other organisms in the environment that have features that are better adapted. Evolutionary epistemology applies this blind variation and selective retention model to the growth of scientific knowledge and to human thought processes in general.

The parallel between biological evolutions and conceptual or we can see 'epistemic' evolution as either literal or analogical. The literal version of evolutionary epistemological biological evolution as the main cause of the growth of knowledge stemmed from this view, called the 'evolution of cognitive mechanic programs', by Bradie (1986) and the 'Darwinian approach to epistemology' by Ruse (1986), that growth of knowledge occurs through blind variation and selective retention because biological natural selection itself is the cause of epistemic variation and selection. The most plausible version of the literal view does not hold that all human beliefs are innate but rather than the mental mechanisms that guide the acquisition of non-innate beliefs are themselves innately and the result of biological natural selection. Ruses (1986) repossess to resume of the insistence of an interlingual rendition of literal evolutionary epistemology that he links to sociology.

Determining the value upon innate ideas can take the path to consider as these have been variously defined by philosophers either as ideas consciously present to the mind priori to sense experience (the non-dispositional sense), or as ideas which we have an innate disposition to form, though we need to be actually aware of them at a particular r time, e.g., as babies - the dispositional sense. Understood in either way they were invoked to account for our recognition of certain verification, such as those of mathematics, or to justify certain moral and religious clams which were held to b capable of being know by introspection of our innate ideas. Examples of such supposed truths might include 'murder is wrong' or 'God exists'.

One difficulty with the doctrine is that it is sometimes formulated as one about concepts or ideas which are held to be innate and at other times one about a source of propositional knowledge, in so far as concepts are taken to be innate the doctrine relates primarily to claims about meaning: Our idea of God, for example, is taken as a source for the meaning of the word God. When innate ideas are understood propositionally, their supposed innateness is taken an evidence for the truth. This latter thesis clearly rests on the assumption that innate propositions have an unimpeachable source, usually taken to be God, but then any appeal to innate ideas to justify the existence of God is circular. Despite such difficulties the doctrine of innate ideas had a long and influential history until the eighteenth century and the concept has in recent decades been revitalized through its employment in Noam Chomsky's influential account of the mind's linguistic capacities.

The attraction of the theory has been felt strongly by those philosophers who have been unable to give an alternative account of our capacity to recognize that some propositions are certainly true where that recognition cannot be justified solely o the basis of an appeal to sense experiences. Thus Plato argued that, for example, recognition of mathematical truths could only be explained on the assumption of some form of recollection, in Plato, the recollection of knowledge, possibly obtained in a previous stat e of existence e draws its topic as most famously broached in the dialogue "Meno," and the doctrine is one attemptive account for the 'innate' unlearned character of knowledge of first principles. Since there was no plausible post-natal source the recollection must refer of a pre-natal acquisition of knowledge. Thus understood, the doctrine of innate ideas supported the views that there were importantly gradatorially innate human beings and it was this sense which hindered their proper apprehension.

The ascetic implications of the doctrine were important in Christian philosophy throughout the Middle Ages and scholastic teaching until its displacement by Locke' philosophy in the eighteenth century. It had in the meantime acquired modern expression in the philosophy of Descartes who argued that we can come to know certain important truths before we have any empirical knowledge at all. Our idea of God must necessarily exist, is Descartes held, logically independent of sense experience. In England the Cambridge Plantonists such as Henry Moore and Ralph Cudworth added considerable support.

Locke's rejection of innate ideas and his alternative empiricist account was powerful enough to displace the doctrine from philosophy almost totally. Leibniz, in his critique of Locke, attempted to defend it with a sophisticated disposition version of theory, but it attracted few followers.

The empiricist alternative to innate ideas as an explanation of the certainty of propositions in the direction of construing with necessary truths as analytic, justly be for Kant's refinement of the classification of propositions with the fourfold analytic/synthetic distention and deductive/inductive did nothing to encourage a return to their innate idea's doctrine, which slipped from view. The doctrine may fruitfully be understood as the genesis of confusion between explaining the genesis of ideas or concepts and the basis for regarding some propositions as necessarily true.

Chomsky's revival of the term in connection with his account of the spoken exchange acquisition has once more made the issue topical. He claims that the principles of language and 'natural logic' are known unconsciously and is a precondition for language acquisition. But for his purposes innate ideas must be taken in a strong dispositional sense - so strong that it is far from clear that Chomsky's claims are as in direct conflict, and make unclear in mind or purpose, as with empiricists accounts of valuation, some (including Chomsky) have supposed. Willard van Orman Quine (1808-2000), for example, sees no disaccording with his own version of empirical behaviourism, in which sees the typical of an earlier time and often replaced by something more modern or fashionable converse [in] views upon the meaning of determining what a thing should be, as each generation has its own standards of mutuality.

Locke' accounts of analytic propositions was, that everything that a succinct account of analyticity should be (Locke, 1924). He distinguishes two kinds of analytic propositions, identity propositions for which 'we affirm the said term of itself', e.g., 'Roses are roses' and predicative propositions in which 'a part of the complex idea is predicated of the name of the whole', e.g., 'Roses are flowers'. Locke calls such sentences 'trifling' because a speaker who uses them 'trifling with words'. A synthetic sentence, in contrast, such as a mathematical theorem, that state of real truth and presents its instructive parallel's of real knowledge'. Correspondingly, Locke distinguishes both kinds of 'necessary consequences', analytic entailments where validity depends on the literal containment of the conclusion in the premise and synthetic entailment where it does not. John Locke (1632-1704) did not originate this concept-containment notion of analyticity. It is discussed by Arnaud and Nicole, and it is safe to say that it has been around for a very long time.

All the same, the analogical version of evolutionary epistemology, called the 'evolution of theory's program', by Bradie (1986). The 'Spenserians approach' (after the nineteenth century philosopher Herbert Spencer) by Ruse (1986), a process analogous to biological natural selection has governed the development of human knowledge, rather than by an instance of the mechanism itself. This version of evolutionary epistemology, introduced and elaborated by Donald Campbell (1974) and Karl Popper, sees the [partial] fit between theories and the world as explained by a mental process of trial and error known as epistemic natural selection.

We have usually taken both versions of evolutionary epistemology to be types of naturalized epistemology, because both take some empirical facts as a starting point for their epistemological project. The literal version of evolutionary epistemology begins by accepting evolutionary theory and a materialist approach to the mind and, from these, constructs an account of knowledge and its developments. By contrast, the analogical version does not require the truth of biological evolution: It simply draws on biological evolution as a source for the model of natural selection. For this version of evolutionary epistemology to be true, the model of natural selection need only apply to the growth of knowledge, not to the origin and development of species. Savagery put, evolutionary epistemology of the analogical sort could still be true even if creationism is the correct theory of the origin of species.

Although they do not begin by assuming evolutionary theory, most analogical evolutionary epistemologists are naturalized epistemologists as well, their empirical assumptions, least of mention, implicitly come from psychology and cognitive science, not evolutionary theory. Sometimes, however, evolutionary epistemology is characterized in a seemingly non-naturalistic fashion. (Campbell 1974) says that 'if one is expanding knowledge beyond what one knows, one has no choice but to explore without the benefit of wisdom', i.e., blindly. This, Campbell admits, makes evolutionary epistemology close to being a tautology (and so not naturalistic). Evolutionary epistemology does assert the analytic claim that when expanding one's knowledge beyond what one knows, one must processed to something that is already known, but, more interestingly, it also makes the synthetic claim that when expanding one's knowledge beyond what one knows, one must proceed by blind variation and selective retention. This claim is synthetic because we can empirically falsify it. The central claim of evolutionary epistemology is synthetic, not analytic, but if the central contradictory of which they are not, then Campbell is right that evolutionary epistemology does have the analytic feature he mentions, but he is wrong to think that this is a distinguishing feature, since any plausible epistemology has the same analytic feature.

Two extra-ordinary issues lie to awaken the literature that involves questions about 'realism', i.e., what metaphysical commitment does an evolutionary epistemologist have to make? (Progress, i.e., according to evolutionary epistemology, does knowledge develop toward a goal?) With respect to realism, many evolutionary epistemologists endorse that is called 'hypothetical realism', a view that combines a version of epistemological 'scepticism' and tentative acceptance of metaphysical realism. With respect to progress, the problem is that biological evolution is not goal-directed, but the growth of human knowledge is. Campbell (1974) worries about the potential dis-analogy here but is willing to bite the stone of conscience and admit that epistemic evolution progress toward a goal (truth) while biological evolution does not. Some have argued that evolutionary epistemologists must give up the 'truth-topic' sense of progress because a natural selection model is in non-teleological in essence alternatively, following Kuhn (1970), and embraced along with evolutionary epistemology.

Among the most frequent and serious criticisms levelled against evolutionary epistemology is that the analogical version of the view is false because epistemic variation is not blind are to argue that, however, that this objection fails because, while epistemic variation is not random, its constraints come from heuristics that, for the most part, are selective retention. Further, Stein and Lipton argue that lunatics are analogous to biological pre-adaptations, evolutionary pre-biological pre-adaptations, evolutionary cursors, such as a half-wing, a precursor to a wing, which have some function other than the function of their discountable structures: The function of descendability may result in the function of their descendable character embodied to its structural foundations, is that of the guideline of epistemic variation is, on this view, not the source of dis-analogy, but the source of a more articulated account of the analogy.

Many evolutionary epistemologists try to combine the literal and the analogical versions, saying that those beliefs and cognitive mechanisms, which are innate results from natural selection of the biological sort and those that are innate results from natural selection of the epistemic sort. This is reasonable as long as the two parts of this hybrid view are kept distinct. An analogical version of evolutionary epistemology with biological variation as its only source of blindness would be a null theory: This would be the case if all our beliefs are innate or if our non-innate beliefs are not the result of blind variation. An appeal to the legitimate way to produce a hybrid version of evolutionary epistemology since doing so trivializes the theory. For similar reasons, such an appeal will not save an analogical version of evolutionary epistemology from arguments to the effect that epistemic variation is blind.

Although it is a new approach to theory of knowledge, evolutionary epistemology has attracted much attention, primarily because it represents a serious attempt to flesh out a naturalized epistemology by drawing on several disciplines. In science is used for understanding the nature and development of knowledge, then evolutionary theory is among the disciplines worth a look. Insofar as evolutionary epistemology looks there, it is an interesting and potentially fruitful epistemological programmed.

What makes a belief justified and what makes true belief knowledge? Thinking that whether a belief deserves one of these appraisals is natural depends on what caused such subjectivity to have the belief. In recent decades many epistemologists have pursued this plausible idea with a variety of specific proposals. Some causal theories of knowledge have it that a true belief that 'p' is knowledge just in case it has the right causal connection to the fact that 'p'. They can apply such a criterion only to cases where the fact that 'p' is a sort that can enter intuit causal relations, as this seems to exclude mathematically and other necessary facts and perhaps any fact expressed by a universal generalization, and proponents of this sort of criterion have usually supposed that it is limited to perceptual representations where knowledge of particular facts about subjects' environments.

For example, Armstrong (1973) initially proposed something which is proposed to another for consideration, as a set before the mind for consideration, as to put forth an intended purpose. That a belief to carry a one's affairs independently and self-sufficiently often under difficult circumstances progress for oneself and makes do and stand on one's own formalities in the transitional form 'This [perceived] objects is 'F' is [non-inferential] knowledge if and only if the belief is a completely reliable sign that the perceived object is 'F', that is, the fact that the object is 'F' contributed to causing the belief and its doing so depended on properties of the believer such that the laws of nature dictated that, for any subject, and the perceived objective 'y', if 'p' had those properties and believed that 'y' is 'F', then 'y' is 'F'. Offers a rather similar account, in terms of the belief's being caused by a signal received by the perceiver that carries the information that the object is 'F'.

This sort of condition fails, however, to be sufficiently for non-inferential perceptivity, for knowledge is accountable for its compatibility with the belief's being unjustified, and an unjustified belief cannot be knowledge. The view that a belief acquires favourable epistemic status by having some kind of reliable linkage to the truth, seems by accountabilities that they have variations of this view which has been advanced for both knowledge and justified belief. The first formulation of a reliable account of knowing notably appeared as marked and noted and accredited to F. P. Ramsey (1903-30), whereby much of Ramsey's work was directed at saving classical mathematics from 'intuitionism', or what he called the 'Bolshevik menace of Brouwer and Weyl'. In the theory of probability he was the first to develop, based on precise behavioural nations of preference and expectation. In the philosophy of language, Ramsey was one of the first thinkers to accept a 'redundancy theory of truth', which he combined with radical views of the function of many kinds of propositions. Neither generalizations, nor causal positions, nor those treating probability or ethics, described facts, but each have a different specific function in our intellectual economy. Additionally, Ramsey, who said that an impression of belief was knowledge if it were true, certain and obtained by a reliable process. P. Unger (1968) suggested that 'S' knows that 'p' just in case it is of at all accidental that 'S' is right about its being the case that drew an analogy between a thermometer that reliably indicates the temperature and a belief interaction of reliability that indicates the truth. Armstrong said that a non-inferential belief qualified as knowledge if the belief has properties that are nominally sufficient for its truth, i.e., guarantees its truth via laws of nature.

They standardly classify reliabilism as an 'externaturalist' theory because it invokes some truth-linked factor, and truth is 'eternal' to the believer the main argument for externalism derives from the philosophy of language, more specifically, from the various phenomena pertaining to natural kind terms, indexicals, etc., that motivate the views that have come to be known as direct reference' theories. Such phenomena seem, at least to show that the belief or thought content that can be properly attributed to a person is dependent on facts about his environment, i.e., whether he is on Earth or Twin Earth, what in fact he is pointing at, the classificatory criteria employed by the experts in his social group, etc. -. Not just on what is going on internally in his mind or brain (Putnam, 175 and Burge, 1979.) Virtually all theories of knowledge, of course, share an externalist component in requiring truth as a condition for knowing. Reliabilism goes further, however, in trying to capture additional conditions for knowledge by means of a nomic, counterfactual or other such 'external' relations between 'belief' and 'truth'.

The most influential counterexample to reliabilism is the demon-world and the clairvoyance examples. The demon-world example challenges the necessity of the reliability requirement, in that a possible world in which an evil demon creates deceptive visual experience, the process of vision is not reliable. Still, the visually formed beliefs in this world are intuitively justified. The clairvoyance example challenges the sufficiency of reliability. Suppose a cognitive agent possesses a reliable clairvoyance power, but has no evidence for or against his possessing such a power. Intuitively, his clairvoyantly formed beliefs are unjustifiably unreasoned, but Reliabilism declares them justified.

Another form of reliabilism, - 'normal worlds', reliabilism, answers to the range problem differently, and treats the demon-world problem in the same fashionable manner, and so permitting a 'normal world', as one that is consistent with our general beliefs about the actual world. Normal-worlds reliabilism says that a belief, in any possible world is justified just in case its generating processes have high truth ratios in normal worlds. This resolves the demon-world problem because the relevant truth ratio of the visual process is not its truth ratio in the demon world itself, but its ratio in normal worlds. Since this ratio is presumably high, visually formed beliefs in the demon world turn out to be justified.

Yet, a different version of reliabilism attempts to meet the demon-world and clairvoyance problems without recourse to the questionable notion of 'normal worlds'. Consider Sosa's (1992) suggestion that justified beliefs is belief acquired through 'intellectual virtues', and not through intellectual 'vices', whereby virtues are reliable cognitive faculties or processes. The task is to explain how epistemic evaluators have used the notion of indelible virtues, and vices, to arrive at their judgments, especially in the problematic cases. Goldman (1992) proposes a two-stage reconstruction of an evaluator's activity. The first stage is a reliability-based acquisition of a 'list' of virtues and vices. The second stage is application of this list to queried cases. Determining has executed the second stage whether processes in the queried cases resemble virtues or vices. We have classified visual beliefs in the demon world as justified because visual belief formation is one of the virtues. Clairvoyance formed, beliefs are classified as unjustified because clairvoyance resembles scientifically suspect processes that the evaluator represents as vices, e.g., mental telepathy, ESP, and so forth

A philosophy of meaning and truth, for which it is especially associated with the American philosopher of science and of language (1839-1914), and the American psychologist philosopher William James (1842-1910), Wherefore the study in Pragmatism is given to various formulations by both writers, but the core is the belief that the meaning of a doctrine is the same as the practical effects of adapting it. Peirce interpreted of theocratical sentences ids only that of a corresponding practical maxim (telling us what to do in some circumstance). In James the position issues in a theory of truth, notoriously allowing that belief, including for examples, belief in God, are the widest sense of the works satisfactorily in the widest sense of the word. On James's view almost any belief might be respectable, and even true, but working with true beliefs is not a simple matter for James. The apparent subjectivist consequences of this were wildly assailed by Russell (1872-1970), Moore (1873-1958), and others in the early years of the 20th-century. This led to a division within pragmatism between those such as the American educator John Dewey (1859-1952), whose humanistic conception of practice remains inspired by science, and the more idealistic route that especially by the English writer F.C.S. Schiller (1864-1937), embracing the doctrine that our cognitive efforts and human needs actually transform the reality that we seek to describe. James often writes as if he sympathizes with this development. For instance, in The Meaning of Truth (1909), he considers the hypothesis that other people have no minds (dramatized in the sexist idea of an 'automatic sweetheart' or female zombie) and remarks' that the hypothesis would not work because it would not satisfy our egoistic craving for the recognition and admiration of others, these implications that make it true that the other persons have minds in the disturbing part.

Modern pragmatists such as the American philosopher and critic Richard Rorty (1931-) and some writings of the philosopher Hilary Putnam (1925-) who has usually tried to dispense with an account of truth and concentrate, as perhaps James should have done, upon the nature of belief and its relations with human attitude, emotion, and need. The driving motivation of pragmatism is the idea that belief in the truth on the one hand must have a close connection with success in action on the other. One way of cementing the connection is found in the idea that natural selection must have adapted us to be cognitive creatures because beliefs have effects, as they work. Pragmatism can be found in Kant's doctrine of the primary of practical over pure reason, and continued to play an influential role in the theory of meaning and of truth.

In case of fact, the philosophy of mind is the modern successor to behaviourism, as do the functionalism that its early advocates were Putnam (1926- ) and Sellars (1912-89), and its guiding principle is that we can define mental states by a triplet of relations they have on other mental stares, what effects they have on behaviour. The definition need not take the form of a simple analysis, but if w could write down the totality of axioms, or postdate, or platitudes that govern our theories about what things of other mental states, and our theories about what things are apt to cause (for example), a belief state, what effects it would have on a variety of other mental states, and what the force of impression of one thing on another, inducing to come into being and carry to as successful conclusions as found a pass that allowed them to affect passage through the mountains. A condition or occurrence traceable to a cause drawing forth the underlying and hidden layers of deep-seated latencies. Very well protected but the digression belongs to the patient, in that, what exists of the back-burners of the mind, slowly simmering, and very much of your self control is intact: Furthering the outcry of latent incestuousness that affects the likelihood of having an influence upon behaviour, so then all that we would have done otherwise, contains all that is needed to make the state a proper theoretical notion. It could be implicitly defied by these theses. Functionalism is often compared with descriptions of a computer, since according to mental descriptions correspond to a description of a machine in terms of software, that remains silent about the underlying hardware or 'realization' of the program the machine is running. The principal advantage of functionalism includes its fit with the way we know of mental states both of ourselves and others, which is via their effects on behaviour and other mental states. As with behaviourism, critics charge that structurally complex items that do not bear mental states might nevertheless, imitate the functions that are cited. According to this criticism functionalism is too generous and would count too many things as having minds. It is also queried whether functionalism is too paradoxical, able to see mental similarities only when there is causal similarity, when our actual practices of interpretations enable us to support thoughts and desires too differently from our own, it may then seem as though beliefs and desires are obtained in the consenting availability of 'variably acquired' causal architecture, just as much as they can be in different Neurophysiologic states.

The philosophical movement of Pragmatism had a major impact on American culture from the late 19th century to the present. Pragmatism calls for ideas and theories to be tested in practice, by assessing whether acting upon the idea or theory produces desirable or undesirable results. According to pragmatists, all claims about truth, knowledge, morality, and politics must be tested in this way. Pragmatism has been critical of traditional Western philosophy, especially the notions that there are absolute truths and absolute values. Although pragmatism was popular for a time in France, England, and Italy, most observers believe that it encapsulates an American faith in know-how and practicality and an equally American distrust of abstract theories and ideologies.

In mentioning the American psychologist and philosopher we find William James, who helped to popularize the philosophy of pragmatism with his book Pragmatism: A New Name for Old Ways of thinking (1907). Influenced by a theory of meaning and verification developed for scientific hypotheses by American philosopher C.S. Peirce, James held that truth is what compellingly works, or has good experimental results. In a related theory, James argued the existence of God is partly verifiable because many people derive benefits from believing.

Pragmatists regard all theories and institutions as tentative hypotheses and solutions. For this reason they believed that efforts to improve society, through such means as education or politics, must be geared toward problem solving and must be ongoing. Through their emphasis on connecting theory to practice, pragmatist thinkers attempted to transform all areas of philosophy, from metaphysics to ethics and political philosophy.

Pragmatism sought a middle ground between traditional ideas about the nature of reality and radical theories of nihilism and irrationalism, which had become popular in Europe in the late 19th century. Traditional metaphysics assumed that the world has a fixed, intelligible structure and that human beings can know absolute or objective truths about the world and about what constitutes moral behaviour. Nihilism and irrationalism, on the other hand, denied those very assumptions and their certitude. Pragmatists today still try to steer a middle course between contemporary offshoots of these two extremes.

The ideas of the pragmatists were considered revolutionary when they first appeared. To some critics, pragmatism's refusal to affirm any absolutes carried negative implications for society. For example, pragmatists do not believe that a single absolute idea of goodness or justice exists, but rather than these concepts are changeable and depend on the context in which they are being discussed. The absence of these absolutes, critics feared, could result in a decline in moral standards. The pragmatists' denial of absolutes, moreover, challenged the foundations of religion, government, and schools of thought. As a result, pragmatism influenced developments in psychology, sociology, education, semiotics (the study of signs and symbols), and scientific method, as well as philosophy, cultural criticism, and social reform movements. Various political groups have also drawn on the assumptions of pragmatism, from the progressive movements of the early 20th century to later experiments in social reform.

Pragmatism is best understood in its historical and cultural context. It arose during the late 19th century, a period of rapid scientific advancement typified by the theories of British biologist Charles Darwin, whose theories suggested too many thinkers that humanity and society are in a perpetual state of progress. During this same period a decline in traditional religious beliefs and values accompanied the industrialization and material progress of the time. In consequence it became necessary to rethink fundamental ideas about values, religion, science, community, and individuality.

The three most important pragmatists are American philosophers' Charles Sanders Peirce, William James, and John Dewey. Peirce was primarily interested in scientific method and mathematics; His objective was to infuse scientific thinking into philosophy and society and he believed that human comprehension of reality was becoming ever greater and that human communities were becoming increasingly progressive. Peirce developed pragmatism as a theory of meaning - in particular, the meaning of concepts used in science. The meaning of the concept 'brittle', for example, is given by the observed consequences or properties that objects called 'brittle' exhibit. For Peirce, the only rational way to increase knowledge was to form mental habits that would test ideas through observation, experimentation, or what he called inquiry. Many philosophers known as logical positivist, a group of philosophers who have been influenced by Peirce, believed that our evolving species was fated to get ever closer to Truth. Logical positivists emphasize the importance of scientific verification, rejecting the assertion of positivism that personal experience is the basis of true knowledge.

James moved pragmatism in directions that Peirce strongly disliked. He generalized Peirce's doctrines to encompass all concepts, beliefs, and actions; he also applied pragmatist ideas to truth as well as to meaning. James was primarily interested in showing how systems of morality, religion, and faith could be defended in a scientific civilization. He argued that sentiment, as well as logic is crucial to rationality and that the great issues of life - morality and religious belief, for example - are leaps of faith. As such, they depend upon what he called 'the will to believe' and not merely on scientific evidence, which can never tell us what to do or what is worthwhile. Critics charged James with relativism (the belief that values depend on specific situations) and with crass expediency for proposing that if an idea or action works the way one intends, it must be right. But James can more accurately be described as a pluralist - someone who believes the world to be far too complex for any-one philosophy to explain everything.

Dewey's philosophy can be described as a version of philosophical naturalism, which regards human experience, intelligence, and communities as ever-evolving mechanisms. Using their experience and intelligence, Dewey believed, human beings can solve problems, including social problems, through inquiry. For Dewey, naturalism led to the idea of a democratic society that allows all members to acquire social intelligence and progress both as individuals and as communities. Dewey held that traditional ideas about knowledge, truth, and values, in which absolutes are assumed, are incompatible with a broadly Darwinian world-view in which individuals and societies are progressing. In consequence, he felt that these traditional ideas must be discarded or revised. Indeed, for pragmatists, everything people know and do depend on a historical context and are thus tentative rather than absolute.

Many followers and critics of Dewey believe he advocated elitism and social engineering in his philosophical stance. Others think of him as a kind of romantic humanist. Both tendencies are evident in Dewey's writings, although he aspired to synthesize the two realms.

The pragmatists' tradition was revitalized in the 1980s by American philosopher Richard Rorty, who has faced similar charges of elitism for his belief in the relativism of values and his emphasis on the role of the individual in attaining knowledge. Interest has renewed in the classic pragmatists - Pierce, James, and Dewey - have an alternative to Rorty's interpretation of the tradition.

One of the earliest versions of a correspondence theory was put forward in the 4th century Bc Greek philosopher Plato, who sought to understand the meaning of knowledge and how it is acquired. Plato wished to distinguish between true belief and false belief. He proposed a theory based on intuitive recognition that true statements correspond to the facts - that is, agree with reality - while false statements do not. In Plato's example, the sentence "Theaetetus flies" can be true only if the world contains the fact that Theaetetus flies. However, Plato - and much later, 20th-century British philosopher Bertrand Russell - recognized this theory as unsatisfactory because it did not allow for false belief. Both Plato and Russell reasoned that if a belief is false because there is no fact to which it corresponds, it would then be a belief about nothing and so not a belief at all. Each then speculated that the grammar of a sentence could offer a way around this problem. A sentence can be about something (the person Theaetetus), yet false (flying is not true of Theaetetus). But how, they asked, are the parts of a sentence related to reality?

One suggestion, proposed by 20th-century philosopher Ludwig Wittgenstein, is that the parts of a sentence relate to the objects they describe in much the same way that the parts of a picture relate to the objects pictured. Once again, however, false sentences pose a problem: If a false sentence pictures nothing, there can be no meaning in the sentence.

In the late 19th-century American philosopher Charles S. Peirce offered another answer to the question "What is truth?" He asserted that truth is that which experts will agree upon when their investigations are final. Many pragmatists such as Peirce claim that the truth of our ideas must be tested through practice. Some pragmatists have gone so far as to question the usefulness of the idea of truth, arguing that in evaluating our beliefs we should rather pay attention to the consequences that our beliefs may have. However, critics of the pragmatic theory are concerned that we would have no knowledge because we do not know which set of beliefs will ultimately be agreed upon; nor are their sets of beliefs that are useful in every context.

A third theory of truth, the coherence theory, also concerns the meaning of knowledge. Coherence theorists have claimed that a set of beliefs is true if the beliefs are comprehensive - that is, they cover everything - and do not contradict each other.

Other philosophers dismiss the question "What is truth?" With the observation that attaching the claim 'it is true that' to a sentence adds no meaning, however, these theorists, who have proposed what are known as deflationary theories of truth, do not dismiss such talk about truth as useless. They agree that there are contexts in which a sentence such as 'it is true that the book is blue' can have a different impact than the shorter statement 'the book is blue'. What is more important, use of the word true is essential when making a general claim about everything, nothing, or something, as in the statement 'most of what he says is true?'

Many experts believe that philosophy as an intellectual discipline originated with the work of Plato, one of the most celebrated philosophers in history. The Greek thinker had an immeasurable influence on Western thought. However, Plato's expression of ideas in the form of dialogues-the dialectical method, used most famously by his teacher Socrates - has led to difficulties in interpreting some of the finer points of his thoughts. The issue of what exactly Plato meant to say is addressed in the following excerpt by author R. M. Hare.

Linguistic analysis as a method of philosophy is as old as the Greeks. Several of the dialogues of Plato, for example, are specifically concerned with clarifying terms and concepts. Nevertheless, this style of philosophizing has received dramatically renewed emphasis in the 20th century. Influenced by the earlier British empirical tradition of John Locke, George Berkeley, David Hume, and John Stuart Mill and by the writings of the German mathematician and philosopher Gottlob Frége, the 20th-century English philosopher's G. E. Moore and Bertrand Russell became the founders of this contemporary analytic and linguistic trend. As students together at the University of Cambridge, Moore and Russell rejected Hegelian idealism, particularly as it was reflected in the work of the English metaphysician F. H. Bradley, who held that nothing is completely real except the Absolute. In their opposition to idealism and in their commitment to the view that careful attention to language is crucial in philosophical inquiry, and they set the mood and style of philosophizing for much of the 20th century English-speaking world.

For Moore, philosophy was first and foremost analysis. The philosophical task involves clarifying puzzling propositions or concepts by indicating fewer puzzling propositions or concepts to which the originals are held to be logically equivalent. Once this task has been completed, the truth or falsity of problematic philosophical assertions can be determined more adequately. Moore was noted for his careful analyses of such puzzling philosophical claims as 'time is unreal', analyses that aided of determining the truth of such assertions.

Russell, strongly influenced by the precision of mathematics, was concerned with developing an ideal logical language that would accurately reflect the nature of the world. Complex propositions, Russell maintained, can be resolved into their simplest components, which he called atomic propositions. These propositions refer to atomic facts, the ultimate constituents of the universe. The metaphysical view based on this logical analysis of language and the insistence that meaningful propositions must correspond to facts constitutes what Russell called logical atomism. His interest in the structure of language also led him to distinguish between the grammatical form of a proposition and its logical form. The statements 'John is good' and 'John is tall' have the same grammatical form but different logical forms. Failure to recognize this would lead one to treat the property 'goodness' as if it were a characteristic of John in the same way that the property 'tallness' is a characteristic of John. Such failure results in philosophical confusion.

