Teleology is from the Greek telos, "end", is the explanation of a thing or event by its purpose, aim, or end- that is, by what it is for. Thus one might say that animals have eyes in order to detect distant enemies or prey- eyes, therefore, exist as a means to that end. Eyes are the "efficient cause" (in Aristotle's term) of detection, or simply its cause, as we normally say; but detection is their "final cause" (from Latin finis, "end"), the reason for them, as we might say.
Purposes indisputably yield at least provisional explanations of conscious human acts. Teleology, or "finalism", as a philosophy emphasizes or magnifies this principle in either or both of two ways. It extends its scope to analogous but nonconscious processes in men, animals, and plants or, indeed, beyond all apparent analogy to inanimate objects. And it asserts the independence or irreducibility of teleology by denying, for instance, that a purpose itself can be explained away as just another efficient cause with causes of its own. This philosophy is supported in part by whatever successes it meets and whatever failures befall nonteleological accounts, but mostly it is supported by examination of what is involved in existence and explanation themselves. Only reasons, it argues, are truly antithetical to "mere chance" and can really explain or "make sense" of things.
Primitive peoples and persons are natural teleologists, ascribing vague purposes to everything. This idea received highly sophisticated expression in Aristotle's doctrine that the essence of anything is the final cause it strives to realize. Such "internal" teleology contrasts but does not conflict with the "external" teleology of the belief, sublimated in Judaism and Christianity, that the world as a whole was designed by a deity. The teleological argument for God (that the order and beauty of nature prove his existence) is thus also a theological argument for teleology. Medieval Scholasticism, combining Aristotelian essences with divine creation, was the acme of teleology.
The causalist, or mechanist, retorts that the charm of finalistic explanation is more sentimental than logical. Finalism, he says, is not only unclear but paradoxical, since it alleges that a present actuality is due to the mere value of a mere future possibility. He may add that, historically, finalism has been either quite unproductive- "a barren virgin" said Francis Bacon- or prone to excesses, as in Voltaire's parody that man's nose was provided to support his spectacles. Causal determinism, on the other hand, is comparatively clear and has succeeded so often in application that it can reasonably be expected to cope with even the most obviously purposive phenomena. The first philosophers of Greece were valiant causalists. Their considerable achievements were not enough to discourage the teleological reaction, as expressed in Socrates, Plato, Aristotle, and Christian theism, but their antiteleology was vigorously reaffirmed as the program of the sciences so triumphant since the Renaissance.
Teleology, nevertheless, does not lack commonsense uses or metaphysical advocates. So-called analytic philosophies are generally hostile to it, but every form of idealism, pragmatism, and existentialism (and even such scientifically oriented systems as those of Samuel Alexander and A. N. Whitehead) finds the universe alive with will, nisus (striving), or "concern". And recent physics, limiting the incidence of causation, cannot complain if teleologists fill the gaps with "reasons". Biologists, who bear the main brunt, mostly reject the special finalistic agencies proposed by vitalism, but they commonly doubt that organic wholes are explicable by the physicochemistry of their parts. Meantime, they face the task of formulating exactly the observable "goal-directedness" of organisms and organs- the coordination of functions by which they survive and flourish- without either asserting or denying that these can eventually be traced to physical causes.
Teleological ethics looks to ends not to explain acts but to define their rightness or wrongness. An act is the right one if it involves, in itself and all its effects, the greatest sum of good that is possible in the circumstances (eg: the most happiness) rather than because of any inherent trait such as conformity with an abstract rule of duty ("formalism" or "deontologism").