Austrian-born philosopher Ludwig Wittgenstein was one of the most influential thinkers of the 20th century. With his fundamental work, Tractatus Logico-philosophicus, published in 1921, he became a central figure in the movement known as analytic and linguistic philosophy.

Russell's work of mathematics attracted towards studying in Cambridge the Austrian philosopher Ludwig Wittgenstein, who became a central figure in the analytic and linguistic movement. In his first major work, Tractatus Logico-Philosophicus (1921; translation 1922), in which he first presented his theory of language, Wittgenstein argued that 'all philosophy is a 'critique of language' and that 'philosophy aims at the logical clarification of thoughts'. The results of Wittgenstein's analysis resembled Russell's logical atomism. The world, he argued, is ultimately composed of simple facts, which it is the purpose of language to picture. To be meaningful, statements about the world must be reducible to linguistic utterances that have a structure similar to the simple facts pictured. In this early Wittgensteinian analysis, only propositions that picture facts - the propositions of science - are considered factually meaningful. Metaphysical, theological, and ethical sentences were judged to be factually meaningless.

Influenced by Russell, Wittgenstein, Ernst Mach, and others, a group of philosophers and mathematicians in Vienna in the 1920s initiated the movement known as logical positivism: Led by Moritz Schlick and Rudolf Carnap, the Vienna Circle initiated one of the most important chapters in the history of analytic and linguistic philosophy. According to the positivists, the task of philosophy is the clarification of meaning, not the discovery of new facts (the job of the scientists) or the construction of comprehensive accounts of reality (the misguided pursuit of traditional metaphysics).

The positivists divided all meaningful assertions into two classes: analytic propositions and empirically verifiable ones. Analytic propositions, which include the propositions of logic and mathematics, are statements the truth or falsity of which depend altogether on the meanings of the terms constituting the statement. An example would be the proposition 'two plus two equals four'. The second class of meaningful propositions includes all statements about the world that can be verified, at least in principle, by sense experience. Indeed, the meaning of such propositions is identified with the empirical method of their verification. This verifiability theory of meaning, the positivists concluded, would demonstrate that scientific statements are legitimate factual claims and that metaphysical, religious, and ethical sentences are factually dwindling. The ideas of logical positivism were made popular in England by the publication of A. J. Ayer's Language, Truth and Logic in 1936.

The positivists' verifiability theory of meaning came under intense criticism by philosophers such as the Austrian-born British philosopher Karl Popper. Eventually this narrow theory of meaning yielded to a broader understanding of the nature of language. Again, an influential figure was Wittgenstein. Repudiating many of his earlier conclusions in the Tractatus, he initiated a new line of thought culminating in his posthumously published Philosophical Investigations (1953, translated 1953). In this work, Wittgenstein argued that once attention is directed to the way language is actually used in ordinary discourse, the variety and flexibility of language become clear. Propositions do much more than simply picture facts.

This recognition led to Wittgenstein's influential concept of language games. The scientist, the poet, and the theologian, for example, are involved in different language games. Moreover, the meaning of a proposition must be understood in its context, that is, in terms of the rules of the language game of which that proposition is a part. Philosophy, concluded Wittgenstein, is an attempt to resolve problems that arise as the result of linguistic confusion, and the key to the resolution of such problems is ordinary language analysis and the proper use of language.

Additional contributions within the analytic and linguistic movement include the work of the British philosopher's Gilbert Ryle, John Austin, and P. F. Strawson and the American philosopher W. V. Quine. According to Ryle, the task of philosophy is to restate 'systematically misleading expressions' in forms that are logically more accurate. He was particularly concerned with statements the grammatical form of which suggests the existence of nonexistent objects. For example, Ryle is best known for his analysis of mentalistic language, language that misleadingly suggests that the mind is an entity in the same way as the body.

Austin maintained that one of the most fruitful starting points for philosophical inquiry is attention to the extremely fine distinctions drawn in ordinary language. His analysis of language eventually led to a general theory of speech acts, that is, to a description of the variety of activities that an individual may be performing when something is uttered.

Strawson is known for his analysis of the relationship between formal logic and ordinary language. The complexity of the latter, he argued, is inadequately represented by formal logic. A variety of analytic tools, therefore, are needed in addition to logic in analysing ordinary language.

Quine discussed the relationship between language and ontology. He argued that language systems tend to commit their users to the existence of certain things. For Quine, the justification for speaking one way rather than another is a thoroughly pragmatic one.

The commitment to language analysis as a way of pursuing philosophy has continued as a significant contemporary dimension in philosophy. A division also continues to exist between those who prefer to work with the precision and rigour of symbolic logical systems and those who prefer to analyse ordinary language. Although few contemporary philosophers maintain that all philosophical problems are linguistic, the view continues to be widely held that attention to the logical structure of language and to how language is used in everyday discourse can many a time have an eye to aid in anatomize Philosophical problems.

A loose title for various philosophies that emphasize certain common themes, the individual, the experience of choice, and the absence of rational understanding of the universe, with the additional ways of addition seems a consternation of dismay or one fear, or the other extreme, as far apart is the sense of the dottiness of 'absurdity in human life', however, existentialism is a philosophical movement or tendency, emphasizing individual existence, freedom, and choice, that influenced many diverse writers in the 19th and 20th centuries.

Because of the diversity of positions associated with existentialism, the term is impossible to define precisely. Certain themes common to virtually all existentialist writers can, however, be identified. The term itself suggests one major theme: the stress on concrete individual existence and, consequently, on subjectivity, individual freedom, and choice.

Most philosophers since Plato have held that the highest ethical good are the same for everyone; Insofar as one approaches moral perfection, one resembles other morally perfect individuals. The 19th-century Danish philosopher Søren Kierkegaard, who was the first writer to call himself existential, reacted against this tradition by insisting that the highest good for the individual are to find his or her own unique vocation. As he wrote in his journal, 'I must find a truth that is true for me . . . the idea for which I can live or die'. Other existentialist writers have echoed Kierkegaard's belief that one must choose one's own way without the aid of universal, objective standards. Against the traditional view that moral choice involves an objective judgment of right and wrong, existentialists have argued that no objective, rational basis can be found for moral decisions. The 19th-century German philosopher Friedrich Nietzsche further contended that the individual must decide which situations are to count as moral situations.



One of the most controversial works of 19th-century philosophy, Thus Spake Zarathustra (1883-1885), which was articulated by the German philosopher Friedrich Nietzsche's theory of the Übermensch, a term translated as "Superman" or "Overman." The Superman was an individual who overcame what Nietzsche termed the 'slave morality' of traditional values, and lived according to his own morality. Nietzsche also advanced his idea that 'God is dead', or that traditional morality was no longer relevant in people's lives. In this passage, the sage Zarathustra came down from the mountain where he had spent the last ten years alone to preach to the people.

Nietzsche, who was not acquainted with the work of Kierkegaard, influenced subsequent existentialist thought through his criticism of traditional metaphysical and moral assumptions and through his espousal of tragic pessimism and the life-affirming individual will that opposes itself to the moral conformity of the majority. In contrast to Kierkegaard, whose attack on conventional morality led him to advocate a radically individualistic Christianity, Nietzsche proclaimed the "death of God" and went on to reject the entire Judeo-Christian moral tradition in favour of a heroic pagan ideal.

The modern philosophy movements of phenomenology and existentialism have been greatly influenced by the thought of German philosopher Martin Heidegger. According to Heidegger, humankind has fallen into a crisis by taking a narrow, technological approach to the world and by ignoring the larger question of existence. People, if they wish to live authentically, must broaden their perspectives. Instead of taking their existence for granted, people should view themselves as part of being (Heidegger's term for that which underlies all existence).

Heidegger, like Pascal and Kierkegaard, reacted against an attempt to put philosophy on a conclusive rationalistic basis - in this case the phenomenology of the 20th-century German philosopher Edmund Husserl. Heidegger argued that humanity finds itself in an incomprehensible, indifferent world. Human beings can never hope to understand why they are here; instead, each individual must choose a goal and follow it with passionate conviction, aware of the certainty of death and the ultimate meaninglessness of one's life. Heidegger contributed to existentialist thought an original emphasis on being and ontology as well as on language.

Twentieth-century French intellectual Jean-Paul Sartre helped to develop existential philosophy through his writings, novels, and plays. Much did of Sartre's works focuses on the dilemma of choice faced by free individuals and on the challenge of creating meaning by acting responsibly in an indifferent world. In stating that 'man is condemned to be free', Sartre reminds us of the responsibility that accompanies human decisions.

Sartre first gave the term existentialism general currency by using it for his own philosophy and by becoming the leading figure of a distinct movement in France that became internationally influential after World War II. Sartre's philosophy is explicitly atheistic and pessimistic; he declared that human beings require a rational basis for their lives but are unable to achieve one and thus human life is a 'futile passion'. Sartre nevertheless insisted that his existentialism is a form of humanism, and he strongly emphasized human freedom, choice, and responsibility. He eventually tried to reconcile these existentialist concepts with a Marxist analysis of society and history.

Although existentialist thought encompasses the uncompromising atheism of Nietzsche and Sartre and the agnosticism of Heidegger, its origin in the intensely religious philosophies of Pascal and Kierkegaard foreshadowed its profound influence on a 20th-century theology. The 20th-century German philosopher Karl Jaspers, although he rejected explicit religious doctrines, influenced contemporary theologies through his preoccupation with transcendence and the limits of human experience. The German Protestant theologian's Paul Tillich and Rudolf Bultmann, the French Roman Catholic theologian Gabriel Marcel, the Russian Orthodox philosopher Nikolay Berdyayev, and the German Jewish philosopher Martin Buber inherited many of Kierkegaard's concerns, especially that a personal sense of authenticity and commitment is essential to religious faith.

Renowned as one of the most important writers in world history, 19th-century Russian author Fyodor Dostoyevsky wrote psychologically intense novels which probed the motivations and moral justifications for his characters' actions. Dostoyevsky commonly addressed themes such as the struggle between good and evil within the human soul and the idea of salvation through suffering. The Brothers Karamazov (1879-1880), generally considered Dostoyevsky's best work, interlaces religious exploration with the story of a family's violent quarrels over a woman and a disputed inheritance.

A number of existentialist philosophers used literary forms to convey their thought, and existentialism has been as vital and as extensive a movement in literature as in philosophy. The 19th-century Russian novelist Fyodor Dostoyevsky is probably the greatest existentialist literary figure. In Notes from the Underground (1864), the alienated antihero rages against the optimistic assumptions of rationalist humanism. The view of human nature that emerges in this and other novels of Dostoyevsky is that it is unpredictable and perversely self-destructive; only Christian love can save humanity from itself, but such love cannot be understood philosophically. As the character Alyosha says in The Brothers Karamazov (1879-80), "We must love life more than the meaning of it."

The opening series of arranged passages in continuous or uniform order, by ways that the progressive course accommodates to arrange in a line or lines of continuity, Wherefore, the Russian novelist Fyodor Dostoyevsky's Notes from Underground (1864) - 'I am a sick man . . . I am a spiteful man' - are among the most famous in 19th-century literature. Published five years after his release from prison and involuntary, military service in Siberia, Notes from Underground is a sign of Dostoyevsky's rejection of the radical social thinking he had embraced in his youth. The unnamed narrator is antagonistic in tone, questioning the reader's sense of morality as well as the foundations of rational thinking. In this excerpt from the beginning of the novel, the narrator describes himself, derisively referring to himself as an 'overly conscious' intellectual.

In the 20th century, the novels of the Austrian Jewish writer Franz Kafka, such as The Trial (1925 translations, 1937) and The Castle (1926 translations, 1930), presents isolated men confronting vast, elusive, menacing bureaucracies; Kafka's themes of anxiety, guilt, and solitude reflect the influence of Kierkegaard, Dostoyevsky, and Nietzsche. The influence of Nietzsche is also discernible in the novels of the French writer's André Malraux and in the plays of Sartre. The work of the French writer Albert Camus is usually associated with existentialism because of the prominence in it of such themes as the apparent absurdity and futility of life, the indifference of the universe, and the necessity of engagement in a just cause. In the United States, the influence of existentialism on literature has been more indirect and diffuse, but traces of Kierkegaard's thought can be found in the novels of Walker Percy and John Updike, and various existentialist themes are apparent in the work of such diverse writers as Norman Mailer and John Barth.

The problem of defining knowledge in terms of true belief plus some favoured relation between the believer and the facts began with Plato's view in the Theaetetus, that knowledge is true belief plus some logos, and epistemology is a beginning for which it is bound to the foundations of knowledge, a special branch of philosophy that addresses the philosophical problems surrounding the theory of knowledge. Epistemology is concerned with the definition of knowledge and related concepts, the sources and criteria of knowledge, the kinds of knowledge possible and the degree to which each is certain, and the exact integrations among the one's who are understandably of knowing and the object known.

Thirteenth-century Italian philosopher and theologian Saint Thomas Aquinas attempted to synthesize Christian belief with a broad range of human knowledge, embracing diverse sources such as Greek philosopher Aristotle and Islamic and Jewish scholars. His thought exerted lasting influence on the development of Christian theology and Western philosophy. And explicated by the author, Anthony Kenny who examines the complexities of Aquinas's concepts of substance and accident.

In the 5th century Bc, the Greek Sophists questioned the possibility of reliable and objective knowledge. Thus, a leading Sophist, Gorgias, argued that nothing really exists, that if anything did exist it could not be known, and that if knowledge were possible, it could not be communicated. Another prominent Sophist, Protagoras, maintained that no person's opinions can be said to be more correct than another's, because each is the sole judge of his or her own experience. Plato, following his illustrious teacher Socrates, tried to answer the Sophists by postulating the existence of a world of unchanging and invisible forms, or ideas, about which it is possible to have exact and certain knowledge. The thing's one sees and touches, they maintained, are imperfect copies of the pure forms studied in mathematics and philosophy. Accordingly, only the abstract reasoning of these disciplines yields genuine knowledge, whereas reliance on sense perception produces vague and inconsistent opinions. They concluded that philosophical contemplation of the unseen world of forms is the highest goal of human life.

Aristotle followed Plato in regarding abstract knowledge as superior to any other, but disagreed with him as to the proper method of achieving it. Aristotle maintained that almost all knowledge is derived from experience. Knowledge is gained either directly, by abstracting the defining traits of a species, or indirectly, by deducing new facts from those already known, in accordance with the rules of logic. Careful observation and strict adherence to the rules of logic, which were first set down in systematic form by Aristotle, would help guard against the pitfalls the Sophists had exposed. The Stoic and Epicurean schools agreed with Aristotle that knowledge originates in sense perception, but against both Aristotle and Plato they maintained that philosophy is to be valued as a practical guide to life, rather than as an end in itself.

After many centuries of declining interest in rational and scientific knowledge, the Scholastic philosopher Saint Thomas Aquinas and other philosophers of the middle Ages helped to restore confidence in reason and experience, blending rational methods with faith into a unified system of beliefs. Aquinas followed Aristotle in regarding perception as the starting point and logic as the intellectual procedure for arriving at reliable knowledge of nature, but he considered faith in scriptural authority as the main source of religious belief.

From the 17th to the late 19th century, the main issue in epistemology was reasoning versus sense perception in acquiring knowledge. For the rationalists, of whom the French philosopher René Descartes, the Dutch philosopher Baruch Spinoza, and the German philosopher Gottfried Wilhelm Leibniz were the leaders, the main source and final test of knowledge was deductive reasoning based on self-evident principles, or axioms. For the empiricists, beginning with the English philosophers Francis Bacon and John Locke, the main source and final test of knowledge was sense perception.

Bacon inaugurated the new era of modern science by criticizing the medieval reliance on tradition and authority and also by setting down new rules of scientific method, including the first set of rules of inductive logic ever formulated. Locke attacked the rationalist belief that the principles of knowledge are intuitively self-evident, arguing that all knowledge is derived from experience, either from experience of the external world, which stamps sensations on the mind, or from internal experience, in which the mind reflects on its own activities. Human knowledge of external physical objects, he claimed, is always subject to the errors of the senses, and he concluded that one cannot have absolutely certain knowledge of the physical world.

Irish-born philosopher and clergyman George Berkeley (1685-1753) argued that of everything a human being conceived of exists, as an idea in a mind, a philosophical focus which is known as idealism. Berkeley reasoned that because one cannot control one's thoughts, they must come directly from a larger mind: that of God. In this excerpt from his Treatise Concerning the Principles of Human Knowledge, written in 1710, Berkeley explained why he believed that it is 'impossible . . . that there should be any such thing as an outward object'.

The Irish philosopher George Berkeley acknowledged along with Locke, that knowledge occurs through ideas, but he denied Locke's belief that a distinction can appear between ideas and objects. The British philosopher David Hume continued the empiricist tradition, but he did not accept Berkeley's conclusion that knowledge was of ideas only. He divided all knowledge into two kinds: Knowledge of relations of ideas - that is, the knowledge found in mathematics and logic, which is exact and certain but provide no information about the world. Knowledge of matters of fact - that is, the knowledge derived from sense perception. Hume argued that most knowledge of matters of fact depends upon cause and effect, and since no logical connection exists between any given cause and its effect, one cannot hope to know any future matter of fact with certainty. Thus, the most reliable laws of science might not remain true - a conclusion that had a revolutionary impact on philosophy.

The German philosopher Immanuel Kant tried to solve the crisis precipitated by Locke and brought to a climax by Hume; His proposed solution combined elements of rationalism with elements of empiricism. He agreed with the rationalists, one can have exact and certain knowledge, but he followed the empiricists in holding that such knowledge is more informative. Adding upon a proposed structure of thought than about the world outside of thought, and distinguishing upon three kinds of knowledge: Analytical deduction, which is exact and certain but uninformative, because it makes clear only what is contained in definitions; synthetic a posterior, which conveys information about the world learned from experience, but is subject to the errors of the senses; and synthetic a priori, which is discovered by pure intuition and is both exact and certain, for it expresses the necessary conditions that the mind imposes on all objects of experience. Mathematics and philosophy, according to Kant, provide this last. Since the time of Kant, one of the most frequently argued questions in philosophy has been whether or not such a thing as synthetic a priori knowledge really exists.

During the 19th century, the German philosopher Georg Wilhelm Friedrich Hegel revived the rationalist claim that absolutely certain knowledge of reality can be obtained by equating the processes of thought, of nature, and of history. Hegel inspired an interest in history and a historical approach to knowledge that was further emphasized by Herbert Spencer in Britain and by the German school of historicisms. Spencer and the French philosopher Auguste Comte brought attention to the importance of sociology as a branch of knowledge and both extended the principles of empiricism to the study of society.

The American school of pragmatism, founded by the philosophers Charles Sanders Peirce, William James, and John Dewey at the turn of this century, carried empiricism further by maintaining that knowledge is an instrument of action and that all beliefs should be judged by their usefulness as rules for predicting experiences.

In the early 20th century, epistemological problems were discussed thoroughly, and subtle shades of difference grew into rival schools of thought. Special attention was given to the relation between the act of perceiving something, the object directly perceived, and the thing that can be said to be known as a result of the perception. The phenomena lists contended that the objects of knowledge are the same as the objects perceived. The neutralists argued that one has direct perceptions of physical objects or parts of physical objects, rather than of one's own mental states. The critical realists took a middle position, holding that although one perceives only sensory data such as colours and sounds, these stand for physical objects and provide knowledge thereof.

A method for dealing with the problem of clarifying the relation between the act of knowing and the object known was developed by the German philosopher Edmund Husserl. He outlined an elaborate procedure that he called phenomenology, by which one is said to be able to distinguish the way things appear to be from the way one thinks they really are, thus gaining a more precise understanding of the conceptual foundations of knowledge.

During the second quarter of the 20th century, two schools of thought emerged, each indebted to the Austrian philosopher Ludwig Wittgenstein. The first of these schools, logical empiricism, or logical positivism, had its origins in Vienna, Austria, but it soon spread to England and the United States. The logical empiricists insisted that there is only one kind of knowledge: scientific knowledge; that any valid knowledge claim must be verifiable in experience; and hence that much that had passed for philosophy was neither true nor false but literally meaningless. Finally, following Hume and Kant, a clear distinction must be maintained between analytic and synthetic statements. The so-called verifiability criterion of meaning has undergone changes as a result of discussions among the logical empiricists themselves, as well as their critics, but has not been discarded. More recently, the sharp distinction between the analytic and the synthetic has been attacked by a number of philosophers, chiefly by American philosopher W.V.O. Quine, whose overall approach is in the pragmatic tradition.

The latter of these recent schools of thought, generally referred to as linguistic analysis, or ordinary language philosophy, seem to break with traditional epistemology. The linguistic analysts undertake to examine the actual way key epistemological terms are used - terms such as knowledge, perception, and probability - and to formulate definitive rules for their use in order to avoid verbal confusion. British philosopher John Langshaw Austin argued, for example, that to say a statement was true, and add nothing to the statement except a promise by the speaker or writer. Austin does not consider truth a quality or property attaching to statements or utterances. However, the ruling thought is that it is only through a correct appreciation of the role and point of this language is that we can come to a better conceptual understanding of what the language is about, and avoid the oversimplifications and distortion we are apt to bring to its subject matter.

Linguistics is the scientific study of language. It encompasses the description of languages, the study of their origin, and the analysis of how children acquire language and how people learn languages other than their own. Linguistics is also concerned with relationships between languages and with the ways languages change over time. Linguists may study language as a thought process and seek a theory that accounts for the universal human capacity to produce and understand language. Some linguists examine language within a cultural context. By observing talk, they try to determine what a person needs to know in order to speak appropriately in different settings, such as the workplace, among friends, or among family. Other linguists focus on what happens when speakers from different language and cultural backgrounds interact. Linguists may also concentrate on how to help people learn another language, using what they know about the learner's first language and about the language being acquired.

Although there are many ways of studying language, most approaches belong to one of the two main branches of linguistics: descriptive linguistics and comparative linguistics.

Descriptive linguistics is the study and analysis of spoken language. The techniques of descriptive linguistics were devised by German American anthropologist Franz Boas and American linguist and anthropologist Edward Sapir in the early 1900s to record and analyse Native American languages. Descriptive linguistics begins with what a linguist hears native speakers say. By listening to native speakers, the linguist gathered a body of data and analyses' it in order to identify distinctive sounds, called phonemes. Individual phonemes, such as /p/ and /b/, are established on the grounds that substitution of one for the other changes the meaning of a word. After identifying the entire inventory of sounds in a language, the linguist looks at how these sounds combine to create morphemes, or units of sound that carry meaning, such as the words push and bush. Morphemes may be individual words such as push; root words, such as the berry in a blueberry; or prefixes (pre- in preview) and suffixes (-ness in openness).

The linguist's next step is to see how morphemes combine into sentences, obeying both the dictionary meaning of the morpheme and the grammatical rules of the sentence. In the sentence "She pushed the bush," the morpheme she, a pronoun, is the subject 'push', a transitive verb, is the verb 'the', a definite article, is the determiner, and bush, a noun, is the object. Knowing the function of the morphemes in the sentence enables the linguist to describe the grammar of the language. The scientific procedures of phonemics (finding phonemes), morphology (discovering morphemes), and syntax (describing the order of morphemes and their function) provides descriptive linguists with a way to write down grammars of languages never before written down or analysed. In this way they can begin to study and understand these languages.

Comparative linguistics is the study and analysis, by means of written records, of the origins and relatedness of different languages. In 1786 Sir William Jones, a British scholar, asserted that Sanskrit, Greek, and Latins were related to each other and had descended from a common source. He based this assertion on observations of similarities in sounds and meanings among the three languages. For example, the Sanskrit word borate for "brother" resembles the Latin word frater, the Greek word phrater, (and the English word brother).

Other scholars went on to compare Icelandic with Scandinavian languages, and Germanic languages with Sanskrit, Greek, and Latin. The correspondences among languages, known as genetic relationships, came to be represented on what comparative linguists refer to as family trees. Family trees established by comparative linguists include the Indo-European, relating Sanskrit, Greek, Latin, German, English, and other Asian and European languages; the Algonquian, relating Fox, Cree, Menomini, Ojibwa, and other Native North American languages; and the Bantu, relating Swahili, Xhosa, Zulu, Kikuyu, and other African languages.

Comparative linguists also look for similarities in the way words are formed in different languages. Latin and English, for example, change the form of a word to express different meanings, as when the English verbs 'go', changes too, 'went' and 'gone' to express a past action. Chinese, on the other hand, has no such inflected forms; the verb remains the same while other words indicate the time (as in "go store tomorrow"). In Swahili, prefixes, suffixes, and infixes (additions in the body of the word) combine with a root word to change its meaning. For example, a single word might be express when something was done, by whom, to whom, and in what manner.

Some comparative linguists reconstruct hypothetical ancestral languages known as proto-languages, which they use to demonstrate relatedness among contemporary languages. A proto-language is not intended to depict a real language, however, and does not represent the speech of ancestors of people speaking modern languages. Unfortunately, some groups have mistakenly used such reconstructions in efforts to demonstrate the ancestral homeland of people.

Comparative linguists have suggested that certain basic words in a language do not change over time, because people are reluctant to introduce new words for such constants as arm, eye, or mother. These words are termed culture free. By comparing lists of culture-free words in languages within a family, linguists can derive the percentage of related words and use a formula to figure out when the languages separated from one another.

By the 1960s comparativists were no longer satisfied with focussing on origins, migrations, and the family tree method. They challenged as unrealistic the notion that an earlier language could remain sufficiently isolated for other languages to be derived exclusively from it over a period of time. Today comparativists seek to understand the more complicated reality of language history, taking language contact into account. They are concerned with universal characteristics of language and with comparisons of grammars and structures.

The field of linguistics, which lends from its own theories and methods into other disciplines, and many subfields of linguistics have expanded our understanding of languages. Linguistic theories and methods are also used in other fields of study. These overlapping interests have led to the creation of several cross-disciplinary fields.

Sociolinguistic study of patterns and variations in language within a society or community. It focuses on the way people use language to express social class, group status, gender, or ethnicity, and it looks at how they make choices about the form of language they use. It also examines the way people use language to negotiate their role in society and to achieve positions of power. For example, sociolinguistic studies have found that the way a New Yorker pronounces the phoneme /r/ in an expression such as "fourth floor" can indicate the person's social class. According to one study, people aspiring to move from the lower middle class to the upper middle class attach prestige to pronouncing /r/. Sometimes they even overcorrect their speech, pronouncing /r/ where those whom they wish to copy may not.

Some Sociolinguists believe that analysing such variables as the use of a particular phoneme can predict the direction of language change. Change, they say, moves toward the variable associated with power, prestige, or other quality having high social value. Other Sociolinguists focus on what happens when speakers of different languages interact. This approach to language change emphasizes the way languages mix rather than the direction of change within a community. The goal of a Sociolinguistical understanding, perhaps, takes a position whereby a communicative competence - what people need to know to use the appropriate language for a given social setting.

Psycholinguistics merge the fields of psychology and linguistics to study how people process language and how language use is related to underlying mental processes. Studies of children's language acquisition and of second-language acquisition are psycholinguistic in nature. Psycholinguists work to develop models for how language is processed and understood, using evidence from studies of what happens when these processes go awry. They also study language disorders such as aphasia (impairment of the ability to use or comprehend words) and dyslexia (impairment of the ability to make out written language).

Computational linguistics involves the use of computers to compile linguistic data, analyse languages, translate from one language to another, and develop and test models of language processing. Linguists use computers and large samples of actual language to analyse the relatedness and the structure of languages and to look for patterns and similarities. Computers also assist in stylistic studies, information retrieval, various forms of textual analysis, and the construction of dictionaries and concordances. Applying computers to language studies has resulted in a machine translated systems and machines that recognize and produce speech and text. Such machines facilitate communication with humans, including those who are perceptually or linguistically impaired.

Applied linguistics employs linguistic theory and methods in teaching and in research on learning a second language. Linguists look at the errors people make as they learn another language and at their strategies for communicating in the new language at different degrees of competence. In seeking to understand what happens in the mind of the learner, applied linguists recognize that motivation, attitude, learning style, and personality affect how well a person learns another language.

Anthropological linguistics, also known as linguistic anthropology, uses linguistic approaches to analyse culture. Anthropological linguists examine the relationship between a culture and its language. The way cultures and languages have moderately changed uninterruptedly through intermittent intervals of time. And how various cultures and languages are related to each other, for example, the present English usage of family and given names arose in the late 13th and early 14th centuries when the laws concerning registration, tenure, and inheritance of property were changed.

Once linguists began to study language as a set of abstract rules that somehow account for speech, other scholars began to take an interest in the field. They drew analogies between language and other forms of human behaviour, based on the belief that a shared structure underlies many aspects of a culture. Anthropologists, for example, became interested in a structuralist approach to the interpretation of kinship systems and analysis of myth and religion. American linguist Leonard Bloomfield promoted structuralism in the United States.

Saussure's ideas also influenced European linguistics, most notably in France and Czechoslovakia (now the Czech Republic). In 1926 Czech linguist Vilem Mathesius founded the Linguistic Circle of Prague, a group that expanded the focus of the field to include the context of language use. The Prague circle developed the field of phonology, or the study of sounds, and demonstrated that universal features of sounds in the languages of the world interrelate in a systematic way. Linguistic analysis, they said, should focus on the distinctiveness of sounds rather than on the ways they combine. Where descriptivist tried to locate and describe individual phonemes, such as /b/ and /p/, the Prague linguists stressed the features of these phonemes and their interrelationships in different languages. In English, for example, the voice distinguishes between the similar sounds of /b/ and /p/, but these are not distinct phonemes in a number of other languages. An Arabic speaker might pronounce the cities Pompeii and Bombay the same way.

As linguistics developed in the 20th century, the notion became prevalent that language is more than speech - specifically, that it is an abstract system of interrelationships shared by members of a speech community. Structural linguistics led linguists to look at the rules and the patterns of behaviour shared by such communities. Whereas structural linguists saw the basis of language in the social structure, other linguists looked at language as a mental process.

The 1957 publication of "Syntactic Structures" by American linguist Noam Chomsky initiated what many views as a scientific revolution in linguistics. Chomsky sought a theory that would account for both linguistic structure and the creativity of language - the fact that we can create entirely original sentences and understand sentences never before uttered. He proposed that all people have an innate ability to acquire language. The task of the linguist, he claimed, is to describe this universal human ability, known as language competence, with a grammar from which the grammars of all languages could be derived. The linguist would develop this grammar by looking at the rules children use in hearing and speaking their first language. He termed the resulting model, or grammar, a transformational-generative grammar, referring to the transformations (or rules) that create (or account for) language. Certain rules, Chomsky asserted, are shared by all languages and form part of a universal grammar, while others are language specific and associated with particular speech communities. Since the 1960s much of the development in the field of linguistics has been a reaction to or against Chomsky's theories.

At the end of the 20th century, linguists used the term grammar primarily to refer to a subconscious linguistic system that enables people to produce and comprehend an unlimited number of utterances. Grammar thus accounts for our linguistic competence. Observations about the actual language we use, or language performance, are used to theorize about this invisible mechanism known as grammar.

The scientific study of language led by Chomsky has had an impact on nongenerative linguists as well. Comparative and historically oriented linguists are looking for the various ways linguistic universals show up in individual languages. Psycholinguists, interested in language acquisition, are investigating the notion that an ideal speaker-hearer is the origin of the acquisition process. Sociolinguists are examining the rules that underlie the choice of language variants, or codes, and allow for switching from one code to another. Some linguists are studying language performance - the way people use language - to see how it reveals a cognitive ability shared by all human beings. Others seek to understand animal communication within such a framework. What mental processes enable chimpanzees to make signs and communicate with one another and how do these processes differ from those of humans?

From these initial concerns came some of the great themes of twentieth-century philosophy. How exactly does language relate to thought? Are the irredeemable problems about putative private thought? These issues are captured under the general label ‘Lingual Turn'. The subsequent development of those early twentieth-century positions has led to a bewildering heterogeneity in philosophy in the early twenty-first century. the very nature of philosophy is itself radically disputed: Analytic, continental, postmodern, critical theory, feminist t, and non-Western, are all prefixes that give a different meaning when joined to ‘philosophy'. The variety of thriving different schools, the number of professional philosophers, the proliferation of publications, the development of technology in helping research as all manifest a radically different situation to that of one hundred years ago.

As with justification and knowledge, the traditional view of content has been strongly Internalist in character. The main argument for externalism derives from the philosophy y of language, more specifically from the various phenomena pertaining to natural kind terms, indexicals, etc. that motivate the views that have come to be known as 'direct reference' theories. Such phenomena seem at least to show that the belief or thought content that can be properly attributed to a person is dependant on facts about his environment, e.g., whether he is on Earth or Twin Earth, what is fact pointing at, the classificatory criterion employed by expects in his social group, etc. - not just on what is going on internally in his mind or brain.

An objection to externalist account of content is that they seem unable to do justice to our ability to know the content of our beliefs or thought 'from the inside', simply by reflection. If content is depending on external factors pertaining to the environment, then knowledge of content should depend on knowledge of these factors - which will not in general be available to the person whose belief or thought is in question.

The adoption of an externalist account of mental content would seem to support an externalist account of justification, apart from all contentful representation is a belief inaccessible to the believer, then both the justifying statuses of other beliefs in relation to that of the same representation are the status of that content, being totally rationalized by further beliefs for which it will be similarly inaccessible. Thus, contravening the Internalist requirement for justification, as an Internalist must insist that there are no justification relations of these sorts, that our internally associable content can also not be warranted or as stated or indicated without the deviated departure from a course or procedure or from a norm or standard in showing no deviation from traditionally held methods of justification exacting by anything else: But such a response appears lame unless it is coupled with an attempt to show that the externalised account of content is mistaken.

Except for alleged cases of thing s that are evident for one just by being true, it has often been thought, anything is known must satisfy certain criteria as well as being true. Except for alleged cases of self-evident truths, it is often thought that anything that is known must satisfy certain criteria or standards. These criteria are general principles that will make a proposition evident or just make accepting it warranted to some degree. Common suggestions for this role include position ‘p', e.g., that 2 + 2 = 4, ‘p' is evident or, if ‘p' coheres wit h the bulk of one's beliefs, ‘p' is warranted. These might be criteria whereby putative self-evident truths, e.g., that one clearly and distinctly conceive s ‘p', ‘transmit' the status as evident they already have without criteria to other proposition s like ‘p', or they might be criteria whereby purely non-epistemic considerations, e.g., facts about logical connections or about conception that need not be already evident or warranted, originally ‘create' p's epistemic status. If that in turn can be ‘transmitted' to other propositions, e.g., by deduction or induction, there will be criteria specifying when it is.

Nonetheless, of or relating to tradition a being previously characterized or specified to convey an idea indirectly, as an idea or theory for consideration and being so extreme a design or quality and lean towards an ecocatorial suggestion that implicate an involving responsibility that include: (1) if a proposition ‘p', e.g., that 2 + 2 = 4, is clearly and distinctly conceived, then ‘p' is evident, or simply, (2) if we can't conceive ‘p' to be false, then ‘p' is evident: Or, (3) whenever are immediately conscious o f in thought or experience, e.g,, that we seem to see red, is evident. These might be criteria whereby putative self-evident truth s, e.g., that one clearly and distinctly conceives, e.g., that one clearly and distinctly conceives ‘p', ‘transmit' the status as evident they already have for one without criteria to other propositions like ‘p'. Alternatively, they might be criteria whereby epistemic status, e.g., p's being evident, is originally created by purely non-epistemic considerations, e.g., facts about how ‘p' is conceived which are neither self-evident is already criterial evident.

The result effect, holds that traditional criteria do not seem to make evident propositions about anything beyond our own thoughts, experiences and necessary truths, to which deductive or inductive criteria ma y be applied. Moreover, arguably, inductive criteria, including criteria warranting the best explanation of data, never make things evident or warrant their acceptance enough to count as knowledge.

Contemporary epistemologists suggest that traditional criteria may need alteration in three ways. Additional evidence may subject even our most basic judgements to rational correction, though they count as evident on the basis of our criteria. Warrant may be transmitted other than through deductive and inductive relations between propositions. Transmission criteria might not simply ‘pass' evidence on linearly from a foundation of highly evident ‘premisses' to ‘conclusions' that are never more evident.

As with justification and knowledge, the traditional view of content has been strongly Internalist in character. The main argument for externalism derives from the philosophy y of language, more specifically from the various phenomena pertaining to natural kind terms, indexicals, etc. that motivate the views that have come to be known as 'direct reference' theories. Such phenomena seem at least to show that the belief or thought content that can be properly attributed to a person is dependant on facts about his environment, e.g., whether he is on Earth or Twin Earth, what is fact pointing at, the classificatory criterion employed by expects in his social group, etc. - not just on what is going on internally in his mind or brain.

An objection to externalist account of content is that they seem unable to do justice to our ability to know the content of our beliefs or thought 'from the inside', simply by reflection. If content is depending on external factors pertaining to the environment, then knowledge of content should depend on knowledge of these factors - which will not in general be available to the person whose belief or thought is in question.

The adoption of an externalist account of mental content would seem to support an externalist account of justification, apart from all contentful representation is a belief inaccessible to the believer, then both the justifying statuses of other beliefs in relation to that of the same representation are the status of that content, being totally rationalized by further beliefs for which it will be similarly inaccessible. Thus, contravening the Internalist requirement for justification, as an Internalist must insist that there are no justification relations of these sorts, that our internally associable content can also not be warranted or as stated or indicated without the deviated departure from a course or procedure or from a norm or standard in showing no deviation from traditionally held methods of justification exacting by anything else: But such a response appears lame unless it is coupled with an attempt to show that the externalised account of content is mistaken.

Except for alleged cases of thing s that are evident for one just by being true, it has often been thought, anything is known must satisfy certain criteria as well as being true. Except for alleged cases of self-evident truths, it is often thought that anything that is known must satisfy certain criteria or standards. These criteria are general principles that will make a proposition evident or just make accepting it warranted to some degree. Common suggestions for this role include position ‘p', e.g., that 2 + 2 = 4, ‘p' is evident or, if ‘p' coheres wit h the bulk of one's beliefs, ‘p' is warranted. These might be criteria whereby putative self-evident truths, e.g., that one clearly and distinctly conceive s ‘p', ‘transmit' the status as evident they already have without criteria to other proposition s like ‘p', or they might be criteria whereby purely non-epistemic considerations, e.g., facts about logical connections or about conception that need not be already evident or warranted, originally ‘create' p's epistemic status. If that in turn can be ‘transmitted' to other propositions, e.g., by deduction or induction, there will be criteria specifying when it is.

Nonetheless, of or relating to tradition a being previously characterized or specified to convey an idea indirectly, as an idea or theory for consideration and being so extreme a design or quality and lean towards an ecocatorial suggestion that implicate an involving responsibility that include: (1) if a proposition ‘p', e.g., that 2 + 2 = 4, is clearly and distinctly conceived, then ‘p' is evident, or simply, (2) if we can't conceive ‘p' to be false, then ‘p' is evident: Or, (3) whenever are immediately conscious o f in thought or experience, e.g,, that we seem to see red, is evident. These might be criteria whereby putative self-evident truth s, e.g., that one clearly and distinctly conceives, e.g., that one clearly and distinctly conceives ‘p', ‘transmit' the status as evident they already have for one without criteria to other propositions like ‘p'. Alternatively, they might be criteria whereby epistemic status, e.g., p's being evident, is originally created by purely non-epistemic considerations, e.g., facts about how ‘p' is conceived which are neither self-evident is already criterial evident.

The result effect, holds that traditional criteria do not seem to make evident propositions about anything beyond our own thoughts, experiences and necessary truths, to which deductive or inductive criteria ma y be applied. Moreover, arguably, inductive criteria, including criteria warranting the best explanation of data, never make things evident or warrant their acceptance enough to count as knowledge.

Contemporary epistemologists suggest that traditional criteria may need alteration in three ways. Additional evidence may subject even our most basic judgements to rational correction, though they count as evident on the basis of our criteria. Warrant may be transmitted other than through deductive and inductive relations between propositions. Transmission criteria might not simply ‘pass' evidence on linearly from a foundation of highly evident ‘premisses' to ‘conclusions' that are never more evident.

A group of statements, some of which purportedly provide support for another. The statements which purportedly provide the support are the premisses while the statement purportedly support is the conclusion. Arguments are typically divided into two categories depending on the degree of support they purportedly provide. Deductive arguments purportedly provide conclusive support for their conclusions while inductively supports the purported provision that inductive arguments purportedly provided only arguments purportedly in the providing probably of support. Some, but not all, arguments succeed in providing support for their conclusions. Successful deductive arguments are valid while successful inductive arguments are valid while successful inductive arguments are strong. An argument is valid just in case if all its premisses are true its conclusion is only probably true. Deductive logic provides methods for ascertaining whether or not an argument is valid whereas, inductive logic provides methods for ascertaining the degree of support the premisses of an argument confer on its conclusion.

Finally, proof, least of mention, is a collection of considerations and reasons that instill and sustain conviction that some proposed theorem-the theorem proved-is not only true, but could not possibly be false. A perceptual observation may instill the conviction that water is cold. But a proof that 2 + 5 = 5 must not only instill the conviction that is true that 2 + 3 = 5, but also that 2 + 3 could not be anything but the digit 5.

Contemporary philosophers of mind have typically supposed (or at least hoped) that the mind can be naturalized -, i.e., that all mental facts have explanations in the terms of natural science. This assumption is shared within cognitive science, which attempts to provide accounts of mental states and processes in terms (ultimately) of features of the brain and central nervous system. In the course of doing so, the various sub-disciplines of cognitive science (including cognitive and computational psychology and cognitive and computational neuroscience) postulate a number of different kinds of structures and processes, many of which are not directly implicated by mental states and processes as commonsensical conceived. There remains, however, a shared commitment to the idea that mental states and processes are to be explained in terms of mental representations.

In philosophy, recent debates about mental representation have centred around the existence of propositional attitudes (beliefs, desires, etc.) and the determination of their contents (how they come to be about what they are about), and the existence of phenomenal properties and their relation to the content of thought and perceptual experience. Within cognitive science itself, the philosophically relevant debates have been focussed on the computational architecture of the brain and central nervous system, and the compatibility of scientific and commonsense accounts of mentality.

Intentional Realists such as Dretske (e.g., 1988) and Fodor (e.g., 1987) note that the generalizations we apply in everyday life in predicting and explaining each other's behaviour (often collectively referred to as 'folk psychology') are both remarkably successful and indispensable. What a person believes, doubts, desires, fears, etc. is a highly reliable indicator of what that person will do. We have no other way of making sense of each other's behaviour than by ascribing such states and applying the relevant generalizations. We are thus committed to the basic truth of commonsense psychology and, hence, to the existence of the states its generalizations refer to. (Some realists, such as Fodor, also hold that commonsense psychology will be vindicated by cognitive science, given that propositional attitudes can be construed as computational relations to mental representations.)

Intentional Eliminativists, such as Churchland, (perhaps) Dennett and (at one time) Stich argue that no such things as propositional attitudes (and their constituent representational states) are implicated by the successful explanation and prediction of our mental lives and behaviour. Churchland denies that the generalizations of commonsense propositional-attitude psychology are true. He (1981) argues that folk psychology is a theory of the mind with a long history of failure and decline, and that it resists incorporation into the framework of modern scientific theories (including cognitive psychology). As such, it is comparable to alchemy and phlogiston theory, and ought to suffer a comparable fate. Commonsense psychology is false, and the states (and representations) it postulates simply don't exist. (It should be noted that Churchland is not an eliminativist about mental representation tout court.

Dennett (1987) grants that the generalizations of commonsense psychology are true and indispensable, but denies that this is sufficient reason to believe in the entities they appear to refer to. He argues that to give an intentional explanation of a system's behaviour is merely to adopt the 'intentional stance' toward it. If the strategy of assigning contentful states to a system and predicting and explaining its behaviour (on the assumption that it is rational -, i.e., that it behaves as it should, given the propositional attitudes it should have in its environment) is successful, then the system is intentional, and the propositional-attitude generalizations we apply to it are true. But there is nothing more to having a propositional attitude than this.

Though he has been taken to be thus claiming that intentional explanations should be construed instrumentally, Dennett (1991) insists that he is a 'moderate' realist about propositional attitudes, since he believes that the patterns in the behaviour and behavioural dispositions of a system on the basis of which we (truly) attribute intentional states to it are objectively real. In the event that there are two or more explanatorily adequate but substantially different systems of intentional ascriptions to an individual, however, Dennett claims there is no fact of the matter about what the system believes (1987, 1991). This does suggest an irrealism at least with respect to the sorts of things Fodor and Dretske take beliefs to be; though it is not the view that there is simply nothing in the world that makes intentional explanations true.

(Davidson 1973, 1974 and Lewis 1974 also defend the view that what it is to have a propositional attitude is just to be interpretable in a particular way. It is, however, not entirely clear whether they intend their views to imply irrealism about propositional attitudes.). Stich (1983) argues that cognitive psychology does not (or, in any case, should not) taxonomize mental states by their semantic properties at all, since attribution of psychological states by content is sensitive to factors that render it problematic in the context of a scientific psychology. Cognitive psychology seeks causal explanations of behaviour and cognition, and the causal powers of a mental state are determined by its intrinsic 'structural' or 'syntactic' properties. The semantic properties of a mental state, however, are determined by its extrinsic properties -, e.g., its history, environmental or intra-mental relations. Hence, such properties cannot figure in causal-scientific explanations of behaviour. (Fodor 1994 and Dretske 1988 are realist attempts to come to grips with some of these problems.) Stich proposes a syntactic theory of the mind, on which the semantic properties of mental states play no explanatory role.

It is a traditional assumption among realists about mental representations that representational states come in two basic varieties (Boghossian 1995). There are those, such as thoughts, which are composed of concepts and have no phenomenal ('what-it's-like') features ('Qualia'), and those, such as sensory experiences, which have phenomenal features but no conceptual constituents. (Non-conceptual content is usually defined as a kind of content that states of a creature lacking concepts but, nonetheless enjoy. On this taxonomy, mental states can represent either in a way analogous to expressions of natural languages or in a way analogous to drawings, paintings, maps or photographs. (Perceptual states such as seeing that something is blue, are sometimes thought of as hybrid states, consisting of, for example, a Non-conceptual sensory experience and a thought, or some more integrated compound of sensory and conceptual components.)

Some historical discussions of the representational properties of mind (e.g., Aristotle 1984, Locke 1689/1975, Hume 1739/1978) seem to assume that Non-conceptual representations - percepts ('impressions'), images ('ideas') and the like - are the only kinds of mental representations, and that the mind represents the world in virtue of being in states that resemble things in it. On such a view, all representational states have their content in virtue of their phenomenal features. Powerful arguments, however, focussing on the lack of generality (Berkeley 1975), ambiguity (Wittgenstein 1953) and non-compositionality (Fodor 1981) of sensory and imagistic representations, as well as their unsuitability to function as logical (Frége 1918/1997, Geach 1957) or mathematical (Frége 1884/1953) concepts, and the symmetry of resemblance (Goodman 1976), convinced philosophers that no theory of mind can get by with only Non-conceptual representations construed in this way.

Contemporary disagreement over Non-conceptual representation concerns the existence and nature of phenomenal properties and the role they play in determining the content of sensory experience. Dennett (1988), for example, denies that there are such things as Qualia at all; while Brandom (2002), McDowell (1994), Rey (1991) and Sellars (1956) deny that they are needed to explain the content of sensory experience. Among those who accept that experiences have phenomenal content, some (Dretske, Lycan, Tye) argue that it is reducible to a kind of intentional content, while others (Block, Loar, Peacocke) argue that it is irreducible.

The representationalist thesis is often formulated as the claim that phenomenal properties are representational or intentional. However, this formulation is ambiguous between a reductive and a non-deductive claim (though the term 'representationalism' is most often used for the reductive claim). On one hand, it could mean that the phenomenal content of an experience is a kind of intentional content (the properties it represents). On the other, it could mean that the (irreducible) phenomenal properties of an experience determine an intentional content. Representationalists such as Dretske, Lycan and Tye would assent to the former claim, whereas phenomenalists such as Block, Chalmers, Loar and Peacocke would assent to the latter. (Among phenomenalists, there is further disagreement about whether Qualia are intrinsically representational (Loar) or not (Block, Peacocke).

Most (reductive) representationalists are motivated by the conviction that one or another naturalistic explanation of intentionality is, in broad outline, correct, and by the desire to complete the naturalization of the mental by applying such theories to the problem of phenomenality. (Needless to say, most phenomenalists (Chalmers is the major exception) are just as eager to naturalize the phenomenal - though not in the same way.)

The main argument for representationalism appeals to the transparency of experience. The properties that characterize what it's like to have a perceptual experience are presented in experience as properties of objects perceived: in attending to an experience, one seems to 'see through it' to the objects and properties it is experiences of. They are not presented as properties of the experience itself. If nonetheless they were properties of the experience, perception would be massively deceptive. But perception is not massively deceptive. According to the representationalist, the phenomenal character of an experience is due to its representing objective, non-experiential properties. (In veridical perception, these properties are locally instantiated; in illusion and hallucination, they are not.) On this view, introspection is indirect perception: one comes to know what phenomenal features one's experience has by coming to know what objective features it represents.

In order to account for the intuitive differences between conceptual and sensory representations, representationalists appeal to their structural or functional differences. Dretske (1995), for example, distinguishes experiences and thoughts on the basis of the origin and nature of their functions: an experience of a property 'P' is a state of a system whose evolved function is to indicate the presence of 'P' in the environment; a thought representing the property 'P', on the other hand, is a state of a system whose assigned (learned) function is to calibrate the output of the experiential system. Rey (1991) takes both thoughts and experiences to be relations to sentences in the language of thought, and distinguishes them on the basis of (the functional roles of) such sentences' constituent predicates. Lycan (1987, 1996) distinguishes them in terms of their functional-computational profiles. Tye (2000) distinguishes them in terms of their functional roles and the intrinsic structure of their vehicles: thoughts are representations in a language-like medium, whereas experiences are image-like representations consisting of 'symbol-filled arrays.' (The account of mental images in Tye 1991.)

Phenomenalists tend to make use of the same sorts of features (function, intrinsic structure) in explaining some of the intuitive differences between thoughts and experiences; but they do not suppose that such features exhaust the differences between phenomenal and non-phenomenal representations. For the phenomenalist, it is the phenomenal properties of experiences - Qualia themselves - that constitute the fundamental difference between experience and thought. Peacocke (1992), for example, develops the notion of a perceptual 'scenario' (an assignment of phenomenal properties to coordinates of a three-dimensional egocentric space), whose content is 'correct' (a semantic property) if in the corresponding 'scene' (the portion of the external world represented by the scenario) properties are distributed as their phenomenal analogues are in the scenario.

Another sort of representation championed by phenomenalists (e.g., Block, Chalmers (2003) and Loar (1996)) is the 'phenomenal concept' -, a conceptual/phenomenal hybrid consisting of a phenomenological 'sample' (an image or an occurrent sensation) integrated with (or functioning as) a conceptual component. Phenomenal concepts are postulated to account for the apparent fact (among others) that, as McGinn (1991) puts it, 'you cannot form [introspective] concepts of conscious properties unless you yourself instantiate those properties.' One cannot have a phenomenal concept of a phenomenal property 'P', and, hence, phenomenal beliefs about P, without having experience of 'P', because 'P' itself is (in some way) constitutive of the concept of 'P'. (Jackson 1982, 1986 and Nagel 1974.)

Though imagery has played an important role in the history of philosophy of mind, the important contemporary literature on it is primarily psychological. In a series of psychological experiments done in the 1970s (summarized in Kosslyn 1980 and Shepard and Cooper 1982), subjects' response time in tasks involving mental manipulation and examination of presented figures was found to vary in proportion to the spatial properties (size, orientation, etc.) of the figures presented. The question of how these experimental results are to be explained has kindled a lively debate on the nature of imagery and imagination.

Kosslyn (1980) claims that the results suggest that the tasks were accomplished via the examination and manipulation of mental representations that they have spatial properties, i.e., pictorial representations, or images. Others, principally Pylyshyn (1979, 1981, 2003), argue that the empirical facts can be explained in terms exclusively of discursive, or propositional representations and cognitive processes defined over them. (Pylyshyn takes such representations to be sentences in a language of thought.)

The idea that pictorial representations are literally pictures in the head is not taken seriously by proponents of the pictorial view of imagery. The claim is, rather, that mental images represent in a way that is relevantly like the way pictures represent. (Attention has been focussed on visual imagery - hence the designation 'pictorial'; Though of course, there may imagery in other modalities - auditory, olfactory, etc. - as well.)

The distinction between pictorial and discursive representation can be characterized in terms of the distinction between analog and digital representation (Goodman 1976). This distinction has itself been variously understood (Fodor & Pylyshyn 1981, Goodman 1976, Haugeland 1981, Lewis 1971, McGinn 1989), though a widely accepted construal is that analog representation is continuous (i.e., in virtue of continuously variable properties of the representation), while digital representation is discrete (i.e., in virtue of properties a representation either has or doesn't have) (Dretske 1981). (An analog/digital distinction may also be made with respect to cognitive processes. (Block 1983.)) On this understanding of the analog/digital distinction, imagistic representations, which represent in virtue of properties that may vary continuously (such for being more or less bright, loud, vivid, etc.), would be analog, while conceptual representations, whose properties do not vary continuously (a thought cannot be more or less about Elvis: either it is or it is not) would be digital.

It might be supposed that the pictorial/discursive distinction is best made in terms of the phenomenal/nonphenomenal distinction, but it is not obvious that this is the case. For one thing, there may be nonphenomenal properties of representations that vary continuously. Moreover, there are ways of understanding pictorial representation that presuppose neither phenomenality nor analogicity. According to Kosslyn (1980, 1982, 1983), a mental representation is 'quasi-pictorial' when every part of the representation corresponds to a part of the object represented, and relative distances between parts of the object represented are preserved among the parts of the representation. But distances between parts of a representation can be defined functionally rather than spatially - for example, in terms of the number of discrete computational steps required to combine stored information about them. (Rey 1981.)

Tye (1991) proposes a view of images on which they are hybrid representations, consisting both of pictorial and discursive elements. On Tye's account, images are '(labelled) interpreted symbol-filled arrays.' The symbols represent discursively, while their arrangement in arrays has representational significance (the location of each 'cell' in the array represents a specific viewer-centred 2-D location on the surface of the imagined object)

The contents of mental representations are typically taken to be abstract objects (properties, relations, propositions, sets, etc.). A pressing question, especially for the naturalist, is how mental representations come to have their contents. Here the issue is not how to naturalize content (abstract objects can't be naturalized), but, rather, how to provide a naturalistic account of the content-determining relations between mental representations and the abstract objects they express. There are two basic types of contemporary naturalistic theories of content-determination, causal-informational and functional.

Causal-informational theories hold that the content of a mental representation is grounded in the information it carries about what does (Devitt 1996) or would (Fodor 1987, 1990) cause it to occur. There is, however, widespread agreement that causal-informational relations are not sufficient to determine the content of mental representations. Such relations are common, but representation is not. Tree trunks, smoke, thermostats and ringing telephones carry information about what they are causally related to, but they do not represent (in the relevant sense) what they carry information about. Further, a representation can be caused by something it does not represent, and can represent something that has not caused it.

The main attempts to specify what makes a causal-informational state a mental representation are Asymmetric Dependency Theories, the Asymmetric Dependency Theory distinguishes merely informational relations from representational relations on the basis of their higher-order relations to each other: informational relations depend upon representational relations, but not vice-versa. For example, if tokens of a mental state type are reliably caused by horses, cows-on-dark-nights, zebras-in-the-mist and Great Danes, then they carry information about horses, etc. If, however, such tokens are caused by cows-on-dark-nights, etc. because they were caused by horses, but not vice versa, then they represent horses.

According to Teleological Theories, representational relations are those a representation-producing mechanism has the selected (by evolution or learning) function of establishing. For example, zebra-caused horse-representations do not mean zebra, because the mechanism by which such tokens are produced has the selected function of indicating horses, not zebras. The horse-representation-producing mechanism that responds to zebras is malfunctioning.

Functional theories, hold that the content of a mental representation are well grounded in causal computational inferential relations to other mental portrayals other than mental representations. They differ on whether relata should include all other mental representations or only some of them, and on whether to include external states of affairs. The view that the content of a mental representation is determined by its inferential/computational relations with all other representations is holism; the view it is determined by relations to only some other mental states is localisms (or molecularism). (The view that the content of a mental state depends on none of its relations to other mental states is atomism.) Functional theories that recognize no content-determining external relata have been called solipsistic (Harman 1987). Some theorists posit distinct roles for internal and external connections, the former determining semantic properties analogous to sense, the latter determining semantic properties analogous to reference (McGinn 1982, Sterelny 1989)

(Reductive) representationalists (Dretske, Lycan, Tye) usually take one or another of these theories to provide an explanation of the (Non-conceptual) content of experiential states. They thus tend to be Externalists, about phenomenological as well as conceptual content. Phenomenalists and non-deductive representationalists (Block, Chalmers, Loar, Peacocke, Siewert), on the other hand, take it that the representational content of such states is (at least in part) determined by their intrinsic phenomenal properties. Further, those who advocate a phenomenology-based approach to conceptual content (Horgan and Tiensen, Loar, Pitt, Searle, Siewert) also seem to be committed to Internalist individuation of the content (if not the reference) of such states.

Generally, those who, like informational theorists, think relations to one's (natural or social) environment are (at least partially) determinative of the content of mental representations are Externalists (e.g., Burge 1979, 1986, McGinn 1977, Putnam 1975), whereas those who, like some proponents of functional theories, think representational content is determined by an individual's intrinsic properties alone, are internalists (or individualists).

This issue is widely taken to be of central importance, since psychological explanation, whether commonsense or scientific, is supposed to be both causal and content-based. (Beliefs and desires cause the behaviours they do because they have the contents they do. For example, the desire that one have a beer and the beliefs that there is beer in the refrigerator and that the refrigerator is in the kitchen may explain one's getting up and going to the kitchen.) If, however, a mental representation's having a particular content is due to factors extrinsic to it, it is unclear how its having that content could determine its causal powers, which, arguably, must be intrinsic. Some who accept the standard arguments for externalism have argued that internal factors determine a component of the content of a mental representation. They say that mental representations have both 'narrow' content (determined by intrinsic factors) and 'wide' or 'broad' content (determined by narrow content plus extrinsic factors). (This distinction may be applied to the sub-personal representations of cognitive science as well as to those of commonsense psychology.

Narrow content has been variously construed. Putnam (1975), Fodor (1982)), and Block (1986) for example, seems to understand it as something like dedictorial content (i.e., Frégean sense, or perhaps character, à la Kaplan 1989). On this construal, narrow content is context-independent and directly expressible. Fodor (1987) and Block (1986), however, has also characterized narrow content as radically inexpressible. On this construal, narrow content is a kind of proto-content, or content-determinant, and can be specified only indirectly, via specifications of context/wide-content pairings. Both, construe of as a narrow content and are characterized as functions from context to (wide) content. The narrow content of a representation is determined by properties intrinsic to it or its possessor such as its syntactic structure or its intra-mental computational or inferential role or its phenomenology.

Burge (1986) has argued that causation-based worries about externalist individuation of psychological content, and the introduction of the narrow notion, are misguided. Fodor (1994, 1998) has more recently urged that there may be no need to narrow its contentual representations, accountable for reasons of an ordering supply of naturalistic (causal) explanations of human cognition and action, since the sorts of cases they were introduced to handle, viz., Twin-Earth cases and Frége cases, are nomologically either impossible or dismissible as exceptions to non-strict psychological laws.

The leading contemporary version of the Representational Theory of Mind, the Computational Theory of Mind, claims that the brain is a kind of computer and that mental processes are computations. According to the computational theory of mind, cognitive states are constituted by computational relations to mental representations of various kinds, and cognitive processes are sequences of such states. The computational theory of mind and the representational theory of mind, may by attempting to explain all psychological states and processes in terms of mental representation. In the course of constructing detailed empirical theories of human and animal cognition and developing models of cognitive processes' implementable in artificial information processing systems, cognitive scientists have proposed a variety of types of mental representations. While some of these may be suited to be mental relata of commonsense psychological states, some - so-called 'subpersonal' or 'sub-doxastic' representations - are not. Though many philosophers believe that computational theory of mind can provide the best scientific explanations of cognition and behaviour, there is disagreement over whether such explanations will vindicate the commonsense psychological explanations of prescientific representational theory of mind.

According to Stich's (1983) Syntactic Theory of Mind, for example, computational theories of psychological states should concern themselves only with the formal properties of the objects those states are relations to. Commitment to the explanatory relevance of content, however, is for most cognitive scientists fundamental. That mental processes are computations, which computations are rule-governed sequences of semantically evaluable objects, and that the rules apply to the symbols in virtue of their content, are central tenets of mainstream cognitive science.

Explanations in cognitive science appeal to a many different kinds of mental representation, including, for example, the 'mental models' of Johnson-Laird 1983, the 'retinal arrays,' 'primal sketches' and '2½ -D sketches' of Marr 1982, the 'frames' of Minsky 1974, the 'sub-symbolic' structures of Smolensky 1989, the 'quasi-pictures' of Kosslyn 1980, and the 'interpreted symbol-filled arrays' of Tye 1991 - in addition to representations that may be appropriate to the explanation of commonsense

Psychological states. Computational explanations have been offered of, among other mental phenomena, belief.

The classicists hold that mental representations are symbolic structures, which typically have semantically evaluable constituents, and that mental processes are rule-governed manipulations of them that are sensitive to their constituent structure. The connectionists, hold that mental representations are realized by patterns of activation in a network of simple processors ('nodes') and that mental processes consist of the spreading activation of such patterns. The nodes themselves are, typically, not taken to be semantically evaluable; nor do the patterns have semantically evaluable constituents. (Though there are versions of Connectionism -, 'localist' versions - on which individual nodes are taken to have semantic properties (e.g., Ballard 1986, Ballard & Hayes 1984).) It is arguable, however, that localist theories are neither definitive nor representative of the Conceptionist program.

Classicists are motivated (in part) by properties thought seems to share with language. Jerry Alan Fodor's (1935-), Language of Thought Hypothesis, (Fodor 1975, 1987), according to which the system of mental symbols constituting the neural basis of thought is structured like a language, provides a well-worked-out version of the classical approach as applied to commonsense psychology. According to the language of a thought hypothesis, the potential infinity of complex representational mental states is generated from a finite stock of primitive representational states, in accordance with recursive formation rules. This combinatorial structure accounts for the properties of productivity and systematicity of the system of mental representations. As in the case of symbolic languages, including natural languages (though Fodor does not suppose either that the language of thought hypotheses explains only linguistic capacities or that only verbal creatures have this sort of cognitive architecture), these properties of thought are explained by appeal to the content of the representational units and their combinability into contentful complexes. That is, the semantics of both language and thought is compositional: the content of a complex representation is determined by the contents of its constituents and their structural configuration.

Connectionists are motivated mainly by a consideration of the architecture of the brain, which apparently consists of layered networks of interconnected neurons. They argue that this sort of architecture is unsuited to carrying out classical serial computations. For one thing, processing in the brain is typically massively parallel. In addition, the elements whose manipulation drive's computation in Conceptionist networks (principally, the connections between nodes) are neither semantically compositional nor semantically evaluable, as they are on the classical approach. This contrast with classical computationalism is often characterized by saying that representation is, with respect to computation, distributed as opposed to local: representation is local if it is computationally basic; and distributed if it is not. (Another way of putting this is to say that for classicists mental representations are computationally atomic, whereas for connectionists they are not.)

Moreover, connectionists argue that information processing as it occurs in Conceptionist networks more closely resembles some features of actual human cognitive functioning. For example, whereas on the classical view learning involves something like hypothesis formation and testing (Fodor 1981), on the Conceptionist model it is a matter of evolving distribution of 'weight' (strength) on the connections between nodes, and typically does not involve the formulation of hypotheses regarding the identity conditions for the objects of knowledge. The Conceptionist network is 'trained up' by repeated exposure to the objects it is to learn to distinguish; and, though networks typically require many more exposures to the objects than do humans, this seems to model at least one feature of this type of human learning quite well.

Further, degradation in the performance of such networks in response to damage is gradual, not sudden as in the case of a classical information processor, and hence more accurately models the loss of human cognitive function as it typically occurs in response to brain damage. It is also sometimes claimed that Conceptionist systems show the kind of flexibility in response to novel situations typical of human cognition - situations in which classical systems are relatively 'brittle' or 'fragile.'

Some philosophers have maintained that Connectionism entails that there are no propositional attitudes. Ramsey, Stich and Garon (1990) have argued that if Conceptionist models of cognition are basically correct, then there are no discrete representational states as conceived in ordinary commonsense psychology and classical cognitive science. Others, however (e.g., Smolensky 1989), hold that certain types of higher-level patterns of activity in a neural network may be roughly identified with the representational states of commonsense psychology. Still others argue that language-of-thought style representation is both necessary in general and realizable within Conceptionist architectures, collect the central contemporary papers in the classicist/Conceptionist debate, and provides useful introductory material as well.

Whereas Stich (1983) accepts that mental processes are computational, but denies that computations are sequences of mental representations, others accept the notion of mental representation, but deny that computational theory of mind provides the correct account of mental states and processes.

Van Gelder (1995) denies that psychological processes are computational. He argues that cognitive systems are dynamic, and that cognitive states are not relations to mental symbols, but quantifiable states of a complex system consisting of (in the case of human beings) a nervous system, a body and the environment in which they are embedded. Cognitive processes are not rule-governed sequences of discrete symbolic states, but continuous, evolving total states of dynamic systems determined by continuous, simultaneous and mutually determining states of the systems components. Representation in a dynamic system is essentially information-theoretic, though the bearers of information are not symbols, but state variables or parameters.

Horst (1996), on the other hand, argues that though computational models may be useful in scientific psychology, they are of no help in achieving a philosophical understanding of the intentionality of commonsense mental states. Computational theory of mind attempts to reduce the intentionality of such states to the intentionality of the mental symbols they are relations to. But, Horst claims, the relevant notion of symbolic content is essentially bound up with the notions of convention and intention. So the computational theory of mind involves itself in a vicious circularity: the very properties that are supposed to be reduced are (tacitly) appealed to in the reduction.

To say that a mental object has semantic properties is, paradigmatically, to say that it may be about, or be true or false of, an object or objects, or that it may be true or false simpliciter. Suppose I think that you took to sniffing snuff. I am thinking about you, and if what I think of you (that they take snuff) is true of you, then my thought is true. According to representational theory of mind such states are to be explained as relations between agents and mental representations. To think that you take snuff is to token in some way a mental representation whose content is that ocelots take snuff. On this view, the semantic properties of mental states are the semantic properties of the representations they are relations to.

Linguistic acts seem to share such properties with mental states. Suppose I say that you take snuff. I am talking about you, and if what I say of you (that they take snuff) is true of them, then my utterance is true. Now, to say that you take snuff is (in part) to utter a sentence that means that you take snuff. Many philosophers have thought that the semantic properties of linguistic expressions are inherited from the intentional mental states they are conventionally used to express. On this view, the semantic properties of linguistic expressions are the semantic properties of the representations that are the mental relata of the states they are conventionally used to express.

It is also widely held that in addition to having such properties as reference, truth-conditions and truth - so-called extensional properties - expressions of natural languages also have intensional properties, in virtue of expressing properties or propositions - i.e., in virtue of having meanings or senses, where two expressions may have the same reference, truth-conditions or truth value, yet express different properties or propositions (Frége 1892/1997). If the semantic properties of natural-language expressions are inherited from the thoughts and concepts they express (or vice versa, or both), then an analogous distinction may be appropriate for mental representations.

Theories of representational content may be classified according to whether they are atomistic or holistic and according to whether they are externalistic or internalistic, whereby, emphasizing the priority of a whole over its parts. Furthermore, in the philosophy of language, this becomes the claim that the meaning of an individual word or sentence can only be understood in terms of its relation to an indefinitely larger body of language, such as à whole theory, or even a whole language or form of life. In the philosophy of mind a mental state similarly may be identified only in terms of its relations with others. Moderate holism may allow the other things besides these relationships also count; extreme holism would hold that a network of relationships is all that we have. A holistic view of science holds that experience only confirms or disconfirms large bodies of doctrine, impinging at the edges, and leaving some leeway over the adjustment that it requires.

Once, again, in the philosophy of mind and language, the view that what is thought, or said, or experienced, is essentially dependent on aspects of the world external to the mind of the subject. The view goes beyond holding that such mental states are typically caused by external factors, to insist that they could not have existed as they now do without the subject being embedded in an external world of a certain kind. It is these external relations that make up the essence or identify of the mental state. Externalism is thus opposed to the Cartesian separation of the mental from the physical, since that holds that the mental could in principle exist as it does even if there were no external world at all. Various external factors have been advanced as ones on which mental content depends, including the usage of experts, the linguistic, norms of the community. And the general causal relationships of the subject. In the theory of knowledge, externalism is the view that a person might know something by being suitably situated with respect to it, without that relationship being in any sense within his purview. The person might, for example, be very reliable in some respect without believing that he is. The view allows that you can know without being justified in believing that you know.

However, atomistic theories take a representation's content to be something that can be specified independent entity of that representation' s relations to other representations. What the American philosopher of mind, Jerry Alan Fodor (1935-) calls the crude causal theory, for example, takes a representation to be a
cow
- a menial representation with the same content as the word 'cow' - if its tokens are caused by instantiations of the property of being-a-cow, and this is a condition that places no explicit constraints on how
cow
's must or might relate to other representations. Holistic theories contrasted with atomistic theories in taking the relations à representation bears to others to be essential to its content. According to functional role theories, a representation is a
cow
if it behaves like a
cow
should behave in inference.

Internalist theories take the content of a representation to be a matter determined by factors internal to the system that uses it. Thus, what Block (1986) calls 'short-armed' functional role theories are Internalist. Externalist theories take the content of a representation to be determined, in part at least, by factors external to the system that uses it. Covariance theories, as well as telelogical theories that invoke an historical theory of functions, take content to be determined by 'external' factors. Crossing the atomist-holistic distinction with the Internalist-externalist distinction.

Externalist theories (sometimes called non-individualistic theories) have the consequence that molecule for molecule identical cognitive systems might yet harbour representations with different contents. This has given rise to a controversy concerning 'narrow' content. If we assume some form of externalist theory is correct, then content is, in the first instance 'wide' content, i.e., determined in part by factors external to the representing system. On the other hand, it seems clear that, on plausible assumptions about how to individuate psychological capacities, internally equivalent systems must have the same psychological capacities. Hence, it would appear that wide content cannot be relevant to characterizing psychological equivalence. Since cognitive science generally assumes that content is relevant to characterizing psychological equivalence, philosophers attracted to externalist theories of content have sometimes attempted to introduce 'narrow' content, i.e., an aspect or kind of content that is equivalent internally equivalent systems. The simplest such theory is Fodor's idea (1987) that narrow content is a function from contents (i.e., from whatever the external factors are) to wide contents.

All the same, what a person expresses by a sentence is often a function of the environment in which he or she is placed. For example, the disease I refer to by the term like 'arthritis', or the kind of tree I refer to as a 'Maple' will be defined by criteria of which I know next to nothing. This raises the possibility of imagining two persons in rather different environments, but in which everything appears the same to each of them. The wide content of their thoughts and sayings will be different if the situation surrounding them is appropriately different: 'situation' may include the actual objects they perceive or the chemical or physical kinds of object in the world they inhabit, or the history of their words, or the decisions of authorities on what counts as an example, of one of the terms they use. The narrow content is that part of their thought which remains identical, through their identity of the way things appear, regardless of these differences of surroundings. Partisans of wide content may doubt whether any content in this sense narrow, partisans of narrow content believer that it is the fundamental notion, with wide content being explicable in terms of narrow content plus context.

Even so, the distinction between facts and values has outgrown its name: it applies not only to matters of fact vs, matters of value, but also to statements that something is, vs. statements that something ought to be. Roughly, factual statements - 'is statements' in the relevant sense - represent some state of affairs as obtaining, whereas normative statements - evaluative, and deontic ones - attribute goodness to something, or ascribe, to an agent, an obligation to act. Neither distinction is merely linguistic. Specifying a book's monetary value is making a factual statement, though it attributes a kind of value. 'That is a good book' expresses a value judgement though the term 'value' is absent (nor would 'valuable' be synonymous with 'good'). Similarly, 'we are morally obligated to fight' superficially expresses a statement, and 'By all indications it ough to rain' makes a kind of ought-claim; but the former is an ought-statement, the latter an (epistemic) is-statement.

Theoretical difficulties also beset the distinction. Some have absorbed values into facts holding that all value is instrumental, roughly, to have value is to contribute - in a factual analysable way - to something further which is (say) deemed desirable. Others have suffused facts with values, arguing that facts (and observations) are 'theory-impregnated' and contending that values are inescapable to theoretical choice. But while some philosophers doubt that fact/value distinctions can be sustained, there persists a sense of a deep difference between evaluating, and attributing an obligation and, on the other hand, saying how the world is.

Fact/value distinctions, may be defended by appeal to the notion of intrinsic value, as a thing has in itself and thus independently of its consequences. Roughly, a value statement (proper) is an ascription of intrinsic value, one to the effect that a thing is to some degree good in itself. This leaves open whether ought-statements are implicitly value statements, but even if they imply that something has intrinsic value - e.g., moral value - they can be independently characterized, say by appeal to rules that provide (justifying) reasons for action. One might also ground the fact value distinction in the attributional (or even motivational) component apparently implied by the making of valuational or deontic judgements: Thus, 'it is a good book, but that is no reason for a positive attribute towards it' and 'you ought to do it, but there is no reason to' seem inadmissible, whereas, substituting, 'an expensive book' and 'you will do it' yields permissible judgements. One might also argue that factual judgements are the kind which are in principle appraisable scientifically, and thereby anchor the distinction on the factual side. This ligne is plausible, but there is controversy over whether scientific procedures are 'value-free' in the required way.

Philosophers differ regarding the sense, if any, in which epistemology is normative (roughly, valuational). But what precisely is at stake in this controversy is no clearly than the problematic fact/value distinction itself. Must epistemologists as such make judgements of value or epistemic responsibility? If epistemology is naturalizable, then even epistemic principles simply articulate under what conditions - say, appropriate perceptual stimulations - a belief is justified, or constitutes knowledge. Its standards of justification, then would be like standards of, e.g., resilience for bridges. It is not obvious, however, that there appropriate standards can be established without independent judgements that, say, a certain kind of evidence is good enough for justified belief (or knowledge). The most plausible view may be that justification is like intrinsic goodness, though it supervenes on natural properties, it cannot be analysed wholly in factual statements.

Thus far, belief has been depicted as being all-or-nothing, however, as a resulting causality for which we have grounds for thinking it true, and, all the same, its acceptance is governed by epistemic norms, and, least of mention, it is partially subject to voluntary control and has functional affinities to belief. Still, the notion of acceptance, like that of degrees of belief, merely extends the standard picture, and does not replace it.

Traditionally, belief has been of epistemological interest in its propositional guise: 'S' believes that 'p', where 'p' is a reposition towards which an agent, 'S' exhibits an attitude of acceptance. Not all belief is of this sort. If I trust you to say, I believer you. And someone may believe in Mr. Radek, or in a free-market economy, or in God. It is sometimes supposed that all belief is 'reducible' to propositional belief, belief-that. Thus, my believing you might be thought a matter of my believing, is, perhaps, that what you say is true, and your belief in free markets or God, is a matter of your believing that free-market economies are desirable or that God exists.

Some philosophers have followed St. Thomas Aquinas (1225-74), in supposing that to believer in God is simply to believer that certain truths hold while others argue that belief-in is a distinctive attitude, on that includes essentially an element of trust. More commonly, belief-in has been taken to involve a combination of propositional belief together with some further attitude.

The moral philosopher Richard Price (1723-91) defends the claim that there are different sorts of belief-in, some, but not all reducible to beliefs-that. If you believer in God, you believer that God exists, that God is good, you believer that God is good, etc. But according to Price, your belief involves, in addition, a certain complex pro-attitude toward its object. Even so, belief-in outruns the evidence for the corresponding belief-that. Does this diminish its rationality? If belief-in presupposes believes-that, it might be thought that the evidential standards for the former must be, at least, as high as standards for the latter. And any additional pro-attitude might be thought to require a further layer of justification not required for cases of belief-that.

Belief-in may be, in general, less susceptible to alternations in the face of unfavourable evidence than belief-that. A believer who encounters evidence against God's existence may remain unshaken in his belief, in part because the evidence does not bear on his pro-attitude. So long as this ids united with his belief that God exists, and reasonably so - in a way that an ordinary propositional belief that would not.

The correlative way of elaborating on the general objection to justificatory externalism challenges the sufficiency of the various externalist conditions by citing cases where those conditions are satisfied, but where the believers in question seem intuitively not to be justified. In this context, the most widely discussed examples have to do with possible occult cognitive capacities, like clairvoyance. Considering the point in application once, again, to reliabilism, the claim is that to think that he has such a cognitive power, and, perhaps, even good reasons to the contrary, is not rational or responsible and therefore not epistemically justified in accepting the belief that result from his clairvoyance, despite the fact that the reliablist condition is satisfied.

One sort of response to this latter sorts of an objection is to 'bite the bullet' and insist that such believers are in fact justified, dismissing the seeming intuitions to the contrary as latent Internalist prejudice. A more widely adopted response attempts to impose additional conditions, usually of a roughly Internalist sort, which will rule out the offending example, while stopping far of a full internalism. But, while there is little doubt that such modified versions of externalism can handle particular cases, as well enough to avoid clear intuitive implausibility, the usually problematic cases that they cannot handle, and also whether there is and clear motivation for the additional requirements other than the general Internalist view of justification that externalist is committed to reject.

A view in this same general vein, one that might be described as a hybrid of internalism and externalism holds that epistemic justification requires that there is a justicatory factor that is cognitively accessible to the believer in question (though it need not be actually grasped), thus ruling out, e.g., a pure reliabilism. At the same time, however, though it must be objectively true that beliefs for which such a factor is available are likely to be true, in addition, the fact need not be in any way grasped or cognitively accessible to the believer. In effect, of the premises needed to argue that a particular belief is likely to be true, one must be accessible in a way that would satisfy at least weak internalism, the Internalist will respond that this hybrid view is of no help at all in meeting the objection and has no belief nor is it held in the rational, responsible way that justification intuitively seems to require, for the believer in question, lacking one crucial premise, still has no reason at all for thinking that his belief is likely to be true.

An alternative to giving an externalist account of epistemic justification, one which may be more defensible while still accommodating many of the same motivating concerns, is to give an externalist account of knowledge directly, without relying on an intermediate account of justification. Such a view will obviously have to reject the justified true belief account of knowledge, holding instead that knowledge is true belief which satisfies the chosen externalist condition, e.g., a result of a reliable process (and perhaps, further conditions as well). This makes it possible for such a view to retain Internalist account of epistemic justification, though the centrality of that concept to epistemology would obviously be seriously diminished.

Such an externalist account of knowledge can accommodate the commonsense conviction that animals, young children, and unsophisticated adults' posse's knowledge, though not the weaker conviction (if such a conviction does exist) that such individuals are epistemically justified in their beliefs. It is, at least, less vulnerable to Internalist counter-examples of the sort discussed, since the intuitions involved there pertain more clearly to justification than to knowledge. What is uncertain is what ultimate philosophical significance the resulting conception of knowledge, for which is accepted or advanced as true or real on the basis of less than conclusive evidence, as can only be assumed to have. In particular, does it have any serious bearing on traditional epistemological problems and on the deepest and most troubling versions of scepticism, which seems in fact to be primarily concerned with justification, and knowledge?`

A rather different use of the terms 'internalism' and 'externalism' have to do with the issue of how the content of beliefs and thoughts is determined: According to an Internalist view of content, the content of such intention states depends only on the non-relational, internal properties of the individual's mind or grain, and not at all on his physical and social environment: While according to an externalist view, content is significantly affected by such external factors and suggests a view that appears of both internal and external elements are standardly classified as an external view.

As with justification and knowledge, the traditional view of content has been strongly Internalist in character. The main argument for externalism derives from the philosophy y of language, more specifically from the various phenomena pertaining to natural kind terms, indexicals, etc. that motivate the views that have come to be known as 'direct reference' theories. Such phenomena seem at least to show that the belief or thought content that can be properly attributed to a person is dependant on facts about his environment, e.g., whether he is on Earth or Twin Earth, what is fact pointing at, the classificatory criterion employed by expects in his social group, etc. - not just on what is going on internally in his mind or brain.

An objection to externalist account of content is that they seem unable to do justice to our ability to know the content of our beliefs or thought 'from the inside', simply by reflection. If content is depending on external factors pertaining to the environment, then knowledge of content should depend on knowledge of these factors - which will not in general be available to the person whose belief or thought is in question.

The adoption of an externalist account of mental content would seem to support an externalist account of justification, apart from all contentful representation is a belief inaccessible to the believer, then both the justifying statuses of other beliefs in relation to that of the same representation are the status of that content, being totally rationalized by further beliefs for which it will be similarly inaccessible. Thus, contravening the Internalist requirement for justification, as an Internalist must insist that there are no justification relations of these sorts, that our internally associable content can also not be warranted or as stated or indicated without the deviated departure from a course or procedure or from a norm or standard in showing no deviation from traditionally held methods of justification exacting by anything else: But such a response appears lame unless it is coupled with an attempt to show that the externalised account of content is mistaken.

Except for alleged cases of thing s that are evident for one just by being true, it has often been thought, anything is known must satisfy certain criteria as well as being true. Except for alleged cases of self-evident truths, it is often thought that anything that is known must satisfy certain criteria or standards. These criteria are general principles that will make a proposition evident or just make accepting it warranted to some degree. Common suggestions for this role include position ‘p', e.g., that 2 + 2 = 4, ‘p' is evident or, if ‘p' coheres wit h the bulk of one's beliefs, ‘p' is warranted. These might be criteria whereby putative self-evident truths, e.g., that one clearly and distinctly conceive s ‘p', ‘transmit' the status as evident they already have without criteria to other proposition s like ‘p', or they might be criteria whereby purely non-epistemic considerations, e.g., facts about logical connections or about conception that need not be already evident or warranted, originally ‘create' p's epistemic status. If that in turn can be ‘transmitted' to other propositions, e.g., by deduction or induction, there will be criteria specifying when it is.

Nonetheless, of or relating to tradition a being previously characterized or specified to convey an idea indirectly, as an idea or theory for consideration and being so extreme a design or quality and lean towards an ecocatorial suggestion that implicate an involving responsibility that include: (1) if a proposition ‘p', e.g., that 2 + 2 = 4, is clearly and distinctly conceived, then ‘p' is evident, or simply, (2) if we can't conceive ‘p' to be false, then ‘p' is evident: Or, (3) whenever are immediately conscious o f in thought or experience, e.g,, that we seem to see red, is evident. These might be criteria whereby putative self-evident truth s, e.g., that one clearly and distinctly conceives, e.g., that one clearly and distinctly conceives ‘p', ‘transmit' the status as evident they already have for one without criteria to other propositions like ‘p'. Alternatively, they might be criteria whereby epistemic status, e.g., p's being evident, is originally created by purely non-epistemic considerations, e.g., facts about how ‘p' is conceived which are neither self-evident is already criterial evident.

The result effect, holds that traditional criteria do not seem to make evident propositions about anything beyond our own thoughts, experiences and necessary truths, to which deductive or inductive criteria ma y be applied. Moreover, arguably, inductive criteria, including criteria warranting the best explanation of data, never make things evident or warrant their acceptance enough to count as knowledge.

Contemporary epistemologists suggest that traditional criteria may need alteration in three ways. Additional evidence may subject even our most basic judgements to rational correction, though they count as evident on the basis of our criteria. Warrant may be transmitted other than through deductive and inductive relations between propositions. Transmission criteria might not simply ‘pass' evidence on linearly from a foundation of highly evident ‘premisses' to ‘conclusions' that are never more evident.

A group of statements, some of which purportedly provide support for another. The statements which purportedly provide the support are the premisses while the statement purportedly support is the conclusion. Arguments are typically divided into two categories depending on the degree of support they purportedly provide. Deductive arguments purportedly provide conclusive support for their conclusions while inductively supports the purported provision that inductive arguments purportedly provided only arguments purportedly in the providing probably of support. Some, but not all, arguments succeed in providing support for their conclusions. Successful deductive arguments are valid while successful inductive arguments are valid while successful inductive arguments are strong. An argument is valid just in case if all its premisses are true its conclusion is only probably true. Deductive logic provides methods for ascertaining whether or not an argument is valid whereas, inductive logic provides methods for ascertaining the degree of support the premisses of an argument confer on its conclusion.

Finally, proof, least of mention, is a collection of considerations and reasonings that instill and sustain conviction that some proposed theorem-the theorem proved-is not only true, but could not possibly be false. A perceptual observation may instill the conviction that water is cold. But a proof that 2 + 5 = 5 must not only instill the conviction that is true that 2 + 3 = 5, but also that 2 + 3 could not be anything but the digit 5.

No one has succeeded in replacing this largely psychological characterization of proofs by a more objective characterization. The representations of reconstructions of proofs as mechanical and semiotical derivation in formal-logical systems all but completely fail to capture ‘proofs' as mathematicians are quite content to give them. For example, formal-logical derivations depend solely on the logical form of the considered proposition, whereas usually proofs depend in large measure on content of propositions other than their logical form

No one has succeeded in replacing this largely psychological characterization of proofs by a more objective characterization. The representations of reconstructions of proofs as mechanical and semiotical derivation in formal-logical systems all but completely fail to capture ‘proofs' as mathematicians are quite content to give them, fas or example, formal-logical .





EVOLVING PRINCIPLES OF THOUGHT



THE HUMAN CONDITION

BOOK TWO





A period of geologic time is an intermittent unit of time that geologists use to divide the earth's history. On the geologic time scale, a period is longer than an epoch and shorter than an era. The earth is about 4.5 billions of year's old. Earth scientists divide its age into shorter blocks of time. The largest of these are eons, of which there are three in the earth's history. The last eon is formally divided into eras, which are made up of periods. Many periods are divided into epochs. Geological or biological events mark the beginnings and ends of some periods, but some are based on a convenient interval of time determined by radiometric dating.

Geologists divide much of the earth's history into periods. Too little is accedingly obscure about pre-Archaean time, from the origin of the earth to 3.8 billion years ago, to divide it into units. The Archaean Eon: 3.8 to 2.5 billion years before present) is not divided into periods. It marks a time in which the structure of the earth underwent many changes and the first life appeared on the earth. Rocks of the Archaean Eon contain some very simple single-cell organisms called Prokaryotes and early blue-green algae colonies called stromatolites. During the Proterozoic Eon (2.5 billion to 570 million years before present) the earth was partially covered alternately by shallow seas and ice sheets. Life advanced from the most basic single-celled organisms into plants and mysteriously to animals that resembled some species that is actively living today. Pre-Archaean time, the Archaean Eon, and the Proterozoic Eon make up what is called Precambrian time. The most recent eon of the earth is the Phanerozoic (570 million years before present to the present). During this eon, the earth and life on it gradually changed to their present state.

Some scientists divide the Proterozoic Eon into four eras and at least ten periods. These divisions are not universally accepted. The eras defined for the Proterozoic is the Huronian Era (2.5 billion to 2.2 billion years before present), the Animikean Era (2.2 billion to 1.65 billion years before present), the Riphean Era (1.65 billion to 800 million years before present), and the Sinian Era (800 million to 570 million years before present). The four informally recognized periods of the Huronian Era are the Elliot Lake Period, the Hough Lake Period, the Quirke Lake Period, and the Cobalt Period, from oldest too youngest. These periods correspond only to deposits in a region of Canada around Lake Superior and have no definite time correlation. A comparison of rocks of the Elliot Lake and Cobalt periods show that oxygen levels in the atmosphere rose during the Huronian Era. The Hough Lake, Quirke Lake, and Cobalt periods all begin with times of glaciation.

The Animikean Era has only one informally acceptant amount of time, in that of a time called the Gunflint Period, which lasted from about 2.2 billion years before present to about two billion years before present. Rocks of the Gunflint Period contain many species of microbes and stromatolites.

The Riphean Era has three informal periods. The oldest period is the Burzian Period (1.65 billion to 1.35 billion years before present), followed by the Yurmatin Period (1.35 billion to 1.05 billion years before present) and then the Karatau Period (from 1.05 billion to 800 million years before present). All three are named from sedimentary rocks in a section of the southern Ural Mountains in Russia.

The Sinian Era is divided into two informal geologic periods-the Sturtian Periods (from 800 million to 610 million years before present) and the Vendian Period (610 million to 570 million years awaiting the presence to the future). The Sturtian is named from rocks in southern Australia that show two distinct glacial episodes. The Vendian is named from rocks in the southern Ural Mountains. The Vendian Period is divided into two epochs, the Varanger Epoch (about 610 million to 590 million years awaiting the presence to the future) and the Ediacara Epoch (590 million to 570 million years awaiting the presence to the future). Rocks from the Ediacara Epoch show the first fossils of complex organisms.

The Phanerozoic Eon is the most recent eon of the earth and is divided into the Paleozoic Era (570 million to 240 million years awaiting the presence to the future), the Mesozoic Era (240 million to 65 million years awaiting the presence to the future), and the Cenozoic Era (65 million years awaiting the presence to the future to the present).

The periods of the Paleozoic Era are the Cambrian Period (570 million to 500 million years awaiting the presence to the future), the Ordovician Period (500 million to 435 million years awaiting the presence to the future), the Silurian Period (435 million to 410 million years awaiting the presence to the future), the Devonian Period (410 million to 360 million years awaiting the presence to the future), the Carboniferous Period (360 million to 290 million years awaiting the presence to the future), and the Permian Period (290 million to 240 million years awaiting). The rocks of the Paleozoic Era contain abundant and diverse fossils, so each period is marked by both geologic and biological events.

The rocks of the Cambrian Period contain many fossils of shelled animals such as trilobites, gastropods, and brachiopods that are not present in earlier rocks. The Ordovician Period is characterized by an abundance of extinct floating marine organisms called graptolites. One of the greatest mass extinctions of the Phanerozoic Eon occurred at the end of the Ordovician Period.

Rocks from the Silurian Period reveal the first evidence of plants and insects on land and the first fossils of fishes with jaws. In the Devonian Period, the first animals with backbones appeared on land. The Devonian was the first period to produce the substantial organic deposits that are used today as energy sources.

The rocks of the Carboniferous Period contain about one-half of the world's coal supplies, created by the remains of the vast population of animals and plants of that period. Besides the abundance of terrestrial vegetation, the first winged insects appeared during the Carboniferous.

During the Permian Period all the continents on the earth came together to form one landmass, called Pangaea. The shallow inland seas of the Permian created an environment in which invertebrate marine life flourished. At the end of the period, one of the greatest extinctions in the earth's history occurred, wiping out most species on the planet.

The Mesozoic Era is composed of the Triassic Period (240 million to 205 million years awaiting the presence to the future), the Jurassic Period (205 million to 138 million years awaiting the presence to the future), and the Cretaceous Period (138 million to 65 million years awaiting the presence to the future). During the Triassic Period, the super-continent of Pangea began to break apart. Dinosaurs first appeared during the Triassic, as did the earliest mammals.

The continents continued to break apart during the Jurassic Period. Reptiles, including the dinosaurs, flourished, taking over ecological niches on the land, in the sea, and in the air, while mammals remained small and rodent like. The continents continued to drift toward their present locations during the Cretaceous Period. Another mass extinction, which killed off the large reptiles such as the dinosaurs, occurred near the end of the Cretaceous.

The Cenozoic Era is divided into the Tertiary Period (65 million to 1.6 million years awaiting the present) and the Quaternary Period (1.6 million years awaiting the presence to the future to the present). During the Tertiary Period the continents assumed their current positions. Mammals became the dominant life forms on the planet during this period, and the direct ancestors of humans appeared at the end of the Tertiary. The most recent ice age occurred during the Quaternary Period. The first humans appeared during the Quaternary. The changing climate and melting of the glaciers, possibly combined with hunting by humans, drove many large mammals of the early Quaternary to extinction, making way for the animal life on the earth today.

The Precambrian is a time span that includes the Archaean and Proterozoic eons that heed of shape as practically as four billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth's history that can still be studied. Notably little outlasted of Earth from the period of 4.6 billion to about four billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.

The Precambrian is also the longest chapter in Earth's history, spanning a period of about 3.5 billion years. During this time frame, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet because of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapour. As Earth continued to cool, the water vapour condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth's water vapour originally came from comets containing frozen water that struck Earth during meteorite bombardment.

By studying 2-billion-year-old rocks found in northwestern Canada, and 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth's surface as early as the middle Precambrian. About a billion years ago, the Earth's plates were entered around the South Pole and formed a super-continent called Rodinia. Slowly, pieces of this super-continent broke away from the central continent and travelled north, forming smaller continents.

Life originated during the Precambrian. The earliest fossil evidence of life consists of Prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote's genetic heritage that had been productively unaltered. The first Prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but of an unequivocal practicality of some 3.5 billion years ago, and where anaerobic-that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.

Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old. As, some rocks found in the Gunflint Chert region of northwest Lake Superior extend over an age of about 2.1 billion years old.

For billions of years, life existed only in the simple form of Prokaryotes. Prokaryotes were referentially followed by an advanced eucaryote, organisms that have a nucleus in their cells and that reproduces by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna, these were first discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequent findings in many locations throughout the world, these strange fossils might be the precursors of many fossil groups that were to explode in Earth's oceans in the Paleozoic Era.

At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Most of all set-groups of animals known today made their initial arrival during the Cambrian explosion. Most of the different ‘body plans' are found in animals today-that is, the way and animal's body is designed, with heads, legs, rear ends, claws, tentacles, or antennae-also originated during this period.

Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species could flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.

Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the super-continent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built many numbers of mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.

Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. They are slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the super-continent Pangaea and resulted within several grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented more than 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouth-like opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today's Mediterranean Sea.

The Paleozoic Ere spread an end with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, resulting among the fragmented contributive factors.

The Mesozoic Era, sprang into formation and are approximately 248 million years ago, is often characterized as the Age of Reptiles because reptiles were the dominant life forms during this era. Reptiles dominated not only on land, as dinosaurs, but also in the sea, as the plesiosaurs and ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles.

The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth's history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.

Mesozoic was an eventful era geologically with many changes to Earth's surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subduction zone extended along North America's western margin. This Subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. Abounding of this focus, called the American Cordillera, exists today as the eastern margin of the Pacific Ring of Fire.

During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.

Birds are members of a group of animals called vertebrates, which possess a spinal column or backbone. Other vertebrates are fish, amphibians, reptiles, and mammals. Many characteristics and behaviours of birds are distinct from all other animals, yet they have noticeable similarities. Like mammals, birds have four-chambered hearts and are warm-blooded-having a proportionally constant body temperature that enables them to live in a variety of environments. Like reptiles, birds develop from embryos in eggs outside the mother's body.

Birds are found worldwide in many habitats. They can fly over some of the highest mountains on earth plus both of the earth's poles, dive through water to depths of more than 250 m.'s (850 ft.), and occupy habitats with the most extreme climates on the planet, including arctic tundra and the Sahara Desert. Certain kinds of seabirds are commonly seen over the open ocean thousands of kilometres from the nearest land, but all birds must come ashore to raise their young.

Highly-developed animals, birds are sensitive and responsive, colourful and graceful, with habits that excite interest and inquiry. People have long been fascinated by birds, in part because birds are found in great abundance and variety in the same habitats in which humans thrive. Like people, most species of birds are active during daylight hours. Humans find inspiration in birds' capacity for flight and in their musical calls. Humans also find birds useful-their flesh and eggs for food, their feathers for warmth, and their companionship. Perhaps a key basis for our rapport with birds is the similarity of our sensory worlds: Both birds and humans rely more heavily on hearing and colour vision than on smell. Birds are useful indicators of the quality of the environment, because the health of bird populations mirrors the health of our environment. The rapid declination of bird populations and the accelerating extinction rates of birds in the world's forests, grasslands, wetlands, and islands are therefore reasons for great concern.

Birds vary in size from the tiny bee hummingbird, which measures about 57 mm. (about 2.25 in.) from a beak tip to tail tips and weigh 1.6 g. (0.06 oz.), to the ostrich, which stands 2.7 m. (9 ft.) tall and weighs up to 156 kg. (345 lb.). The heaviest flying bird is the great bustard, which can weigh up to 18 kg. (40 lb.).

All birds are covered with feathers, collectively called plumage, which are specialized structures of the epidermis, or outer layer of skin. The main component of feathers is keratin, a flexible protein that also forms the hair and fingernails of mammals. Feathers provide the strong yet lightweight surface area needed for powered, aerodynamic flight. They also serve as insulation, trapping pockets of air to help birds conserve their body heat. The varied patterns, colours, textures, and shapes of feathers help birds to signal their age, sex, social status, and species that identity of one another. Some birds have plumage that blends in with their surroundings to provide camouflage, helping these birds escape notice by their predators. Birds use their beaks to preen their feathers, often using oil from a gland at the base of their tails. Preening removes dirt and parasites and keeps feathers waterproof and supple. Because feathers are nonliving structures that cannot repair themselves when bigeneric or broken, and they must be renewed periodically. Most adult birds molt-lose and replace their feathers-at least once a year.

Bird wings are highly modified forelimbs with a skeletal structure resembling that of arms. Wings may be long or short, round or pointed. The shape of a bird's wings influences its style of flight, which may consist of gliding, soaring, or flapping. Wings are powered by flight muscles, which are the largest muscles in birds that fly. Flight muscles are found in the chest and are attached to the wings by large tendons. The breastbone, a large bone shaped like the keel of a boat, supports the flight muscles.

Nearly all birds have a tail, which helps them control the direction in which they fly and play a role in landing. The paired flight feathers of the tail, called retrices, extend from the margins of a bird's tail. Smaller feathers called coverts' lie on top of the retrices. Tails may be square, rounded, pointed, or forked, depending on the lengths of the retrices and the way they end. The shapes of bird tails vary more than the shapes of wings, possibly because tail shape is less critical to flight than wing shape. Many male birds, such as pheasants, have ornamental tails that they use to attract mating partners.

Birds have two legs; the lower part of each leg is called the tarsus. Most birds had four toes on each foot, and in many birds, including all songbirds, the first toe, called a hallux, points backwards. Bird toes are adapted in various species for grasping perches, climbing, swimming, capturing prey, and carrying and manipulating food.

Instead of heavy jaws with teeth, modern birds have toothless, lightweight jaws, called beaks or bills. Unlike humans or other mammals, birds can move their upper jaws independently of the rest of their heads. This helps them to open their mouths extremely wide. Beaks occur in a wide range of shapes and sizes, depending on the type of food a bird eats.

The eyes of birds are large and provide excellent vision. They are protected by three eyelids: An upper lid resembling that of humans, a lower lid that closes when a bird sleeps, and a third lid, called a nictitating membrane, that sweeps across the eye sideways, starting from the side near the beak. This lid is a thin, translucent fold of skin that moistens and cleans the eye and protects it from wind and bright light.

The ears of birds are completely internal, with openings placed just behind and below the eyes. In most birds, textured feathers called auriculars form a protective screen that prevents objects from entering the ear. Birds rely on their ears for hearing and for balance, which is especially critical during flight. Two groups of birds, cave swiftlets and oilbirds, find their way in dark places by echolocation-making clicks or rattle calls and interpreting the returning echoes to obtain clues about their environment.

The throats of nearly all birds contain a syrinx (plural, syringes), an organ that is comparable to the voice box of mammals. The syrinx has two membranes that produce sound when they vibrate. Birds classified as songbirds have a peculiarly greater extent-developed syringe. Some songbirds, such as the wood thrush, can control each membrane independently; in this way they can sing two songs simultaneously.

Birds have well-developed brains, which provide acute sensory perception, keen balance and coordination, and instinctive behaviours, along with a surprising degree of intelligence. Parts of the bird brain that are especially developed are the optic lobes, where nerve impulses from the eyes are processed, and the cerebellum, which coordinates muscle actions. The cerebral cortex, the part of the brain responsible for thought in humans, is primitive in birds. However, birds have a hyperstriatum -a forebrain component that mammals lack. This part of the brain helps songbirds to learn their songs, and scientists believe that it may also be the source of bird intelligence.

The internal body parts of all birds, including flightless ones, reflects the evolution of birds as flying creatures. Birds have lightweight skeletons in which many major bones are hollow. A unique feature of birds is the furculum, or wishbone, which is comparable to the collarbones of humans, although in birds the left and right portions are fused. The furculum absorbs the shock of wing motion and acts as a spring to help birds breathe while they fly. Several anical adaptations help to reduce weight and concentrate it near the centre of gravity. For example, modern birds are toothless, which helps reduce the weight of their beaks, and food grinding is carried out in the muscular gizzard, a part of the stomach near the body's core. The egg-laying habit of birds enables their young to develop outside the body of the female, significantly lightening her load. For further weight reduction, the reproductive organs of birds atrophy, or become greatly reduced in size, except the breeding season.

Flight, especially taking off and landing, requires a huge amount of energy-more than humans need even for running. Taking flight is less demanding for small birds than it is for large ones, but small birds need more energy to stay warm. In keeping with their enormous energy needs, birds have an extremely fast metabolism, which includes the chemical reactions involved in releasing stored energy from food. The high body temperature of birds-40° to 42° C.'s (104° to about 108° F.'s)-provides an environment that supports rapid chemical reactions.

To sustain this high-speed metabolism, birds need an abundant supply of oxygen, which combines with food molecules within cells to release energy. The respiratory, or breathing, system of birds is adapted to meet their special needs. Unlike humans, birds have lungs with an opening at each end. New air entered the lungs from one end, and used air goes out the other end. The lungs are connected to a series of air sacs, which simplify the movement of air. Birds breathe faster than any other animal. For example, a flying pigeon breathes 450 times each minute, whereas a human, when running, might breathe only about 30 times each minute.

The circulatory system of birds also functions at high speed. Blood vessels pick up oxygen in the lungs and carry it, along with nutrients and other substances essential to life, to all of a bird's body tissues. In contrast to the human heart, which beats about 160 times per minute when a person runs, a small bird's heart beats between 400 and 1,000 times per minute. The hearts of birds are proportionately larger than the hearts of other animals. Birds that migrate and those that live at high altitudes have larger hearts, compared with their body size, than other birds.

The characteristic means locomotion in birds is flight. However, birds are also variously adapted for movement on land, and some are excellent swimmers and divers.

Like aeroplanes, birds rely on lift-an upward force that counters gravity-to fly. Birds generate lift by pushing down on the air with their wings. This action causes the air, in return, to push the wings up. The shape of wings, which have an upper surface that is convex and a lower surface that is concave, contributes to this effect. To turn, birds often tilt so that one wing is higher than the other.

Different wing shapes adapt birds for different styles of flight. The short, rounded wings and strong breast muscles of quail are ideal for short bursts of powered flight. Conversely, the albatross's long narrow wings enable these birds to soar effortlessly over windswept ocean surfaces. The long, broad wings of storks, vultures, and eagles provide excellent lift on rising air currents.

Feathers play a crucial role in flight. The wings and tails of birds have detailed Flight feathers-the largest and strongest type of feathers-that contribute to lift. Because each of the flight feathers is connected to a muscle, birds can adjust their position individually. As a bird pushes down on the air with its wings, its flight feathers overlap to prevent air from passing through. The same feathers twist open on the upstroke, so that air flows between them and less effort is needed to lift the wings.

Feathers also help to reduce drag, a force of resistance that acts on solid bodies moving through air. Contour feathers, which are the most abundant type of feather, fill and cover angular parts of a bird's body, giving birds a smooth, aerodynamic form.

Bird tails are also important to flight. Birds tip their tail feathers in different directions to achieve stability and to help change direction while flying. When soaring, birds spread their tail feathers to obtain more lift. When landing, birds turn their tails downward, so that their tails act like brakes.

Most birds can move their legs alternately to walk and run, and some birds are adept at climbing trees. Birds' agility on land varies widely among different species. The American robin both hops and walks, while the starling usually walks. The ostrich can run as fast as

64 km./h. (40 mph.). Swifts, however, can neither hop nor run; their weak feet are useful only for clinging to vertical surfaces, such as the walls of caves and houses.

Birds that walk in shallow water, such as herons and stilts, have long legs that simplify wading. Jacanas, which walk on lily pads and mud, have long toes and nails that disperse their weight to help prevent them from sinking. Penguins have stubby legs placed far back from their centre of gravity. So, they can walk only with an upright posture and a short-stepping gait. When penguins need to move quickly, they ‘toboggan' on their bellies, propelling themselves across ice with their wings and feet.

Many birds are excellent swimmers and divers, including such distantly related types of birds as grebes, loons, ducks, auks, cormorants, penguins, and diving petrels. Most of these birds have webbed or lobed toes that act as paddles, which they use to propel themselves underwater. Others, including auks and penguins, use their wings to propel themselves through the water. Swimming birds have broad, raft-like bodies that provide stability. They have dense feather coverings that hold pockets of air for warmth, but they can compress the air out of these pockets to reduce buoyancy when diving.

Many fish-catching birds can dive to great depths, either from the air or from the water's surface. The emperor penguin can plunge into depths of more than 250 m. (850 ft.) and remain submerged for about 12 minutes. Some ducks, swans, and geese perform an action called dabbling, in which they tip their tails up and reach down with their beaks to forage on the mud beneath shallow water.

Like other animals, birds must eat, rest, and defend themselves against predators to survive. They must also reproduce and raise their young to contribute to the survival of their species. For many bird species, migration is an essential part of survival. Birds have acquired remarkably diverse and effective strategies for achieving these ends.

Birds spend much of their time feeding and searching for food. Most birds cannot store large reserves of food internally, because the extra weight would prevent them from flying. Small birds need to eat even more frequently than large ones, because they have a greater surface area in proportion to their weight and therefore lose their body heat more quickly. Some extremely small birds, such as hummingbirds, have so little food in reserve that they enter a state resembling hibernation during the night and rely on the warmth of the sun to energize them in the morning.

Depending on the species, birds eat insects, fish, meat, seeds, nectar, and fruit. Most birds are either carnivorous, meaning they eat other animals, or herbivorous, meaning they eat plant material. Many birds, including crows and gulls, are omnivorous, eating almost anything. Many herbivorous birds feed protein-rich animal material to their undergoing maturation. Some bird species have highly abstractive diets, such as the Everglade kite, which feeds exclusively on snails.

Two unusual internal organs help birds to process food. The gizzard, which is part of a bird's stomach, has thick muscular walls with hard inner ridges. It can crush large seeds and even shellfish. Some seed-eating birds swallow small stones so that the gizzard will grind food more efficiently. Birds that feed on nectar and soft fruits have poorly developed gizzards.

Most birds have a crop-a sac-like extension of the esophagus, the tubular organ through which food passes after leaving the mouth. Some birds store food in their crops and transport it to the place where they sleep. Others use the crop to carry food that they will later regurgitate to their offspring.

The bills of birds are modified in ways that help birds obtain and handle food. Nectar-feeders, such as hummingbirds, have long thin bills, which they insert into flowers, and particularized expansible or brushlike tongues, through which they draw up nectar. Meat-eating birds, including hawks, owls, and shrikes, have strong, hooked bills that can tear flesh. Many fish-eating birds, such as merganser ducks, have tooth-like ridges on their bills that help them to hold their slippery prey. The thick bills and strong jaw muscle of various finches and sparrows are ideal for crushing seeds. Woodpeckers use their bills as chisels, working into dead or living wood to find insect larvae and excavate nest cavities.

At least two species of birds use tools in obtaining food. One is the woodpecker finch, which uses twigs or leaf stalks to extract insects from narrow crevices in trees. The other is the Egyptian vulture, which picks up large stones in its bill and throws them at ostrich eggs to crack them open.

Birds need far less sleep than humans do. Birds probably sleep to relax their muscles and conserve energy but not to refresh their brains. Many seabirds, in particular, sleep very little. For example, the sooty tern, which rarely plummet into settling on water, may fly for several years with only brief periods of sleep lasting a few seconds each. Flying is so effortless for the sooty tern and other seabirds that it takes virtually no energy at all.

Most birds are active during the day and sleep at night. Exceptions are birds that hunt at night, such as owls and night jars. Birds use nests for sleeping only during the breeding season. The rest of the year, birds sleep in shrubs, on tree branches, in holes in trees, and on the bare ground. Most ducks sleep on the water. Many birds stand while they sleep, and some birds sleep while perched on some branch-sometimes using only one foot. These birds can avoid falling over because of a muscle arrangement that causes their claws to tighten when they bend their legs to relax.

To reproduce, birds must find a suitable mate, or mates, and the necessary resources-food, water, and nesting materials-for caring for their eggs and raising the hatched young to independence. Most birds mate during a specific season in a particular habitat, although some birds may reproduce in varied places and seasons, provided environmental conditions are suitable.

Most of all birds have monogamous mating patterns, meaning that one male and one female mate exclusively with each other for at least one season. However, some bird species is either polygynous, that is, the males mate with more than one female, or polyandrous, in which case the females mate with more than one male. Among many types of birds, including some jays, several adults, rather than a single breeding pair, often help to raise the young within an individual nest.

Birds rely heavily on their two main senses, vision and hearing, in courtship and breeding. Among most songbirds, including the nightingale and the sky lark, males use song to establish breeding territories and attract mates. In many species, female songbirds may be attracted to males that sing the loudest, longest, or most varied songs. Many birds, including starlings, mimic the sounds of other birds. This may help males to achieve sufficiently varied songs to attract females.

Many birds rely on visual displays of their feathers to obtain a mating partner. For example, the blue bird of paradise hangs upside down from a tree branch to show off the dazzling feathers of its body and tail. A remarkable courtship strategy is exhibited by male bowerbirds of Australia and New Guinea. These birds attract females by building bowers for shelter, which they decorate with colourful objects such as flower petals, feathers, fruit, and even human-made items such as ribbons and tinfoil.

Among some grouse, cotingas, the small wading birds called shorebirds, hummingbirds, and other groups, males gather in areas called leks to attract mates through vocal and visual displays. Females visiting the leks select particularly impressive males, and often only one or a very few males effectively mate. Among western grebes, both males and females participate in a dramatic courtship ritual called rushing, in which mating partners lift their upper bodies far above the water and paddle rapidly to race side by side over the water's surface. Although male birds usually court females, there are some types of birds, including the phalaropes, in which females court males.

Many birds establish breeding territories, which they defend from rivals of the same species. In areas where suitably nesting habitats is limited, birds may nest in large colonies. An example is the crab plover, which sometimes congregates by the thousands in areas of only about 0.6 hectares (about 1.5 acres).

For breeding, most birds build nests, which help them to incubate, or warm, the developing eggs. Nests sometimes offer camouflage from predators and physical protection from the elements. Nests may be elaborate constructions or some mere scrapes on the ground. Some birds, including many shorebirds, incubate their eggs without any type of nest at all. The male emperor penguin of icy Antarctica incubates the single egg on top of its feet under a fold of skin.

Bird nests range in size from the tiny cups of hummingbirds to the huge stick nests of eagles, which may weigh a ton or more. Some birds, such as the mallee-fowl of southern Australia, use external heat sources, such as decaying plant material, to incubate their eggs. Many birds, including woodpeckers, use tree cavities for nests. Others, such as cowbirds and cuckoos, are brood parasites; they neither build nests nor care for their young. Instead, females of these species lay their eggs in the nests of birds of other species, so that the eggs are incubated-and hatchling duration's bobbed up to raised birds, in so, that they and other hatchling chicks are than the hatchlings' unfeigned by the same rearing nest.

Incubation by one or both parents works with the nest structure to provide an ideal environment for the eggs. The attending parent may warm the eggs with a part of its belly called the brood patch. Bird parents may also wet or shade the eggs to prevent them from overheating.

The size, shape, colour, and texture of a bird egg are specific to each species. Eggs provide an ideal environment for the developing embryo. The shells of eggs are made from calcium carbonates. They contain thousands of pores through which water can evaporate and air can seep in, enabling the developing embryo to breathe. The number of eggs in a clutch (the egg or eggs laid by a female bird in one nesting effort) may be 15 or more for some birds, including pheasants. In contrast, some large birds, such as condors and albatross, may lay only a single egg every two years. The eggs of many songbirds hatch after developing for as few as ten days, whereas those of an albatross and kiwis may require 80 days or more.

Among some birds, including songbirds and pelicans, newly hatched younkers that are without feathers, blind, and incapable of regulating their body temperature. Many other birds, such as ducks, are born covered with down and can feed themselves within hours after hatching. Depending on the species, young birds may remain in the nest for as little as part of a day or as long as several months. Grown older from their young (those that has left the nest) may still rely on parental care for many days or weeks. Only about 10 percent of birds survive their first year of life; the rest die of starvation, disease, predators, or inexperience with the behaviours necessary for survival. The age at which birds begin to breed varies from less than a year in many songbirds and some quail to ten years or more in some albatross. The life spans of birds in the wild are poorly known. Many small songbirds live only three to five years, whereas some albatrosses are known to have survived more than 60 years in the wild.

The keen eyesight and acute hearing of birds help them react quickly to predators, which may be other birds, such as falcons and hawks, or other types of animals, such as snakes and weasels. Many small birds feed in flocks, where they can benefit from the observing power of multiple pairs of eyes. The first bird in a flock to spot a predator usually warns the others with an alarm call.

Birds that feed alone commonly rely on camouflage and rapid flight as means of evading predators. Many birds have highly specific and unusual defence strategies. The burrowing owl in North America, which lives in the burrows of ground squirrels, frightens away predators by making a call that sounds much like a rattlesnake. The snipe, a wading bird, flees from its enemies with a zigzag flight pattern that is hard for other birds to follow.

Many bird species undergo annual migrations, travelling between seasonally productive habitats. Migration helps birds to have continuous sources of food and water, and to avoid environments that are too hot or too cold. Most spectacular of bird migrations are made by seabirds, in which they fly across oceans and along coastlines, sometimes travelling 32,000 km. (20,000 mi.) or more in a single year. Migrating birds use a variety of cues to find their way. These include the positions of the sun during the day and the stars at night; the earth's magnetic field; and visual, olfactory, and auditory landmarks. The strict formations in which many birds fly help them on the journey, for example, migrating geese travel in a V-shaped formation, which enables all of the geese except the leader to take advantage of the updrafts generated by the flapping wings of the goose in front. Young birds of many species undertake their first autumn migration with no guidance from experienced adults. These inexperienced birds do not necessarily reach their destinations; many birds stray in the wrong direction and are sometimes observed thousands of kilometres away from their normal route.

There are nearly 10,000 known species of modern or recently extinct birds. Traditionally, taxonomists (those who classify living things based on evolutionary relationships) have looked at bird characteristics such as skeletal structure, plumage, and bill shape to determine which birds have a shared evolutionary history. More recently, scientists have turned to deoxyribonucleic acid (DNA)-the genetic information found in the cells of all living organisms-for clues about relationships among birds. DNA is useful to volaille bird taxonomists because closely related birds have more similar DNA than do groups of birds that are distantly related. DNA comparisons have challenged some of scientists' previous ideas about relationships among birds. For example, these studies have revealed that vultures of the Americas are more closely related to storks than to the vultures of Europe, Asia, or Africa.

Another method of categorizing birds focuses on adaptive types, or lifestyles. This system groups together birds that live in similar environments or have similar methods for obtaining food. Even among a given adaptive types, birds show tremendous diversity.

Aquatic birds obtain most or all of their food from the water. All aquatic birds that live in saltwater environments have salt glands, which enable them to drink seawater and excrete the excess salt. Albatross, shearwaters, storm petrels, and diving petrels are considered the most exclusively aquatic of all birds. These birds spend much of their time over the open ocean, well away from land.

Many other birds have aquatic lifestyles but live closer to land. Among these are penguins, which live in the southernmost oceans near the Antarctic. Some species of penguins spend most of their lives in the water, coming on land only to reproduce and molt. Grebes and divers, or loons, are found on or near lakes. Grebes are unusual among birds because they make their nests on the water, using floating plant materials that they hide among reeds. Pelicans, known for their long bills and huge throat pouches, often switch between salt water and fresh water habitats during the year. Gulls are generalists among the aquatic birds, feeding largely by scavenging over open water, along shores, or even inland areas. Waterfowls, a group that includes ducks, geese, and swans, often breed on freshwater lakes and marshes, although they sometimes make their homes in marine habitats.

Many long-legged, long-billed birds are adapted to live at the junction of land and water. Large wading birds, including herons, storks, ibises, spoonbills, and flamingoes, are found throughout the world, except near the poles. These birds wade in shallow water or across mudflats, wet fields, or similar environments to find food. Depending on the species, large wading birds may eat fish, frogs, shrimp, or microscopic marine life. Many large wading birds gather in enormous groups to feed, sleep, or nest. Shorebirds often inhabit puddles or other shallow bodies of water. The diversity of shorebirds is reflected in their varied bill shapes and leg lengths. The smallest North American shorebirds, called stints or peeps, have short, thin bills that enable them to pick at surface prey, whereas curlews probe with their long bills for burrowing shellfish and marine worms that are beyond the reach of most other shore feeders. Avocets and stilts have long legs and long bills, both of which help them to feed in deeper water.

Among the best-known birds are the birds of prey. Some, including hawks, eagles, and falcons, are active during the daytime. Others, notably owls, are nocturnal, or active at night. Birds of prey have hooked beaks, strong talons or claws on their feet, and keen eyesight and hearing. The larger hawks and eagles prey on small mammals, such as rodents and other vertebrates. Some birds of prey, such as the osprey and many eagles, eat fish. Falcons eat mainly insects, and owls, depending on the species, have diets ranging from insects to fish and mammalians. Scavengers that feed on dead animals are also considered birds of prey. These include relatives of eagles called Old World vultures, which live in Eurasia and Africa, and the condors and vultures of North and South America.

Some birds, including the largest of all living birds, have lost the ability to fly. The ostriches and their relatives-rheas, emus, cassowaries, and kiwis-are flightless birds settling in Africa, South America, and Australia, including New Guinea and New Zealand. The tinamous of Central and South America are related to the ostrich group, but they have a limited ability to fly. Other birds that feed primarily on the ground and exceed as excellent runners include the bustards (relatives of the cranes) and megapodes, members of a group of chicken-like birds that includes quail, turkeys, pheasants, and grouse. Vegetation is an important part of the diets of running birds.

More than half of all living species of birds are perching birds. Perching birds have been successful in all terrestrial habitats. Typically small birds, perching birds have a distinctive arrangement of toes and leg tendons that enables them to perch acrobatically on small twigs. They have the most satisfactorially-developed and complex vocalizations of all birds. They are divided into two main groups: the sub-oscines, which are mainly tropical and include tyrant flycatchers, antbirds, and oven-birds, and the oscines or songbirds, which make up about 80 percent of all perching bird species, among them the familiar sparrows, finches, warblers, crows, blackbirds, thrushes, and swallow. Some birds of this group catch and feed upon flying insects. An example is the swallow, which opens its mouth in a large trap-like gape to gather food. One apparent group, the dippers, is aquatic; its members obtain their food during short dives in streams and rivers.

Many other groups of birds thrive in terrestrial habitats. Parrots, known for their brilliantly coloured plumage, form a distinctive group of tropical and southern temperate birds that inhabit woodlands and grasslands. Doves and pigeons, like parrots, are seed and fruit eaters but are more widespread and typically more subdued in colour. The cuckoos-including the tree-dwelling species such as the European cuckoo, whose call is mimicked by the cuckoo clock, and ground-inhabiting species, such as roadrunners-are land birds. Hummingbirds are a group of nectars and insect-feeding land birds whose range extends from Alaska to the tip of South America. Woodpeckers and their relatives thrive in forests. Kingfishers are considered land birds despite their habit of eating fish.

Although birds collectively occupy most of the earth's surfaces, most individual species are found only in particular regions and habitats. Some species are quite restricted, occurring only on a single oceanic island or an isolated mountaintop, whereas others are cosmopolitan, living in suitable habitats on most continents. The greatest species diversity occurs in the tropics in North and South America, extending from Mexico to South America. This part of the world is especially rich in tyrant flycatchers, oven-birds, antbirds, tanagers, and hummingbirds. The Australia and New Guinea region have possibly the most distinguishing groups of birds, because its birds have long been isolated from those of the rest of the world. Emus, cassowaries, and several songbird groups, including birds-of-paradise, are found nowhere else. Africa is the unique home to many bird families, including turacos, secretary birds, and helmet-shrikes. Areas that are further from the equator have fewer diverse birds. For example, about 225 bird species breed in the British Isles-approximately half the number of breeding strains that inhabit a single reserve in Ecuador or Peru. Despite the abundance of seabirds at its fringes, Antarctica is the poorest bird continent, with only about 20 species.

The habitats occupied by birds are also diverse. Tropical rain forests have high species diversity, as do savannas and wetlands. Fewer species generally occupy extremely arid habitats and very high elevations. A given species might be a habitat specialist, such as the marsh wren, which lives only in marshes of cattails or tules, or a generalist, such as the house sparrow, which can thrive in a variety of environments.

Many habitats are only seasonally productive for birds. The arctic tundra, for example, teams with birds during the short summer season, when food and water are plentiful. In the winter, however, this habitat is too cold and dry for all but a few species. Many bird species respond to such seasonal changes by undergoing annual migrations. Many bird species that breed in the United States and Canada move south to winter in Central or northern South America. Similar migrations from temperate regions to tropical ones exist between Europe and Africa, northeastern Asia and southeast Asia and India and, to a lesser degree, from southern Africa and southern South America to the equatorial parts of those continents.

Scientists disagree about many aspects of the evolution of birds. Many paleontologists (scientists who study fossils to learn about prehistoric life) believe that birds evolved from small, pillaging dinosaurs called theropods. These scientists say that many skeletal features of birds, such as light, hollow bones and a furculum, were present in theropod dinosaurs before the evolution of birds. Others, however, think that birds evolved from an earlier type of reptile called the codonts-a group that ultimately produced dinosaurs, crocodiles, and the flying reptiles known as pterosaurs. These scientists assert that similarities between birds and theropod dinosaurs are due to a phenomenon called convergent evolution-the evolution of similar traits among groups of organisms that are not necessarily related.

Scientists also disagree about how flight evolved. Some scientists believe that flight first occurred when the ancestors of birds climbed trees and glided down from branches. Others theorize that bird flight began from the ground up, when dinosaurs or reptiles ran along the ground and leaped into the air to catch insects or to avoid predators. Continued discovery and analysis of fossils will help clarify the origins of birds.

Despite uncertainties about bird evolution, scientists do know that many types of birds lived during the Cretaceous Period, which dates to about 138 million to 65 million years ago. Among these birds was Ichthyornis Victor, which resembled a gull and had vertebrae similar to those of a fish, and Hesperonis regalis, which was nearly wingless and had vertebrae like those of today's birds. Most birds of the Cretaceous Period are thought to have died out in the mass extinctions-deaths of many animal species, which took place at the end of the Cretaceous Period.

The remains of prehistoric plants and animals buried and preserved in sedimentary rock or trapped in amber or other deposits of ancient organic matter, provided a record of the history of life on Earth. Scientists who subject in the field of fossil records are called paleontologists, for which having learnt those extinguishing natural archeological remains, are an ongoing phenomenons. In fact, the hundreds of millions of species that have lived on Earth over the past 3.8 billion years, more than 99 percent are already extinct. Some of this happens as the natural result of competition between species and is known as natural selection. According to natural selection, living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment. Those species incapable of adapting are faced with imminent extinction. This constant rate of extinction, sometimes called background extinction, is like a slowly ticking clock. First one species, then another becomes extinct, and new species appear almost at random as geological time goes by. Normal rates of background extinction are usually about five families of organisms lost per million years.

More recently, paleontologists have discovered that not all extinction is slow and gradual. At various times in the fossil record, many different, unrelated species became extinct at nearly the same time. The cause of these large-scale extinctions is always dramatic environmental change that produces conditions too severe for organisms to endure. Environmental changes of this caliber result from extreme climatic change, such as the global cooling observed during the ice ages, or from catastrophic events, such as meteorite impacts or widespread volcanic activity. Whatever their causes, these events dramatically alter the composition of life on Earth, as entire groups of organisms disappear and entirely new groups rise to take their place.

In its most general sense, the term mass extinction refers to any episode of multiple loss of species. Nonetheless, the term is generally reserved for truly global extinction events-events in which extensive species loss occurs in all ecosystems on land and in the sea, affecting every part of the Earth's surface. Scientists recognize five such mass extinctions in the past 500 million years. The first occurred around 438 million years ago in the Ordovician Period. At this time, more than 85 percent of the species on Earth became extinct. The second took place 367 million years ago, near the end of the Devonian Period, when 82 percent of all species were lost. The third and greatest mass extinction to date occurred 245 million years ago at the end of the Permian Period. In this mass extinction, as many as 96 percent of all species on Earth were lost. The devastation was so great that paleontologists use this event to mark the end of the ancient, or Paleozoic Era, and the beginning of the middle, or Mesozoic Era, when many new groups of animals evolved.

About 208 million years ago near the end of the Triassic Period, the fourth in mass extinction claimed 76 percent of the species alive at the time, including many species of amphibians and reptiles. The fifth and most recent mass extinction occurred about 65 million years ago at the end of the Cretaceous Period and resulted in the loss of 76 percent of all species, most notably the dinosaurs.

Many geologists and paleontologists speculate that this fifth mass extinction occurred when one or more meteorites struck the Earth. They believe the impact created a dust cloud that blocked much of the sunlight-seriously altering global temperatures and disrupting photosynthesis, the process by which plants derive energy. As plants died, organisms that relied on them for food also disappeared. Supporting evidence for this theory comes from a buried impact crater in the Yucatán Peninsula of Mexico. Measured at 200 km. (124 mi.) in diameter, this huge crater is thought to be the result of a large meteorite striking the Earth. A layer of the element iridium in the geologic sediment from this time provides additional evidence. Unusual in such quantities on Earth, iridium is common in extraterrestrial bodies, and theory supporters suggest iridium travelled to Earth on a meteorite.

Other scientists suspect that widespread volcanic activity in what is now India and the Indian Ocean may have been the source of the atmospheric gases and dust that blocked sunlight. Ancient volcanoes could have been the source of the unusually high levels of iridium, and advocates of this theory point out that iridium is still being released today by at least one volcano in the Indian Ocean. No matter what the cause, the extinction at the end of the Cretaceous Period was so great that scientists use this point in time to divide the Mesozoic Era (also called the Age of Reptiles) from the Cenozoic Era (otherwise known as the Age of Mammals).

Historically biologists-most famous among them British naturalist Charles Darwin-assumed that extinction is the natural outcome of competition between newly evolved, adaptively superior species and their older, more primitive ancestors. These scientists believed that newer, and more peremptorily evolved species simply drove less well-adapted species to extinction. That is, historically, extinction was thought to result from evolution. It was also thought that this process happens in a slow and regular manner and occurs at different times in different groups of organisms.

In the case of background extinction, this holds true. An average of three species becomes extinct every million years, usually because of the forces of natural selection. When this happens, appraisive species, characteristically differs only slightly from the organisms that disappeared-rise to take their places, creating evolutionary lineages of related species. The modern horse, for example, comes from a long evolutionary lineage of related, but now extinct, species. The earliest known horse had four toes on its front feet, three toes on its rear feet, and weighed just 36 kg. (80 lb.). About 45 million years ago, this horse became extinct. It was succeeded by other types of horses with different characteristics, such as teeth better shaped for eating different plants, which made them better in agreement or accorded with their owing environments. This pattern of extinction and the ensuing rise of related species continued for more than 55 million years, ultimately resulting in the modern horse and its relatives the zebras and asses.

In mass extinctions, entire groups of species-such as families, orders, and classes-die out, creating opportunities for the survivors to exploit new habitats. In their new niches, the survivors evolve new characteristics and habits and, consequently, develop into entirely new species. What this course of events means is that mass extinctions are not the result of the evolution of new species, but actually a cause of evolution. Fossils from periods of mass extinction suggest that most new species evolve after waves of extinction. Mass extinctions cause periodic spurts of evolutionary change that shake up the dynamics of life on Earth.

This is perhaps best shown in the development of our own ancestors, the early mammals. Before the fall of the dinosaurs, which had dominated Earth for more than 150 million years, mammals were small, nocturnal, and secretive. They devoted much of their time and energy to evading meat-eating dinosaurs. With the extinction of dinosaurs, the remaining mammals moved into habitats and ecological niches previously dominated by the dinosaurs. Over the next 200 million years, those early mammals evolved into a variety of species, assuming many ecological roles and rising to dominate the Earth as the dinosaurs had before them.

Most scientists agree that life on Earth is now faced with the most severe extinction episode since the event that drove the dinosaurs extinct. No one knows exactly how many species are being lost because no one knows exactly how many species exist on Earth. Estimates vary, but the most widely accepted figure lies between 10 and 13 million species. Of these, biologists estimate that as many as 27,000 species are becoming extinct each year. This translates into an astounding three varieties every hour.

Instead of global climate change, humans are the cause of this latest mass extinction. With the invention of agriculture some 10,000 years ago, humans began destroying the world's terrestrial ecosystems to produce farmland. Today pollution destroys ecosystems even in remote deserts and in the world's deepest oceans. In addition, we have cleared forests for lumber, pulp, and firewood. We have harvested the fish and shellfish of the world's largest lakes and oceans in volumes that make it impossible for populations to recover fast enough to meet our harvesting needs. Everywhere we go, whether on purpose or by accident, we have brought along species that disrupt local ecosystems and, in many cases, drive native species extinct. For instance, Nile perch were intentionally introduced to Lake Victoria for commercial fishing in 1959. This fish proved to be an efficient predator, driving 200 rare species of cichlid fishes to extinction.

This sixth extinction, as it has become known, poses a great threat to our continued existence on the planet. As the sum of all species living in the world's ecosystems, knew as biodiversity, least of mention, losing substance under which a great deal is excessively generically set to one side by much of the resourcefulness from which we depend. Humans use at least 40,000 different plants, animals, fungi, bacteria, and virus species for food, clothing, shelter, and medicines. In addition, the fresh airs we breathe, the water we drink, cook, and wash with, as many chemicals cycles-including the nitrogen cycle and the carbon cycle, so vital to sustain life-depend on the continued health of ecosystems and the species within them.

The list of victims of the sixth extinction grows by the year. Forever lost are the penguin-like great auk, the passengers' pigeon, the zebra-like quagga, the thylacine, the Balinese tiger, the ostrich-like moa, and the tarpan, a small species of wild horse, to name but a few. More than 1,000 plants and animals are threatened by extinction. Each of these organisms has unique attributes-some of which may hold the secrets to increasing world food supplies, eradicating water pollution, or curing disease. A subspecies of the endangered chimpanzee, for example, has recently been identified as the probable origin of the human immunodeficiency virus, the virus that causes acquired immunodeficiency syndrome (AIDS). All the same, these animals are widely hunted in their west African habitat, and just as researchers learn of their significance to the AIDS epidemic, the animals face extinction. If they become extinct, they will take with them many secrets surrounding this devastating disease.

In the United States, legislation to protect endangered species from impending extinction includes the Endangered Species Act of 1973. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), established in 1975, enforces the prohibition of trading of threatened plants and animals between countries. The Convention on Biological Diversity, an international treaty developed in 1992 at the United Nations Conference on the Environment and Development, obligates more than 160 countries to take action to protect plant and animal species.

Scientists meanwhile are intensifying their efforts to describe the species of the world. So far biologists have identified and named around 1.75 million species-a mere fraction of the species believed to exist today. Of those identified, special attention is given to species at or near the brink of extinction. The World Conservation Union (IUCN) maintains an active list of endangered plants and animals called the Red List. In addition, captive breeding programs at zoos and private laboratories are dedicated to the preservation of endangered species. Participants in these programs breed members of different populations of endangered species to increase their genetic diversity, thus enabling the species to cope with further threats to their numbers better.

All these programs together have had some notable successes. The peregrine falcon, nearly extinct in the United States due to the widespread use of the pesticide DDT, rebounded strongly after DDT was banned in 1973. The brown pelican and the bald eagle offer similar success stories. The California condor, a victim of habitat destruction, was bred in captivity, and small numbers of them are now being released back into the wild.

Growing numbers of legislators and conservation biologists, scientists who specialize in preserving and nurturing biodiversity, are realizing that the primary cause of the current wave of extinction is habitat destruction. Efforts have accelerated to identify ecosystems at greatest risk, including those with high numbers of critically endangered species. Programs to set aside large tracts of habitats, often interconnected by narrow zones or corridors, offer the best hope yet of sustaining ecosystems, and by that most of the world's species.

Nevertheless, the Tertiary Period directly following the Cretaceous witnessed an explosive evolution of birds. One bird that lived during the Tertiary Period was Diatryma, which stood 1.8 to 2.4 m. (about six to 8 ft.) tall and had massive legs, a huge bill, and very small, underdeveloped wings. Most modern families of birds can be traced back in the fossil record to the early or mid-Eocene Epoch-a stage within the Tertiary Period that occurred about 50 million years ago. Perching birds, called passerines, experienced a tremendous growth in species diversity in the latter part of the Tertiary; today this group is the most diverse order of birds.

During the Pleistocene Epoch, from 1.6 million to 10,000 years ago, also known as the Ice Age, glacier ice spread over more than one-fourth of the land surfaces of the earth. These glaciers isolated many groups of birds from other groups with which they had previously interbred. Scientists have long assumed that the resulting isolated breeding groups evolved into the species of birds that exist today. This assumption has been modified from studies involving bird DNA within cellular components called mitochondria. Pairs of species that only recently diverged from a shared ancestry are expected to have more similar mitochondrial DNA than are pairs that diverged in the more distant past. Because mutations in mitochondrial DNA are thought to occur at a fixed rate, some scientists believe that this DNA can be interpreted as a molecular clock that reveals the approximate amount of time that has elapsed since two species diverged from one another. Studies of North American songbirds based on this approach suggest that only the earliest glaciers of the Pleistocene are likely to have played a role in shaping bird species.

The evolution of birds has not ended with the birds that we know today. Some bird species are dying out. In addition, the process of Speciation, evolutionary changes that resultant products in some newer species-continues always.

Birds have been of ecological and economic importance to humans for thousands of years. Archaeological sites reveal that prehistoric people used many kinds of birds for food, ornamentation, and other cultural purposes. The earliest domesticated bird was probably the domestic fowl or chicken, derived from jungle fowls of Southeast Asia. Domesticated chickens existed even before 3000 Bc. Other long-domesticated birds are ducks, geese, turkeys, guineas-fowl, and pigeons.

Today the adults, young, and eggs of both wild and domesticated birds provide humans with food. People in many parts of Asia even eat nests that certain swiftlets in southeastern Asia construct out of saliva. Birds give us companionship as pets, assume religious significance in many cultures, and, with hawks and falcons, perform work for us as hunters. People in maritime cultures have learned to monitor seabird flocks to find fish, sometimes even using cormorants to do the fishing.

Birds are good indicators of the quality of our environment. In the 19th century, coal miners brought caged canaries with them into the mines, knowing that if the birds stopped singing, dangerous mine gases had escaped into the air and poisoned them. Birds provided a comparable warning to humans in the early 1960s, when the numbers of peregrine falcons in the United Kingdom and raptors in the United States suddenly declined. This decline was caused by organochlorine pesticides, such as DDT, which were accumulating in the birds and causing them to produce eggs with overly fragile shells. This decline in the bird populations alerted humans to the possibility that pesticides can harm people as well. Today certain species of birds are considered indicators of the environmental health of their habitats. An example of an indicator bird is the northern spotted owl, which can only reproduce within old growth forests in the Pacific Northwest.

Many people enjoy bird-watching. Equipped with binoculars and field guides, they identify birds and their songs, often keeping lists of the various species they have witnessed. Scientists who study birds are known as ornithologists. These experts investigate many behaviours, and, evolutionary histories, ecology, set-classification, and species distributed of both domesticated and rashly or wild birds.

Overall, birds pose little direct danger to humans. A few birds, such as the cassowaries of New Guinea and northeastern Australia, can kill humans with their strong legs and bladelike claws, but actual attacks are extremely rare. Many birds become quite aggressive when defending a nest site; humans are routinely attacked, and occasionally killed, by hawks engaging in such defence. Birds pose a greater threat to human health as carriers of diseases. Diseases carried by birds that can affect humans include influenza and psittacosis.

Negative impacts by birds on humans are primarily economic. Blackbirds, starlings, sparrows, weavers, crows, parrots, and other birds may seriously deplete crops of fruit and grain. Similarly, fish-eating birds, such as cormorants and herons, may adversely influence aquacultural production. However, the economic benefits of wild birds to humans are well documented. Many birds help humans, especially farmers, by eating insects, weeds, slugs, and rodents.

Although birds, with some exceptions, are tremendously beneficial to humans, humans have a long history of causing harm to birds. Studies of bone deposits on some Pacific islands, including New Zealand and Polynesia, suggest that early humans hunted many hundreds of bird species to extinction. Island birds have always been particularly susceptible to predation by humans. Because these birds have largely evolved without land-based predators, they are tame and in many descriptions are flightless. They are therefore easy prey for humans and the animals that accompany them, such as rats. The dodo, a flightless pigeon-like bird on the island of Mauritius in the Indian Ocean, was hunted to extinction by humans in the 1600s.

With colonial expansion and the technological advances of the 18th and 19th centuries, humans hunted birds on an unprecedented scale. This time period witnessed the extinction of the great auk, a large flightless seabird of the North Atlantic Ocean that was easily killed by sailors for food and oil. The Carolina parakeet also became extinct in this intermittent interval of time, although the last one of these birds survived in the Cincinnati Zoo until 1918.

In the 20th century, a time of explosive growth in human populations, the major threats to birds have been the destruction and modification of their habitats. The relentless clearing of hardwood forests outweighed even relentless hunting as the cause of the extinction of the famous passengers' pigeon, whose eastern North American populations may have once numbered in the billions. The fragmentation of habitats into small parcels is also harmful to birds, because it increases their vulnerability to predators and parasites.

Habitat fragmentation and reduction particularly affect songbirds that breed in North America in the summer and migrate to Mexico, the Caribbean, Central America, and Colombia for the winter. In North America, these birds suffer from forest fragmentation caused by the construction of roads, housing developments, and shopping malls. In the southern part of their range, songbirds are losing traditional nesting sites as tropical forests are destroyed and shade trees are removed from coffee plantations.

Pesticides, pollution, and other poisons also threaten today's birds. These substances may kill birds outright, limit their ability to reproduce, or diminish their food supplies. Oil spills have killed thousands of aquatic birds, because birds with oil-drenched feathers cannot fly, float, or stay warm. Acid rain, caused by chemical reactions between airborne pollutants and water and an oxygen providence in the atmosphere, has decreased the food supply of many birds that feed on fish or other aquatic life in polluted lakes. Many birds are thought to be harmed by selenium, mercury, and other toxic elements present in agricultural runoff and in drainage from mines and power plants. For example, loons in the state of Maine may be in danger due to mercury that drifts into the state from unregulated coal-fired power plants in the Midwest and other sources. Global warming, an increase in the earth's temperature due to a buildup of greenhouse gases, is another potential threat to birds.

Sanctuaries for birds exist all over the world-two examples are the Bharatpur Bird Sanctuaries in India's Keoladeo National Park, which protects painted storks, gray herons, and many other bird species, and the National Wildlife Refuge system of the United States. In North America, some endangered birds are bred in settings such as zoos and specialized animal clinics and later released into the wild. Such breeding programs have added significantly to the numbers of whooping cranes, peregrine falcons, and California condors. Many countries, including Costa Rica, are finding they can reap economic benefits, including the promotion of tourism, by protecting the habitats of birds and other wildlife.

The protection of the earth's birds will require more than a single strategy. Many endangered birds need a combination of legal protections, habitat management, and control of predators and competitors. Ultimately, humans must decide that the bird's world is worth preserving along with our own.

Most people did not understand the true nature of fossils until the beginning of the 19th century, when the basic principles of modern geology were established. Since about 1500 AD., scholars had engaged in a bitter controversy over the origin of fossils. One group held that fossils are the remains of prehistoric plants and animals. This group was opposed by another, which declared that fossils were either freaks of nature or creations of the devil. During the 18th century, many people believed that all fossils were relics of the great flood recorded in the Bible.

Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how the earth evolved. The study and comparison of different strata are called stratigraphy.

Fossils sustain most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one-that is, they represent a species that was widespread but existed for a brief period of time. The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 570 million years of the earth's history into eras, periods, and epochs. The part of the earth's history before about 570 million years ago is called Precambrian time, which began with the earth's birth, probably more than four billion years ago.

The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.6 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils, the first abundant fossils of larger animals had been dated from around 600 million years ago. The Paleozoic era lasted to about 330 million years. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Index fossils of the first half of the Paleozoic era are those of the invertebrates, such as trilobites, graptolites, and crinoids. Remains of plants and such vertebrates as fish and reptiles make up the index fossils of the second half of this era.

At the beginning of the Cambrian period (570 million to 500 million years ago) animal life was entirely confined to the seas. By the end of the period, all the phyla of the animal kingdom existed, except vertebrates. The characteristic animals of the Cambrian period were the trilobites, a primitive form of arthropod, which reached their fullest development in this period and became extinct by the end of the Paleozoic era. The earliest snails appeared in this period, as did the cephalopod mollusks. Other groups represented in the Cambrian period were brachiopods, bryozoans, and foraminifers. Plants of the Cambrian period included seaweeds in the oceans and lichens on land.

The most characteristic animals of the Ordovician period (500 million to 435 million years ago) were the graptolites, which were small, colonial hemichordates (animals possessing an anical structure suggesting part of a spinal cord). The first vertebrates-primitive fish-and the earliest corals emerged during the Ordovician period. The largest animal of this period was a cephalopod mollusk that had a shell about three m.'s (about 10 ft.) in length. Plants of this period resembled those of the Cambrian periods.

The most important evolutionary development of the Silurian period (435 million to 410 million years ago) was that of the first air-breathing animal, a scorpion. Fossils of this creature have been found in Scandinavia and Great Britain. The first fossil records of vascular plants-that are, land plants with tissue that carries food-appeared in the Silurian period. They were simple plants that had not developed separate stems and leaves.

The dominant forms of animal life in the Devonian period (410 million to 360 million years ago) were fish of various types, including sharks, lungfish, armoured fish, and primitive forms of ganoid (hard-scaled) fish that were probably the evolutionary ancestors of amphibians. Fossil remains found in Pennsylvania and Greenland suggest that early forms of amphibia may already have existed during the Devonian period. Early animal forms included corals, starfish, sponges, and trilobites. The earliest known insect was found in Devonian rock.

The Devonian is the first period from which any considerable number of fossilized plants have been preserved. During this period, the first woody plants developed, and by the end of the period, land-growing forms included seed ferns, ferns, scouring rushes, and scale trees, the modern relative of club moss. Although the present-day equivalents of these groups are mostly small plants, they developed into treelike forms in the Devonian period. Fossil evidence shows that forests existed in Devonian times, and petrified stumps of some larger plants from the period measure about 60 cm. (about 24 in.) in diameter.

The Carboniferous period lasted from 360 million to 290 million years ago. During the first part of this period, sometimes called the Mississippian period (360 million to 330 million years ago), the seas contained a variety of echinoderms and foraminifers, and most forms of animal life that appeared in the Devonian. A group of sharks, the Cestraciontes, or shell-crushers were dominant among the larger marine animals. The predominant group of land animals was the Stegocephalia, an order of primitive, lizard-like amphibians that developed from the lungfish. The various forms of land plants became diversified and grew larger, particularly those that grew in low-laying swampy areas.

The second part of the Carboniferous, sometimes called the Pennsylvanian period (330 million to 290 million years ago), saw the evolution of the first reptiles, a group that developed from the amphibians and lived entirely on land. Other land animals included spiders, snails, scorpions, more than 800 species of cockroaches, and the largest insect ever evolved, a species resembling the dragonfly, with a wingspread of about 74 cm. (about 29 in.). The largest plants were the scale trees, which had tapered trunks that measured as much as 1.8 m.'s (6 ft.) in diameter at the base and 30 m.'s (100 ft.) in height. Primitive gymnosperms known as cordaites, which had pithy stems surrounded by a woody shell, were more slender but even taller. The first true conifers, forms of advanced gymnosperms, also developed during the Pennsylvanian period.

The chief events of the Permian period (290 million to 240 million years ago) were the disappearance of many forms of marine animals and the rapid spread and evolution of the reptiles. Usually, Permian reptiles were of two types: lizard-like reptiles that lived entirely on land, and sluggish, semiaquatic types. A comparatively small group of reptiles that evolved in this period, the Theriodontia, were the ancestors of mammals. Most vegetation of the Permian period was composed of ferns and conifers.

The Mesozoic era is often called the Age of Reptiles, because the reptile class was dominant on land throughout the age The Mesozoic era lasted to about 175 million years, and includes the Triassic, Jurassic, and Cretaceous periods. Index fossils from this era include a group of extinct cephalopods called ammonites, and extinct forms of sand dollars and sea urchins.

The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic period (240 million to 205 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with heavy, fleshy tails, and seldom exceeded 4.5 m. (15 ft.) in length. Other reptiles of the Triassic period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.

The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.

During the Jurassic period (205 million to 138 million years ago), dinosaurs continued to evolve in a wide range of size and diversity. Types included heavy four-footed sauropods, such as Apatosaurus (formerly Brontosaurus); two-footed carnivorous dinosaurs, such as Allosaurus; two-footed vegetarian dinosaurs, such as Camptosaurus; and four-footed armoured dinosaurs, such as Stegosaurus. Winged reptiles included the pterodactyl, which, during this period, ranged in size from extremely small species to those with wingspreads of 1.2 m.'s (4 ft.). Marine reptiles included plesiosaurs, a group that had broad, flat bodies like those of turtles, with long necks and large flippers for swimming, Ichthyosauria, which resembled dolphins, or at times they appear like primitive crocodiles.

The mammals of the Jurassic period consisted of four orders, all of which were smaller than small modern dogs. Many insects of the modern orders, including moths, flies, beetles, grasshoppers, and termites appeared during the Jurassic period. Shellfish included lobsters, shrimp, and ammonites, and the extinct group of belemnites, which resembled squid and had cigar-shaped internal shells. Plant life of the Jurassic period was dominated by the cycads, which resembled thick-stemmed palms. Fossils of most species of Jurassic plants are widely distributed in temperate zones and polar regions, suggesting that the climate was uniformly mild.

The reptiles were still the dominant form of animal life in the Cretaceous period (138 million to 65 million years ago). The four types of dinosaurs found in the Jurassic also lived during this period, and a fifth type, the horned dinosaurs, also appeared. By the end of the Cretaceous, about 65 million years ago, all these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 15.5 m's (50 ft). Other reptiles of the period include the first snakes and lizards. Several types of Cretaceous birds have been discovered, including Hesperornis, a diving bird about 1.8 m's (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved.

The most important evolutionary advance in the plant kingdom during the Cretaceous period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations. By the end of the period, many modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut. Some paleontologists believe that these deciduous woody plants first evolved in Jurassic times but grew only in upland areas, where conditions were unfavourable for fossil preservation.

The Cenozoic era (65 million years ago to the present time) is divided into the Tertiary period (65 million to 1.6 million years ago) and the Quaternary period (1.6 million years ago to the present). However, because scientists have so much more information about this era, they tend to focus on the epochs that make up each period. During the first part of the Cenozoic era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic era disappeared.

The Paleocene epoch (65 million to 55 million years ago) marks the beginning of the Cenozoic era. Seven groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Most of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, which were the ancestors of modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents.

During the Eocene epoch (55 million to 38 million years ago), several ancestors direct evolutionary modern animals appeared. Among these animals-all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern birds as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.

During the Oligocene epoch (38 million to 24 million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of several modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.

The development of mammals during the Miocene epoch (24 million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.

The paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were fundamentally forward-moving in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, and now-extinct species such as the giant saber-toothed tigers. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the musk-ox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.

Earth is one of nine planets in the solar system, the only planet known to harbor life, and the ‘home' of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth's surface is covered by water, which is essential to life. The rest is land, mostly as continents that rise above the oceans.

Earth's surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.

Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets-Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth's atmosphere includes just the right degree of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth's atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth, not either too far or nearer from the Sun. Nevertheless, because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot-462°C's (864°F)-hot enough to melt lead and too hot for life to exist.

Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, suggesting that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.

For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas, for example, that the Earth was a sphere and that it travelled around the Sun were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. With the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.

In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings can study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.

From this recent space exploration, we now know that Earth is one of the most geological activities unbound of all the planets and moons in the solar system. Earth is constantly changing. Over long periods land is built up and worn away, oceans are formed and re-formed. Continents move around, break up, and merge.

Life itself contributes to changes on Earth, especially in, and the way living things can alter Earth's atmosphere. For example, Earth at once had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth's atmosphere and made it possible for animal life to evolve on land.

A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth's atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.

Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km. (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km./h. (about 67,000 mph.) as it orbits the Sun. All but one planet orbit the Sun in the same plane-that is, if an imaginary line were extended from the centre of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.

Earth's orbital path is not quite a perfect circle but instead is elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km. (about 95 million mi.) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi.) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.

The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has arm-like discs of stars that spiral out from its centre. The solar system is found in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the centre of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.

Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is in a measure less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets-Mercury, Venus, Earth, and Mars-that makes up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system-Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it.

Earth has one natural satellite, the Moon. The Moon orbits the Earth, undivided and compelling of one revolution in an elliptical path in 27 days 7 hr 43 min 11.5-sec. The Moon orbits the Earth because of the force of Earth's gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon's gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than four billion years ago when a large meteorite or small planet struck the Earth.

As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min's 4.1-sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr. 48 min.'s 46-sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It indirectly takes by more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy's centre.

Earth's axis of rotation is inclined (tilted) 23.5° on its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur between these orientations.

The atmosphere is a layer of different gases that extends from Earth's surface to the exosphere, the outer limit of the atmosphere, about 9,600 km. (6,000 mi.) above the surface. Near Earth's surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consist of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapour; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi.) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi.) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C's, or about 2200°F). The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth's surface to altitudes of 1,000 km (600 mi) and more.

Earth's atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet's climate and weather. The atmosphere plays a key role in supporting life. Most life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth's climate by trapping radiation from the Sun that is reflected from Earth's surface. Water vapour, carbon dioxide, methane, and nitrous oxide in the atmosphere act as ‘greenhouse gases'. Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and by that help warm Earth's surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.

Other gases in the atmosphere are also essential to life. The trace amount of ozone based in Earth's stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozone layer, life as we know it could not survive on land. Earth's atmosphere is also an important part of a phenomenon known as the water cycle or the hydrologic cycle.

The water cycle simply means that Earth's water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time. Water is always shifting on the Earth's surface and changing back and forth between ice, liquid water, and water vapour.

The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapour. Some of this water vapour falls as precipitation directly back into the oceans, completing a short cycle. Some water vapour, however, reaches land, where it may fall as snow or rain. Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventual melts or is warmed by the Sun and turned into water vapour, entering the atmosphere and falling again as precipitation. All water that falls on land eventually return to the ocean, completing the water cycle.

The hydrosphere consists of the bodies of water that cover 71 percent of Earth's surface. The largest of these are the oceans, which hold more than 97 percent of all water on Earth. Glaciers and the polar ice caps encircle just more than 2 percent of Earth's water as solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapour. Only 0.017 percent of all the water on Earth is found in lakes and rivers. A mere 0.001 percent is found in the atmosphere as water vapour. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture. Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.

The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth's surface is called the continental crust. It is about 15 to 75 km (nine to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is five to 10 km (three to 6 mi). The crust has a definite boundary called the Mohorovicic discontinuity, or simply the Moho. The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth's interior.

Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by the breakdown of igneous rocks. They have a tendency to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused over time. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.

Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter coloured, less dense igneous rock, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.

The biosphere can support life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated. They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight. Some scientists believe that the biosphere may extend deeply into the Earth's crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.

Earth's surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years. Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge super-continents, and the breakup of super-continents into smaller continents.

The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth's surface. Another principal factor is the movement of Earth's continents and sea-floors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all these changes. Heat in Earth's interior is believed to be responsible for continental movement, mountain building, and the creation of new sea-floor in ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth's interior helps build up Earth's surface while heat from the Sun helps wear down the surface.

Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water. Once the rocks reached Earth's surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change. These changes make up the process of weathering. There are two types of weathering: physical weathering and chemical weathering.

Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.

Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar. Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture. Chemical feed weathering also causes the exit of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface and groundwater water. These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.

Erosion is the process that removes lose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.

Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can lend support to lose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes. The forces of the running water removes lose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods. Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens ever more material so the process continues.

Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds of a functional dynamic spread of ice. For major glaciers to form, this lack of snowmelt has to occur for many years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), Arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the results of glacial erosion. Wind is an important cause of erosion only in arid (dry) regions. Wind carries sand and dust, which can scour even solid rock. Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering. An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water. The higher the elevation the more energetic water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.

A landscape's geologic age-that is, how long current conditions of weathering and erosion have affected the area-determines its overall appearance. Younger landscapes tend to be more rugged and angular in appearance. Older landscapes have a tendency to have more rounded slopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.

Opposing this tendency toward a levelling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth's surface occur in the outermost solid portion of Earth, known as the lithosphere. The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km. (40 to 60 mi.) thick. Compared with the interior of the Earth, however, this region is moderately thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.

Scientists believe that the lithosphere is broken into a series of plates, or segments. According to the theory of plate tectonics, these plates move around on Earth's surface over long periods. Tectonics comes from the Greek word, tektonikos, which means ‘builder'.

According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Coco's plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.

These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways. The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions and mountain building and the formation of the oceans and continents.

When the plates pull apart, two types of phenomena come about, depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form. An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new sea-floor is created at the site of these ridges. Rift valleys are also present along the crests of the mid-ocean ridges.

Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new sea-floor. According to this explanation, molten rock known as magma rises from Earth's interior to form hot spots beneath the ocean floor. As two oceanic plates pull apart from each other in the middle of the oceans, a crack, or rupture, appear and forms the mid-ocean ridges. These ridges exist in all the worlds' ocean basins and resemble the seams of a baseball. The molten rock rises through these cracks and creates new sea-floor.

When plates collide or push against each other, regions called convergent plate margins form. Along these margins, one plate is usually forced to dive below the other. As that plate dives, it triggers the melting of the surrounding lithosphere and a region just below is known as the asthenosphere. These pockets of molten crust rise behind the margin through the overlying plate, creating curved chains of volcanoes known as arcs. This process is called Subduction.

If one plate consists of oceanic crust and the other consists of continental crust, the denser oceanic crust will dive below the continental crust. If both plates are oceanic crust, then either may be sub-ducted. If both are continental crust, Subduction can continue for a brief while but will eventually ends because continental crust is not dense enough to be forced very far into the upper mantle.

The results of this Subduction process are readily visible on a map showing that 80 percent of the world's volcanoes rim the Pacific Ocean where plates are colliding against each other. The Subduction zone created by the collision of two oceanic plates-the Pacific plate and the Philippine plate-can also create a trench. Such a trench resulted in the formation of the deepest point on Earth, the Mariana Trench, which is estimated to be 11,033 m's (36,198 ft) below sea level.

On the other hand, when two continental plates collide, mountain building occurs. The collision of the Indo-Australian plate with the Eurasian plate has produced the Himalayan Mountains. This collision resulted in the highest point of Earth, Mount Everest, which is 8,850 m's (29,035 ft) above sea level.

Finally, some of Earth's plates neither collide nor pull apart yet slips past each other. These regions are convened by the transforming margins. Few volcanoes occur in these areas because neither plate is forced down into Earth's interior and little melting occurs. Earthquakes, however, are abundant as the two rigid plates slide past each other. The San Andreas Fault in California is a well-known example of a transformed margin.

The movement of plates occurs at a slow pace, at an average rate of only 2.5 cm (one in) per year. Still, over millions of years this gradual movement results in radical changes. Current plate movement is making the Pacific Ocean and Mediterranean Sea smaller, the Atlantic Ocean larger, and the Himalayan Mountains higher.

The interior of Earth plays an important role in plate tectonics. Scientists believe it is also responsible for Earth's magnetic field. This field is vital to life because it shields the planet's surface from harmful cosmic rays and from a steady stream of energetic particles from the Sun known as the solar wind.

Earth's interior consists of the mantle and the core. The mantle and core make up by far the largest part of Earth's mass. The distance from the base of the crust to the centre of the core is about 6,400 km (about 4,000 mi).

Scientists have learned about Earth's interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth's interior. Finally, seismic waves generated by earthquakes send geophysicists information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth as waves. The way these waves proceed through the interior of Earth reveals the nature of materials inside the planet.

The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km (about 1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock. The term asthenosphere is derived from Greek and means ‘a weak layer'. The asthenosphere's soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere's plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.

The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a ‘sticky' liquid when it is subjected to enough heat and pressure.

The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km (about 1,404 mi) thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km (about 758 mi) thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one source of heat in Earth's interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C's (12,000°F).

Scientists believe that Earth's liquid iron core aids to make over a magnetic field that surrounds Earth and shields the planet from harmful cosmic rays and the Sun's solar wind. The idea that Earth is like a giant magnet was first proposed in 1600 by English physician and natural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetized needle in a compass point north. According to Gilbert, Earth's magnetic field creates a magnetic north pole and a magnetic south pole. The magnetic poles do not correspond to the geographic North and South poles, however. Moreover, the magnetic poles wander and are not always in the same place. The north magnetic pole is currently close to Ellef Ringnes Island in the Queen Elizabeth Islands near the boundary of Canada's Northwest Territories with Nunavut. The magnetic south poles lies just off the coast of Wilkes Land, Antarctica.

Not only do the magnetic poles wander, but they also reverse their polarity-that is, the north magnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals have occurred at least 170 times over the past 100 million years. The reversals occur on average about every 200,000 years and take place gradually over a period of several thousand years. Scientists still do not understand why these magnetic reversals occur but think they may be related to Earth's rotation and changes in the flow of liquid iron in the outer core.

Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth's magnetic field. Known as the dynamo theory, this theory may be the best explanation yet for the origin of the magnetic field. Earth's magnetic field operates in a region above Earth's surface known as the magnetosphere. The magnetosphere is shaped in some respects like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.

Inside the magnetosphere are the Van Allen's radiation belts, named for the American physicist James A. Van Allen who discovered them in 1958. The Van Allen belts are regions where charged particles from the Sun and from cosmic rays are trapped and sent into spiral paths resembling Earth's magnetic field. The radiation belts by that shield Earth's surface from these highly energetic particles. Occasionally, however, due to extremely strong magnetic fields on the Sun's surface, which are visible as sunspots, a brief burst of highly energetic particles streams along with the solar wind. Because Earth's magnetic field lines converge and are closest to the surface at the poles, some of these energetic particles sneak through and interact with Earth's atmosphere, creating the phenomenon known. Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system took form of about 4.6 billion years. Originating endurably in some lengthily endurance from dust and giant gaseous particles-wave substances known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in an explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.

Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the s of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen s in the Sun is the source of its energy.

Many scientists favour the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.

According to this theory, temperatures decreased with increasing distance from the centre of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense of departing-that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets-Jupiter and Saturn-are by and large made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.

Within the planetesimal Earth, heavier matter sank to the centre and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the centre contained radioactive material, an important source of Earth's internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.

Not long after they formed, more than four billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon's surface, which is unchanged. Earth's craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.

Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By four billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so existing was impossible for life.

Geologists divide the history of the Earth into three eons: the Archaean Eon, which lasted from around four billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.

The Archaean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archaean is generally dated as the age of the oldest terrestrial rocks, which are about four billion years old. The Archaean Eon came to an end 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.

Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Residing until the late 1800s scientists could only determine the relative ages of rock strata. They knew that overall the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.

The study of fossils also enabled geologists to set the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.

Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could detect an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists can assemble Earth's geologic time scale from the Archaean Eon to the present.

The Precambrian is a time span that includes the Archaean and Proterozoic eons began roughly four billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth's history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about four billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.

The Precambrian is also the longest chapter in Earth's history, spanning a period of about 3.5 billion years. During this time frame, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet because of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapour. As Earth continued to cool, the water vapour condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth's water vapour originally came from comets containing frozen water that struck Earth during meteorite bombardment.

By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth's surface as early as the middle Precambrian. About a billion years ago, the Earth's plates were entered around the South Pole and formed a super-continent called Rodinia. Slowly, pieces of this super-continent broke away from the central continent and travelled north, forming smaller continents.

Life originated during the Precambrian. The earliest fossil evidence of life consists of Prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote's hereditary material was copied unchanged. The first Prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, by 3.5 billion years ago, and where anaerobic—that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.

Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue

-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old.

For billions of years, life existed only in the simple form of Prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduces by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The inordinate branch of Precambrian time occurred some 560 million to 545 million years ago and seeing an appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils may be the precursors of many fossil groups that were to explode in Earth's oceans in the Paleozoic Era.

At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Most of the major groups of animals we know today made their first appearance during the Cambrian explosion. Most of the different ‘body plans' found in animals today-that is, the way of an animal's body is designed, with heads, legs, rear ends, claws, tentacles, or antennae-also originated during this period.

Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species can flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.

Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the super-continent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built several mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.

Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. Their slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the super-continent Pangaea and resulted in some grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented more than 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouth-like opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today's Mediterranean Sea.

The Paleozoic ended with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.

The Mesozoic Era, beginning 248 million years ago, is often characterized as the Age of Reptiles because reptiles were the dominant life forms during this era. Reptiles dominated not only on land, as dinosaurs, but also in the sea, as the plesiosaurs and ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles.

The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth's history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first birds and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.

The Mesozoic was an eventful era geologically with many changes to Earth's surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subduction zone extended along North America's western margin. This Subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. A great deal of this featured characteristic is called the American Cordillera, and exists today as the eastern margin of the Pacific Ring of Fire.

During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.

The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.

A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.

The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.

Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today shows that sea-floor spreading is still causing the country to grow.

Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.

The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.

With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid-1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth's immediate future depends largely on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet's surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth's early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.

Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.

In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun's mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth's oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun's outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth's rocks. Earth will end its existence as a burnt cinder.

Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey through and beyond the solar system and begin to colonize other planets in our galaxy, and find yet of another place to call ‘home'.

The Cenozoic era (65 million years ago to the present time) is divided into the Tertiary period (65 million to 1.6 million years ago) and the Quaternary period (1.6 million years ago to the present). However, because scientists have so much more information about this era, they tend to focus on the epochs that make up each period. During the first part of the Cenozoic era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic era disappeared

Index fossils of the Cenozoic tend to be microscopic, such as the tiny shells of foraminifera. They are commonly used, along with varieties of pollen fossils, to date the different rock strata of the Cenozoic era.

The Paleocene epoch (65 million to 55 million years ago) marks the beginning of the Cenozoic era. Seven groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Most of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, which were the ancestors of modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents

During the Eocene epoch (55 million to 38 million years ago), most direct evolutionary ancestors of modern animals appeared. Among these animals-all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern birds as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.

During the Oligocene epoch (38 million to 24 million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of many of modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.

The development of mammals during the Miocene epoch (24 million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.

The paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were basically modern in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, as well as now-extinct species such as the giant saber-toothed tiger. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the musk-ox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.

The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.

A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.

The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.

Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today suggests that sea-floor spreading be still causing the country to grow.

Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two or 1.7 million years ago.

The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.

Most biologists agree that animals evolved from simpler single-celled organisms. Exactly how this happened is unclear, because few fossils have been left to record the sequence of events. Faced with this lack of fossil evidence, researchers have attempted to piece together animal origins by examining the single-celled organisms alive today.

Modern single-celled organisms are classified into two kingdoms: the Prokaryotes and protists. Prokaryotes, which include bacteria, are very simple organisms, and lack many features seen in animal cells. Protists, on the other hand, are more complex, and their cells contain all the specialized structures, or organelles, found in the cells of animals. One protist group, the choanoflagellates or collar flagellates, contains organisms that bear a striking resemblance to cells that are found in sponges. Most choanoflagellates live on their own, but significantly, some form permanent groups or colonies.

This tendency to form colonies are widely believed to have been an important stepping stone on the path to animal life. The next step in evolution would have involved a transition from colonies of independent cells to colonies containing specialized cells that were dependent on each other for survival. Once this development had occurred, such colonies would have effectively become single organisms. Increasing specialization among groups of cells could then have created tissues, triggering the long and complex evolution of animal bodies.

This conjectural sequence of events probably occurred along several parallel paths. One path led to the sponges, which retain a collection of primitive features that set them apart from all animals. Another path led to two major subdivisions of the animal kingdom: the Protostomes, which include arthropods, annelid worms, mollusks, and cnidarians; and the deuterostomes, which include echinoderms and chordates. Protostomes and deuterostomes differ fundamentally in the way they develop as embryos, strongly suggesting that they split from each other a long time ago.

Animal life first appeared perhaps a billion years ago, but for a long time after this, the fossil record remains almost blank. Fossils exist that seem to show burrows and other indirect evidence for animal life, but the first direct evidence of animals themselves appears about 650 million years ago, toward the end of the Precambrian period. At this time, the animal kingdom stood on the threshold of a great explosion in diversity. By the end of the Cambrian Period, 150 million years later, all of the main types of animal life existing today had become established.

When the first animals evolved, dry land was probably without any kind of life, except possibly bacteria. Without terrestrial plants, land-based animals would have had nothing to eat. Nevertheless, when plants took up life on land more than 400 million years ago, that situation changed, and animals evolved that could use this new source of food. The first land animals included primitive wingless insects and probably a range of soft-bodied invertebrates that have not left fossil remains. The first vertebrates to move onto land were the amphibians, which appeared about 370 million years ago.

For all animals, life on land involved meeting some major challenges. Foremost among these was the need to conserve water and the need to extract oxygen from the air. Another problem concerned the effects of gravity. Water buoys of living things, but air, which is 750 times less dense than water, generates almost no buoyancy at all. To function effectively on land, animals needed support.

In soft-bodied land animals such as earthworms, this support is provided by a hydrostatic skeleton, which works by internal pressure. The animal's body fluids press out against its skin, giving the animal its shape. In insects and other arthropods, support is provided by the exoskeleton (external skeletons), while in vertebrates it is provided by bones. Exoskeletons can play a double role by helping animals to conserve water, but they have one important disadvantage: unlike an internal bony skeleton, their weight increases very rapidly as they get bigger, eventually making them too heavy to move. This explains why insects have all remained relatively small, while some vertebrates have reached very large sizes.

Like other living things, animals evolve by adapting to and exploiting their surroundings. In the billion-year history of animal life, this process could use resources in a different way. Some of these species are surviving today, but these are a minority; an even greater number are extinct, having lost the struggle for survival

Speciation, the birth of new species, usually occurs when a group of living things becomes isolated from others of their kind. Once this has occurred, the members of the group follow their own evolutionary path and adapt in ways that make them increasingly distinct. After a long period-typically thousand of the years-unique features were to mean that they can no longer breed within the former circle of relative relations. At this point, a new species comes into being.

In animals, this isolation can come about in several different ways. The simplest form, geographical isolation, occurs when members of an original species become separated by a physical barrier. One example of such a barrier is the open sea, which isolates animals that have been accidentally stranded on remote islands. As the new arrivals adapt to their adopted home, they become ever more distinct from their mainland relatives. Sometimes the result is a burst of adaptive radiation, which produces several different species. In the Hawaiian Islands, for example, 22 species of honey-creepers have evolved from a single pioneering species of a finch-like bird.

Another type of isolation is thought to occur where there is no physical separation. Here, differences in behaviour, such as mate selection, may sometimes help to split a single species into distinct groups. If the differences persist for a some duration, in that they live long enough new species are created.

The fate of a new species depends very much on the environment in which it evolved. If the environment is stable and no new competitors appear on the scene, an animal species may change very little in hundreds of thousands of years. Nevertheless, if the environment changes rapidly and competitors arrive from outside, the struggle for survival is much more intense. In these conditions, either a species change, or it eventually becomes extinct.

During the history of animal life, on at least five occasions, sudden environmental change has triggered simultaneous extinction on a massive scale. One of these mass extinctions occurred at the end of the Cretaceous Period, about 65 million years ago, killing all dinosaurs and perhaps two-thirds of marine species. An even greater mass extinction took place at the end of the Permian Period, about 200 million years ago. Many biologists believe that we are at present living in a sixth period of mass extinction, this time triggered by human beings.

Compared with plants, animals make up only a small part of the total mass of living matter on earth. Despite this, they play an important part in shaping and maintaining natural environments.

Many habitats are directly influenced by the way animals live. Grasslands, for example, exist partly because grasses and grazing animals have evolved a close partnership, which prevents other plants from taking hold. Tropical forests also owe their existence to animals, because most of their trees rely on animals to distribute their pollen and seeds. Soil is partly the result of animal activity, because earthworms and other invertebrates help to break down dead remains and recycle the nutrients that they contain. Without its animal life, the soil would soon become compacted and infertile.

By preying on each other, animals also help to keep their own numbers in check. This prevents abrupt population peaks and crashes and helps to give living systems a built-in stability. On a global scale, animals also influence some of the nutrient cycles on which almost all life depends. They distribute essential mineral elements in their waste, and they help to replenish the atmosphere's carbon dioxide when they breathe. This carbon dioxide is then used by plants as they grow.

Until relatively recently in human history, people existed as nomadic hunter-gatherers. They used animals primarily as a source of food and for raw materials that could be used for making tools and clothes. By today's standards, hunter-gatherers were equipped with rudimentary weapons, but they still had a major impact on the numbers of some species. Many scientists believe, for example, that humans were involved in a cluster of extinctions that occurred about 12,000 years ago in North America. In less than a millennium, two-thirds of the continent's large mammal species disappeared.

This simple relationship between people and animals changed with domestication, which also began about 12,000 years ago. Instead of being actively hunted, domesticated animals were slowly brought under human control. Some were kept for food or for clothing, others for muscle power, and some simply for companionship.

The first animal to be domesticated was almost certainly the dog, which was bred from wolves. It was followed by species such as the cat, horse, camel, llama, and aurochs (a species of wild cattle), and by the Asian jungle fowl, which is the ancestor of today's chickens. Through selective breeding, each of these animals has been turned into forms that are particularly suitable for human use. Today, many domesticated animals, including chickens, vastly outnumber their wild counterparts. Sometimes, such as the horse, the original wild species has died out together.

Over the centuries, many domesticated animals have been introduced into different parts of the world only to escape and establish themselves in the wild. With stowaway pests such as rats, these ‘feral' animals have often affected native wildlife. Cats, for example, have inflicted great damage on Australia's smaller marsupials, and feral pigs and goats continue to be serious problems for the native wildlife of the Galápagos Islands.

Despite the growth of domestication, humans continue to hunt some wild animals. Some forms of hunting are carried out mainly for sport, but others provide food or animal products. Until recently, one of the most significant of these forms of hunting was whaling, which reduced many whale stocks to the brink of extinction. Today, highly efficient sea fishing threatens some species of fish with the same fate since the beginning of agriculture. The human population has increased by more than two thousand times. To provide the land needed for growing food and housing people, large areas of the earth's landscapes have been completely transformed. Forests have been cut down, wetlands drained, and deserts irrigated, reducing these natural habitats to a fraction of their former extent.

Some species of animals have managed to adapt to these changes. A few, such as the brown rat, raccoon, and house sparrow, have benefited by exploiting the new opportunities that have opened and have successfully taken up life on farms, or in towns and cities. Nonetheless, most animals have specialized ways of life that make them dependent on a particular kind of habitat. With the destruction of their habitats, their number inevitably declines.

In the 20th century, animals have also had to face additional threats from human activities. Foremost among these are environmental pollution and the increasing demand for resources such as timber and fresh water. For some animals, the combination of these changes has proved so damaging that their numbers are now below the level needed to guarantee survival.

Across the world, efforts are currently underway to address this urgent problem. In the most extreme cases, gravely threatened animals can be helped by taking them into captivity and then releasing them once breeding programs have increased their number. One species saved in this way is the Hawaiian mountain goose or n ? n ? . In 1951, its population had been reduced to just 33. Captive breeding has since increased the population to more than 2500, removing the immediate threat of extinction.

While captive breeding is a useful emergency measure, it cannot assure the long-term survival of a species. Today animal protection focuses primarily on the preservation of entire habitats, an approach that maintains the necessary links between the different species the habitats support. With the continued growth in the world's human population, habitat preservation will require a sustained reduction in our use of the world's resources to minimize our impact on the natural world.

Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how the earth evolved. The study and comparison of different strata are called stratigraphy.

Fossils provide for most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one-that is, they represent a species that was widespread but existed for a brief period of time. The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 570 million years of the earth's history into eras, periods, and epochs. The part of the earth's history before about 570 million years ago is called Precambrian time, which began with the earth's birth, probably more than four billion years ago.

The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.6 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils. The first abundant fossils of larger animals date from about 600 million years ago.

At first glance, the sudden jump from 8000 Bc to 10,000 years ago looks peculiar. On reflection, however, the time-line has clearly not lost 2,000 years. Rather, the time-line has merely shifted from one convention of measuring time to another. To understand the reasons for this shift, it will help to understand some of the different conventions used to measure time.

All human societies have faced the need to measure time. Today, for most practical purposes, we keep track of time with the aid of calendars, which are widely and readily available in printed and computerized forms throughout the world. However, long before humans developed any formal calendar, they measured time based on natural cycles: the seasons of the year, the waxing and waning of the moon, the rising and setting of the sun. Understanding these rhythms of nature was necessary for humans so they could be successful in hunting animals, catching fish, and collecting edible nuts, berries, roots, and vegetable matter. The availability of these animals and plants varied with the seasons, so early humans needed at least a practical working knowledge of the seasons to eat. When humans eventually developed agricultural societies, it became crucial for farmers to know when to plant their seeds and harvest their crops. To ensure that farmers had access to reliable knowledge of the seasons, early agricultural societies in Mesopotamia, Egypt, China, and other lands supported specialists who kept track of the seasons and created the world's first calendars. The earliest surviving calendars date from around 2400 Bc.

As societies became more complex, they required increasingly precise ways to measure and record increments of time. For example, some of the earliest written documents recorded tax payments and sales transactions, and indicating when they took place was important. Otherwise, anyone reviewing the documents later would find it impossible to determine the status of an individual account. Without any general convention for measuring time, scribes (persons who wrote documents) often dated events by the reigns of local rulers. In other words, a scribe might indicate that an individual's tax payment arrived in the third year of the reign (or third regnal years) of the Assyrian ruler Tiglath-Pileser. By consulting and comparing such records, authorities could determine if the individual were up to date in tax payments.

These days, scholars and the public alike refer to time on many different levels, and they consider events and processes that took place at any times, from the big bang to the present. Meaningful discussion of the past depends on some generally observed frames of reference that organize time coherently and allow us to understand the chronological relationships between historical events and processes.

For contemporary events, the most common frame of reference is the Gregorian calendar, which organizes time around the supposed birth date of Jesus of Nazareth. This calendar refers to dates before Jesus' birth as Bc (‘before Christ') and those afterwards as ad (anno Domini, Latin for ‘in the year of the Lord'). Scholars now believe that Jesus was born four to six years before the year recognized as ad one in the Gregorian calendar, so this division of time is probably off its intended mark by a few years. Nonetheless, even overlooking this point, the Gregorian calendar is not meaningful or useful for references to events in the so-called deep past, a period so long ago that to be very precise about dates is impossible. Saying that the big bang took place in the year 15,000,000,000 Bc would be misleading, for example. No one knows exactly when the big bang took place, and even if someone did, there would be little point in dating that moment and everything that followed from it according to an event that took place some 14,999,998,000 years later. For purposes of dating events and processes in the deep past and remote prehistory, then, scientists and historians have adopted different principles of measuring time.

In conventional usage, prehistory refers to the period before humans developed systems for writing, while the historical era refers to the period after written documents became available. This usage became common in the 19th century, when professional historians began to base their studies of the past largely on written documentation. Historians regarded written source materials as more reliable than the artistic and artifactual evidence studied by archaeologists working on prehistoric times. Recently, however, the distinction between prehistory and the historical era has become much more blurred than it was in the 19th century. Archaeologists have unearthed rich collections of artifacts that throw considerable light on so-called prehistoric societies. When, contemporary historians realize much better than did their predecessors that written documentary evidence raises as many questions as it does answers. In any case, written documents illuminate only selected dimensions of experience. Despite these nuances of historical scholarship, for purposes of dating events and processes in times past, the distinction between the term's prehistory and the historical era remains useful. For the deep past and prehistory, establishing precise dates is rarely possible: Only in the cases of a few natural and celestial phenomena, such as eclipses and appearances of comets, are scientists able to infer relatively precise dates. For the historical era, on the other hand, precise dates can be established for many events and processes, although certainly not for all.

Since the Gregorian calendar is not especially useful for dating events in the distant period long before the historical era, many scientists who study the deep past refer not to years ‘Bc' or AD' but to years ‘before the present'. Astronomers and physicists, for example, believe the big bang took place between 10 billion and 20 billion years ago, and that planet Earth came into being about 4.65 billion years ago. When dealing with Earth's physical history and life forms, geologists often dispense with year references together and divide time into alternate spans of time. These time spans are conventionally called eons (the longest span), eras, periods, and epochs (the shortest span). Since obtaining precise dates for distant times is impossible, they simply refer to the Proterozoic Eon (2.5 billion to 570 million years ago), the Mesozoic Era (240 million to 65 million years ago), the Jurassic Period (205 million to 138 million years ago), or the Pleistocene Epoch

(1.6 million to 10,000 years ago).

Because the Pleistocene Epoch is a comparatively recent time span, archaeologists and pre historians are frequently able to assign at least approximate year dates to artifacts from that period. As with all dates in the distant past, however, it would be misleading to follow the principles of the Gregorian calendar and refer to dates' Bc. As a result, archaeologists and pre-historians often call these dates' bp (‘before the present'), with the understanding that all dates bp are approximate. Thus, scholars date the evolution of The Homo sapiens to about 130,000 bp and the famous cave paintings at Lascaux in southern France to about 15,000 Bc.

The Dynamic Timeline, of which all date before 8000 Bc refers to dates before the present, and all dates since 8000 Bc categorizes time according to the Gregorian calendar. Thus, a backward scroll in the time-line will take users from 7700 Bc to 7800 Bc, 7900 Bc, and 8000 Bc to 10,000 years ago. Note that the time-line has not lost 2,000 years! To date events this far back in time, the Dynamic Timeline has simply switched to a different convention of designating the dates of historical events.

Written documentation enables historians to establish relatively precise dates of events in the historical era. However, placing these events in chronological order requires some agreed upon starting points for a frame of reference. For purposes of maintaining proper tax accounts in a Mesopotamian city-state, dating an event in relation to the first year of a king's reign might be sufficient. For purposes of understanding the development of entire peoples or societies or regions, however, a collection of dates according to the regnal years of many different local rulers would quickly become confusing. Within a given region there might be many different local rulers, so efforts to establish the chronological relationship between events may entail an extremely tedious collation of all the rulers' regnal years. Thus, to facilitate the understanding of chronological relationships between events in different jurisdictions, some larger frame of reference is necessary. Most commonly these larger frames of reference take the form of calendars, which not only make it possible to predict changes in the seasons but also enable users to organize their understanding of time and appreciate the relationships between datable events.

Different civilizations have devised thousands of different calendars. Of the 40 or so calendars employed in the world today, the most widely used is the Gregorian calendar, introduced in 1582 by Pope Gregory XIII. The Gregorian calendar revised the Julian calendar, instituted by Julius Caesar in 45 Bc, to bring it closer in line with the seasons. Most Roman Catholic lands accepted the Gregorian calendar upon its promulgation by Gregory in 1582, but other lands adopted it much later: Britain in 1752, Russia in 1918, and Greece in 1923. During the 20th century it became the dominant calendar throughout the world, especially for purposes of international business and diplomacy.

Despite the prominence of the Gregorian calendar in the modern world, millions of people use other calendars as well. The oldest calendar still in use is the Jewish calendar, which dates' time from the creation of the world in the (Gregorian) year 3761 Bc, according to the Hebrew scriptures. The year 2000Bc. in the Gregorian calendar thus corresponding to the year am 5761 in the Jewish calendar (am stands for anno Mundi, Latin for ‘the year of the world'). The Jewish calendar is the official calendar of Israel, and it also serves as a religious calendar for Jews worldwide.

The Chinese use another calendar, which, as tradition holds, takes its point of departure in the year 2697 Bc in honour of a beneficent ruler's work. The year AD 2000 of the Gregorian calendar, and with that it corresponds to the year 4697 in the Chinese calendar. The Maya calendar began even earlier than the Chinese-August 11, 3114 Bc. Maya scribes calculated that this is when the cycle of time began. The Maya actually used two interlocking calendars-one a 365-day calendar based on the cycles of the sun, the other a sacred almanac used to calculate auspicious or unlucky days. Despite the importance of these calendars to the Maya civilization, the calendars passed out of general use after the Spanish conquest of Mexico in the 16th century AD.

The youngest calendar in widespread use today is the Islamic lunar calendar, which begins the day after the Hegira, Muhammad's migration from Mecca to Medina in ad 622. The Islamic calendar is the official calendar in many Muslim lands, and it governs religious observances for Muslims worldwide. Since it reckons time according too lunar rather than solar cycles, the Islamic calendar does not neatly correspond to the Gregorian and other solar calendars. For example, although there were 1,378 solar years between Muhammad's Hegira and AD 2000, that year corresponds to the year 1420 in the Islamic calendar. Like the Gregorian calendar and despite their many differences, the Jewish, Chinese, and Islamic calendars all make it possible to place individual datable events in proper chronological order.

Recently, controversies have arisen concerning the Gregorian calendar's designation of Bc and ad to indicate years before and after the birth of Jesus Christ. This practice originated in the 6th century ad with a Christian monk named Dionysius Exiguus. Like other devout Christians, Dionysius regarded the birth of Jesus as the singular turning point of history. Accordingly, he introduced a system that referred to events in time based on the number of years they occurred before or after Jesus' birth. The system caught on very slowly. Saint Bede the Venerable, a prominent English monk and historian, employed the system in his own works in the 8th century ad, but the system came into general use only about AD 1400. (Until then, Christians generally calculated time according to regnal years of prominent rulers.) When Pope Gregory XIII ordered the preparation of a new calendar in the 16th century, he intended it to serve as a religious calendar as well as a tool for predicting seasonal changes. As leader of the Roman Catholic Church, Pope Gregory considered it proper to continue recognizing Jesus' birth as the turning point of history.

As lands throughout the world adopted the Gregorian calendar, however, the specifically Christian implications of the term's Bc and ad did not seem appropriate for use by non-Christians. Really, they did not even seem appropriate to many Christians when dates referred to events in non-Christian societies. Why should Buddhists, Hindus, Muslims, or others date time according to the birth of Jesus? In saving the Gregorian calendar as a widely observed international standard for reckoning time, while also avoiding the specifically Christian implications of the qualification's Bc and ad, scholars replaced the birth of Jesus with the notion of ‘the common era' and began to qualify dates as BCE (‘before the common era') or Ce ("in the common era"). For the practical purpose of organizing time, BCE is the exact equivalent of Bc, and Ce is the exact equivalent of AD, but the term's BCE and Ce have very different connotations than do Bc and AD.

The qualification's BCE and Ce first came into general use after World War II (1939-1945) among biblical scholars, particularly those who studied Judaism and early Christianity in the period from the 1st century Bc (or BCE) and the 1st century ad (or Ce). From their viewpoint, this "common era" was an age when proponents of Jewish, Christian, and other religious faiths intensively interacted and debated with one another. Using the designations, BCE and Ce enabled them to continue employing a calendar familiar to them all while avoiding the suggestion that all historical time revolved around the birth of Jesus Christ. As the Gregorian calendar became prominent throughout the world in the 20th century, many peoples were eager to find terms more appealing to them than Bc and ad, and accordingly, the BCE and Ce usage became increasingly popular. This usage represents only the most recent of many efforts by the world's peoples to devise meaningful frameworks of time.

Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system formed about 4.6 billion years ago from a giant cloud of gas and dust known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in an explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.

Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the s of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen s in the Sun is the source of its energy.

Many scientists favour the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.

According to this theory, temperatures decreased with increasing distance from the centre of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense out-that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets-Jupiter and Saturn-are mostly made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.

Within the planetesimal Earth, heavier matter sank to the centre and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the centre contained radioactive material, an important source of Earth's internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.

Not long after they formed, more than four billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon's surface, which is unchanged. Earth's craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.

Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By four billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so existing was impossible for life.

Geologists divide the history of the Earth into three eons: the Archaean Eon, which lasted from around four billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.

The Archaean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archaean is generally dated as the age of the oldest terrestrial rocks, which are about four billion years old. The Archaean Eon ended 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.

Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Until the late 1800s scientists could only determine the relative age of rock strata, or layering. They knew that overall the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.

The study of fossils also enabled geologists to determine the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.

Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could determine an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists were able to assemble Earth's geologic time scale from the Archaean Eon to the present.

The Precambrian is a time span that includes the Archaean and Proterozoic eons and began about four billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth's history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about four billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to

4.2 billion years.

The Precambrian is also the longest chapter in Earth's history, spanning a period of about 3.5 billion years. During this time frame, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet because of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapour. As Earth continued to cool, the water vapour condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth's water vapour originally came from comets containing frozen water that struck Earth during meteorite bombardment.

By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth's surface as early as the middle Precambrian. About a billion years ago, the Earth's plates were entered around the South Pole and formed a super-continent called Rodinia. Slowly, pieces of this super-continent broke away from the central continent and travelled north, forming smaller continents.

Life originated during the Precambrian. The earliest fossil evidence of life consists of Prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote's hereditary material was copied unchanged. The first Prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but certainly by 3.5 billion years ago, and where anaerobic-that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.

Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old

The colonization of Australia/New Guinea was not achieved until the time to which took off around 50,000 years ago. Another extension of human range that soon followed as the one into th coldest parts of Eurasia. While Neanderthals lived in glacial times and were adapted to the cold, they penetrated no farther north than Germany and Kiev. That's not surprising, since Neanderthals apparently lacked needles, sewn clothing, warm houses, and other technological essentials of survival in the coldest climates. Anatomically modern peoples who possess such technology had expanded into Siberia by around 20,000 years ago (there are the usual much olde disputed claims). That expansion may have been responsible for the extinctions of Eurasia's wooly mammoth and wooly rhinoceroses.

With the settlement of Austral/New Guinea, humans now occupied three of the five habitable continents. However, Antartica because it was not reached by humans until the 19th century and has two continents, North America and South America. That left only two continents, North America and South America. They were surely the last ones settled, for the obvious reason tat reaching the Americas fro the Old World required either boats (for which there is no evidence even in Indonesia until 40,000 years ago and nine in Europe until much later) in order to cross by sea, or else it required the occupation of Siberia (unoccupied until about 20,000 years ago) ib order to cross the Bering land bridge.

However, it is uncertain when, between about 14,000 and 35,000 years ago, the Americas were first colonized. The oldest unquestionable human remains in the Americas are at sites in Alaska dated around 12,000 Bc., followed by a profusion of sites in the United States south of the Canadian border and in Mexico in the centuries just before 11,000 Bc. The latter sites are called Clovis sites, named just after the type site near the town of Clovis, New Mexico, where there characteristic large stone spearpoints were first recognized. Hundreds of Clovis sites are now known, blanketing all 48 of the lower U.S. states south into Mexico. Unquestioned and in Patagonia. These facts suggest the interpretation that Clovis sites document the America's first colonized by people, who quickly multiplied, expanded, and filled the two continents.

Nevertheless, it may be all the same, that differences between the long-term histories of peoples of the different continents have been due not to innate differences in the people themselves but to differences in their environments. That is to say, that if the populations of Aboriginal Australia and Eurasia could have been interchanged during the Late Pleistocene, the original Aboriginal Australia would no be the ones occupying most of the Americas and Australia, we well as Eurasia, while the original Aboriginal and Australia, as well as Eurasia, while the original Aboriginal Eurasians would be the ones now reduced to a downtrodden population fragment in Australia. One might at first be inclined to dismiss this assertion as meaningless, because the excrement is imaginary and claims itself its outcome that cannot be verified, but historians are nonetheless able to evaluate related hypotheses by retrospective tests. For instance, one can examine what did happen when European farmers were transplanted to Greenland or the U.S. Great Plains, and when farmers stemming ultimately from China emigrated to the Chatha Islands, the rain forests of Borneo, or the volcanic soil o Java or Hawaii. These tests confirm that the same ancestral peoples either ended up extinct, or returned to living as hunter-gatherers, or went on to build complex states, depending on their environments., similarly, Aboriginal Australian hunter-gatherers, variously transplanted to Finders Island, Tasmania, or southeastern Australia, ended up extinct, or as canal builders intensively managing a productive fishery, depending on their continents.

Of course, the continents differ in innumerable environmental features affecting trajectories of human societies. But merely a laundry list of ever possible difference does not constitute any one answer. Just four sets of differences appear as considered being the most important ones.

The fist set consists of continental difference in the wild plant and anal species available as starting materials for domestication. That's because food production was critical for the accumulation of food surpluses that could feed non-food producing specialists, and for the buildup of large populations enjoying a military advantage though mere numbers even before they had developed any technological or political advantage.

On each continent, animal and plant domestication was concentrated in a few especially favourable homelands' accounting for only a small fraction of the continent's total area. In the case of technical innovations and political institutions as well, most societies acquire much more from other societies than they invent themselves. Thus diffusion and migration within a continent contribute importantly the development of its societies, which tend in the log run to share each other's development (insofar as environments permit) because of the processes illustrated in much more form by Maori New Zealand's Musket Wars. That is, societies initially lacking an advantage ether acquire it from societies possessing it or (if they fail to do so) are replaced by those other society.

Even so, for billions of years, life existed only in the simple form of Prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduces by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils are the precursors of many fossil groups that were to explode in Earth's oceans in the Paleozoic Era.

At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Almost all of the major groups of animals we know today made their first appearance during the Cambrian explosion. Almost all of the different ‘body plans' found in animals today-that is, the way and animal's body is designed, with heads, legs, rear ends, claws, tentacles, or antennae-also originated during this period.

Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species were able to flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.

Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the super-continent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some of the continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built many mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.

Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. They're slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the super-continent Pangaea and resulted in some of the grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented more than 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouth like opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today's Mediterranean Sea.

The Paleozoic ended with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.

The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic period (240 million to 205 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with heavy, fleshy tails, and seldom exceeded 4.5 m's (15 ft) in length. Other reptiles of the Triassic period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.

The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small in size and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.

The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth's history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.

The Mesozoic was an eventful era geologically with many changes to Earth's surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subductions zone extended along North America's western margin. This Subductions zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. A great deal of this feature, called the American Cordillera, exists today as the eastern margin of the Pacific Ring of Fire.

During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.

The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.

A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.

The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.

Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that sea-floor spreading is still causing the country to grow.

Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.

The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.

With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid 1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth's immediate future depends largely on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet's surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth's early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.

Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.

In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun's mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth's oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun's outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth's rocks. Earth will end its existence as a burnt cinder.

Or, perhaps, that a single catastrophic event had been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico's Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.

The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.

Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today's Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that sea-floor spreading is still causing the country to grow.

Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.

The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth's 4.6-billion-year history, within the past 200,000 years.

With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid 1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth's immediate future depends mainly on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet's surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth's early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.

Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.

In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun's mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth's oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun's outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth's rocks. Earth will end its existence as a burnt cinder.

Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey beyond the solar system to colonize other planets in the Milky Way Galaxy and find among other different places to call ‘home'.

The dinosaurs were one of a group of extinct reptiles that lived from about 230 million to about sixty-five million years ago. British anist Sir Richard Owen coined the word dinosaur in 1842, derived from the Greek words' deinos, meaning ‘marvellous' or ‘terrible', and sauros, meaning ‘lizard'. For more than 140 million years, dinosaurs reigned as the dominant on land.

Owen distinguished dinosaurs from other prehistoric reptiles by their upright rather than sprawling legs and by the presence of three or more vertebrae supporting the pelvis, or hipbone. They classify dinosaurs into two orders according to differences in pelvic structure: Saurischia, or lizard-hipped dinosaurs, and Ornithischia, or bird-hipped dinosaurs. Dinosaur bones occur in sediments deposited during the Mesozoic Era, the so-called era of middle animals, also known as the age of reptiles. This era is divided into three periods: the Triassic (240 million to 205 million years ago), the Jurassic (205 million to 138 million years ago), and the Cretaceous (138 million to sixty-five million years ago).

Historical references to dinosaur bones may extend as far back as the 5th century Bc. Some scholars think that Greek historian Herodotus was referring to fossilized dinosaur skeletons and eggs when he described griffins—legendary beasts that were part eagle and part lions-guarding nests in central Asia. ‘Dragon bones' mentioned in a 3rd century ad text from China are thought to refer to bones of dinosaurs.

The first dinosaurs studied by paleontologists (scientists who study prehistoric life) were Megalosaurus and Iguanodon, whose partial bones were discovered early in the 19th century in England. The shape of their bones shows that these animals resembled large, land-dwelling reptiles. The teeth of Megalosaurus, which are pointed and have serrated edges, suggest that this animal was a flesh eater, while the flattened, grinding surfaces of Iguanodon teeth suggest that it was a plant eater. Megalosaurus lived during the Jurassic Period, and Iguanodon lived during the early part of the Cretaceous Period. Later in the 19th century, paleontologists collected and studied more comprehensive skeletons of related dinosaurs found in New Jersey. From these finds they learned that Megalosaurus and Iguanodon walked on two legs, not four, as had been thought.

Some ornithischians quickly became quadrupedal (four-legged) and relied on body armour and other physical defences rather than fleetness for protection. Plated dinosaurs, such as the massive Stegosaurus of the late Jurassic Period, bore a double row of triangular bony plates along their backs. These narrow plates contained tunnels through which blood vessels passed, allowing the animals to radiate excess body heat or to warm themselves in the sun. Many also bore a large spined plate over each shoulder. Stegosaurs resembled gigantic porcupines, and they probably defended themselves by turning their spined tails toward aggressors.

During the Cretaceous Period, stegosaurs were supplanted by armoured dinosaurs such as Ankylosaurus. These animals were similar in size to stegosaurs but otherwise resembled giant horned toads. Some even possessed a bony plate in each eyelid and large tail clubs. Their necks were protected by heavy, bony rings and spines, showing that these areas needed protection from the attacks of carnivorous dinosaurs.

The reptiles were still the dominant form of animal life in the Cretaceous period (138 million to 65 million years ago). The four types of dinosaurs found in the Jurassic also lived during this period, and a fifth type, the horned dinosaurs, also appeared. By the end of the Cretaceous, about 65 million years ago, all these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 15.5 m's (50 ft). Other reptiles of the period include the first snakes and lizards. Several types of Cretaceous have been discovered, including Hesperornis, a diving bird about 1.8 m's (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved. The most important evolutionary advance in the plant kingdom during the Cretaceous period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations. By the end of the period, many modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut. Some paleontologists believe that these deciduous woody plants first evolved in Jurassic times but grew only in upland areas, where conditions were unfavourable for fossil preservation. Becoming the most abundant plant-eating dinosaurs. They ranged in size from small runners that were two m's (6 ft) long and weighed 15 kg (33 lb), such as Hypsilophodon, to elephantine cows that were (32 ft) long and weighed 4 metric tons, such as Edmontosaurus. These animals had flexible jaws and grinding teeth, which eventually surpassed those of the modern cows in their suitability for chewing fibrous plants. The beaks of ornithopods became broader, earning them the name duck-billed dinosaur. Their tooth batteries became larger, their backs became stronger, and their forelimbs lengthened until their arms became elongated walking sticks, although ornithopods remained bipedal. The nose supported cartilaginous sacks or bony tubes, suggesting that these dinosaurs may have communicated by trumpeting. Fossil evidence from the late Cretaceous Period includes extensive accumulations of bones from ornithopods drowned in floods, indicating that duck-billed dinosaurs often migrated in herds of thousands. A few superbly preserved Edmontosaurus skeletons encased within impressions of skin have been discovered in southeastern Wyoming.

Pachycephalosaurs were small bipedal ornithischians with thickened skulls, flattened bodies, and tails surrounded by a latticework of bony rods. In many of these dinosaurs, such as the Pachycephalosaurus, -a large specimen up to eight m's (26 ft) a long-the skull was capped by a rounded dome of solid bone. Some paleontologists suggest that males may have borne the thickest domes and butted heads during mating contests. Eroded pachycephalosaur domes are often found in stream deposits from late in the Cretaceous Period.

The quadrupedal ceratopsians, or horned dinosaurs, typically bore horns over the nose and eyes, and had a saddle-shaped bony frill that extended from the skull over the neck. These bony frills were well developed in the late Cretaceous Triceratops, of which is a dinosaur that could reach lengths of up to eight m's (26 ft) and weighing more than 12 metric tons. The frill served two purposes: It protected the vulnerable neck, and it contained a network of blood vessels on its undersurface to radiate excess heat. Large accumulations of fossil bones suggest that ceratopsians lived in herds.

Controversy surrounds the extinction of the dinosaurs. According to one theory, dinosaurs were slowly driven to extinction by environmental changes linked to the gradual withdrawal of shallow seas from the continents at the end of the dinosaurian era. Proponents of this theory postulate that dinosaurs dwindled in number and variety over several million years.

An opposing theory proposes that the impact of an asteroid or comet caused catastrophic destruction of the environment, leading to the extinction of the dinosaurs. Evidence to support this theory includes the discovery of a buried impact crater (thought to be the result of a large comet striking the earth) that is 200 km (124 mi) in diameter in the Yucatán Peninsula of Mexico. A spray of debris, called an ejecta sheet, which was blown from the edge of the crater, has been found over vast regions of North America. Comet-enriched material from the impact's fiery explosion was distributed all over the world. With radiometric dating (Radiometric Dating), scientists have used the decay rates of certain s to date the crater, ejecta sheet, and fireball layer. Using similar techniques to date the dramatic changes in the record of microscopic fossils, they have found that the impact and the dinosaur extinction occurred nearly simultaneously.

Although large amounts of ash suggest that most of North and South America was devastated by fire from the impact, the longer-term planetwide environmental effects of the impact were ultimately more lethal to life than the fire. Dust blocked sunlight from the earth's surface for many months. Scorched sulfur from the impact site, water vapour and chlorine from the oceans, and nitrogen from the air combined to produce a worldwide fallout of intensely acidic rain. Scientists postulate that darkness and acid rain caused plant growth to cease. As a result, both the herbivorous dinosaurs, which were dependent on plants for food, and the carnivorous dinosaurs, which fed on the herbivores, were exterminated. On the other hand, animals such as frogs, lizards, and small insect-eating turtles and mammals, which were dependent on organisms that fed on decaying plant material, were more likely to survive. Their survival indicates that, in most areas, the surface of Earth did not freeze.

Fossilized dinosaur remains are usually buried in sediments deposited on land. These remains are likely to be found in regions where the silt and sands spread by rivers of the Mesozoic Era are exposed. Fossils are easier to find in arid badlands-rugged, rocky areas with little vegetation, where the sediments are not covered by soil. The excavation of large skeletal fossils involves painstaking procedures to protect the fossils from damage. Fewer than 3,000 dinosaur specimens have been collected to date, and only fifty skeletons of the 350 known varieties of dinosaurs are completely known. Probably less than 10 percent of the varieties of dinosaurs that once lived have been identified.

The shape of dinosaur bones provides clues to how these animals interacted with each other. These bones also reveal information about body form, weight, and posture. Surface ridges and hollows on bones indicate the strength and orientation of muscles, and rings within the bones indicate growth rates. Diseased, broken, and bitten bones bear witnesses to the hazards of life during the dinosaurian age. Cavities in bones reflect the shape of the brain, spinal cord, and blood vessels. Delicate ossicles, or small bony structures in the skull, reveal the shape of the eyeball and its pupil. The structure of the skull and fossilized contents of the abdominal region provide clues to diet.

Organic molecules are also preserved within bones in trace quantities. By studying isotopes of s within these molecules, scientists can gather evidence about body-heat flow and about the food and water consumed by dinosaurs. Impressions in sediment depict skin texture and foot shape, and trackways provide evidence about speed and walking habits.

A 113-million-year-old fossil called Scipionyx samniticus, discovered in southern Italy in the late 1980s, is the first fossil identified that clearly shows the structure and placement of internal organs, including the intestines, colon, liver, and muscles. The fossilized internal organs of Scipionyx samniticus give paleontologists information about how dinosaurs metabolized their food, and other general information about dinosaurs.

Beginning in the late 19th century, the field of paleontology grew as scientific expeditions to find fossil remains became more frequent. American paleontologist Othniel Charles Marsh and his collectors explored the western United States for dinosaurian remains. They identified many genera that have since become household names, including Stegosaurus and Triceratops. In the early part of the 20th century, American paleontologist's Barnum Brown and Charles Sternberg demonstrated that the area now known as Dinosaur Provincial Park in Alberta, Canada, is the richest site for dinosaur remains in the world. Philanthropist Andrew Carnegie-sponsored excavations in the great Jurassic quarry-pits in Utah, which subsequently turned into Dinosaur National Monument. Beginning in 1922, explorer Roy Chapman Andrews led expeditions to Mongolia that resulted in the discovery of dinosaur eggs. More recently, Luis Alvarez, a particle physicist and Nobel laureate, and his son, geologist Walter Alvarez, discovered evidence of the impact of an asteroid or comet debris that coincided with the extinction of the dinosaurs. Among foreign scholars, German paleontologist Werner Janensch, beginning in 1909, led well-organized dinosaur collecting expeditions to German East Africa (modern Tanzania), where the complete skeletal anatomy of the gigantic Brachiosaurus was documented.

One of the most important fossil-rich sites is located in China. In a small town about 400 km (about 200 mi) northeast of Beijing, there is a fossil formation, called the Yixian formation. Of which have yielded many fossilized specimens of primitive and bird-like dinosaurs, and soft parts such as feathers and fur. Some scientists believe these fossils provide evidence that may have evolved from dinosaurs. Among the recent finds in the Yixian formation is an eagle-sized animal with barracuda-like teeth and very long claws named Sinornithosaurus millenii. Although this dinosaur could not fly, it did have a shoulder blade structure in which allowed a wide range of arm motion similar to flapping. Featherlike structures covered most of the animal's body.

Another important dinosaur discovery made in 1993 strengthens the evolutionary relationship between dinosaurs and, A 14-year-old boy who was hunting for fossils near Glacier National Park in northern Montana found a fossil of a nearly complete skeleton of a small dinosaur, later named Bambiraptor feinbergi. The fossil is of a juvenile dinosaur only one m (3 ft) long with a body that resembles that of a roadrunner. It has several physical features similar to those of early, including long, winglike arms, bird-like shoulders, and a wishbone. Some scientists propose that Bambiraptor feinbergi may be a type of dinosaur similar to those from which evolved. Other scientists believe that the animal lived too late in time to be ancestral to, while still other scientists hypothesize that dinosaurs may have led to flying ancestral dinosaurs, from which more than once in evolutionary time.

Argentina is another area rich in fossils. In 1995 a local auto mechanic in Nequén, a province on the eastern slopes of the Andes in Argentina, found the fossils of Giganotosaurus, a meat-eating dinosaur that may have reached a length of more than 13 m's (43 ft). Five years later, in a nearby location, a team of researchers unearthed the bones of what could be the largest meat-eating dinosaur. The newly discovered species is related to the Giganotosaurus, but it was larger, reaching a length of 14 m's (45 ft). This dinosaur was heavier and had shorter legs than the Tyrannosaurus rex. The fossilized bones indicate that the dinosaur's jaw was shaped like scissors, suggesting it used its teeth to dissect prey.

In early 2000 AD., scientists used X-rays to view the chest cavity of a dinosaur fossil found in South Dakota. Computerized three-dimensional imaging revealed the remains of what is thought to be the first example of a dinosaur heart ever discovered. The heart appears to contain four chambers with a single aorta, a structure that more closely resembles the heart of a bird or mammal than the heart of any living reptile. The structure of the heart suggests that the dinosaur may have had a high metabolic rate that is more like that of an active warm-blooded animal than that of a cold-blooded reptile.

Many unusual dinosaur fossils found in the Sahara in northern Africa might be related to dinosaur fossils discovered in South America, indicating that the two continents were connected through most of the dinosaurian period. These findings, along with other studies of the environments of dinosaurs and the plants and animals in their habitats, help scientists learn how the world of dinosaurs resembled and differed from the modern world.

The ancestors of dinosaurs were crocodile-like-creatures called archosaurs. They appeared early in the Triassic Period and diversified into a variety of forms that are popularly known as the thecodont group of reptiles. Many of these creatures resembled later Cretaceous dinosaurs. Some archosaurs led to true crocodiles. Others produced pterosaurs, flying reptiles that possessed slender wings supported by a single spar-like finger. Still other archosaurs adopted a bipedal (two-legged) posture and developed S-shaped necks, and it was certain species of these reptiles that eventually evolved into dinosaurs.

Fossil evidence of the earliest dinosaurs dates from about 230 million years ago. This evidence, found in Madagascar in 1999, consists of bones of an animal about the size of a kangaroo. This dinosaur was a type of saurischian and was a member of the plant-eating prosauropods, which were related to ancestors of the giant, long-necked sauropods that included the Apatosaurus. Before this discovery, the earliest known dinosaur on record was the Eoraptor, which lived 227 million years ago. Discovered in Argentina in 1992, the Eoraptor was an early saurischian, one m (3 ft) long, with a primitive skull.

Scientists have identified the isolated bones and teeth of a few tiny dinosaurs representing ornithischians dating from the beginning of the Jurassic Period, around 205 million years ago. By the middle of the Jurassic Period, around 180 million years ago, most of the basic varieties of saurischian and ornithischian dinosaurs had appeared, including some that far surpassed modern elephants in size. Dinosaurs had evolved into the most abundant large animals on land, and the dinosaurian age had begun.

Earth's environment during the dinosaurian era was far different from it is today. The days were several proceeding moments shorter than they are today because the gravitational pull of the sun and the moon have over time had a braking influence on Earth's rotation. Radiation from the Sun was not as strong as it is today because the Sun has been slowly brightening over time.

Other changes in the environment may be linked to the atmosphere. Carbon dioxide, a gas that traps heat from the Sun in Earth's atmosphere-the so-called greenhouse effect-was several times more abundant in the air during the dinosaurian age. As a result, surface temperatures were warmer and no polar ice caps could form.

The pattern of continents and oceans was also very different during the age of dinosaurs. At the beginning of the dinosaurian era, the continents were united into a gigantic super-continent called Pangaea (all lands), and the oceans formed a vast world ocean called Panthalassa (all seas). About 200 million years ago, movements of Earth's crust caused the super-continent to begin slowly separating into northern and southern continental blocks, which broke apart further into the modern continents by the end of the dinosaurian era.

Because of these movements of Earth's crust, there was less land in equatorial regions than there is at present. Deserts, possibly produced by the warm, greenhouse atmosphere, were widespread across equatorial land, and the tropics were not as rich an environment for life forms as they are today. Plants and animals may have flourished instead in the temperate zones north and south of the equator.

The most obvious differences between dinosaurian and modern environments are the types of life forms present. There were fewer than half as many species of plants and animals on land during the Mesozoic Era than there are today. Bushes and trees appear to have provided the most abundant sources of food for dinosaurs, rather than the rich grasslands that feed most animals today. Although flowering plants appeared during the dinosaurian era, few of them bore nuts or fruit.

The animals of the period had slower metabolisms and smaller brains, suggesting that the pace of life was relatively languid and the behaviour were simple. The more active animals-such as ants, wasps, and mammals-first made their appearance during the dinosaurian era but was not as abundant as they are now.

The behaviour of dinosaurs was governed by their metabolism and by their central nervous system. The dinosaurs' metabolism-the internal activities that supply the body's energy needs-affected their activity level. It is unclear whether dinosaurs were purely endothermic (warm-blooded), like modern mammals, or ectothermic (cold-blooded), like modern reptiles. Endotherms regulate their body temperature internally by means of their metabolism, rather than by using the temperature of their surroundings. As a result, they have higher activity levels and higher energy needs than ectotherms. Ectotherms have a slower metabolism and regulate their body temperature by means of their behaviour, taking advantage of external temperature variations by sunning themselves to stay warm and resting in the shade to cool down. By determining whether dinosaurs were warm or cold-blooded, paleontologists could discover whether dinosaurs behaved more like modern mammals or more like modern reptiles.

Gradual changes in dinosaur anatomy suggest that the metabolic rates and activity levels of dinosaurs increased as they evolved, and some scientists believe this indicates that dinosaurs became progressively more endothermic. Overall, dinosaur body size decreased throughout the latter half the dinosaurian era, increasing the dinosaurs' need for activity and a higher metabolism to maintain warmth. Smaller animals have more surface area in proportion to their volume, which causes them to lose more heat as it radiates from their skin. Well-preserved fossils show that many small dinosaurs were probably covered with hair or feather-like fibres. Dinosaurs' tooth batteries (many small teeth packed together) became larger, enabling them to chew their food more efficiently, their breathing passages became separated from their mouth cavity, allowing them to chew and breathe while, and their nostrils became larger, making their breathing more efficient. These changes may have helped the dinosaurs digest their food and change it into energy more quickly and efficiently, thereby helping them maintain a higher metabolism.