Teleology is the position that there is design, purpose, directive principle, or finality in the works and processes of nature, and the philosophical study of that purpose.

Teleology stands in contrast to philosophical naturalism, and both ask questions separate from the questions of science. While science investigates natural laws and phenomena, Philosophical naturalism and teleology investigate the existence or non-existence of an organizing principle behind those natural laws and phenonema. Philosophical naturalism asserts that there are no such principles. Teleology asserts that there are.

Thus, within philosophical naturalism, man sees because he has eyes. Within teleology, however, man both sees because he has eyes, and has eyes so he can see. As Aristotle wrote in support of teleology, "Nature adapts the organ to the function, and not the function to the organ" (De partib., animal., IV, xii, 694b; 13). Lucretius replied in support of philosophical naturalism: "Nothing in the body is made in order that we may use it. What happens to exist is the cause of its use." (De nat. rerum, IV, 833; cf. 822-56)

Classical Greek teleology

Plato summarized the argument for teleology as follows in Phaedo, arguing that it is error to fail to distinguish between the ultimate Cause, and the mere means by which the ultimate Cause acts:

"Imagine not being able to distinguish the real cause from that without which the cause would not be able to act as a cause. It is what the majority appear to do, like people groping in the dark; they call it a cause, thus giving it a name that does not belong to it. That is why one man surrounds the earth with a vortex to make the heavens keep it in place, another makes the air support it like a wide lid. As for their capacity of being in the best place they could possibly be put, this they do not look for, nor do they believe it to have any divine force, but they believe that they will some time discover a stronger and more immortal Atlas to hold everything together more, and they do not believe that the truly good and "binding" binds and holds them together. (Plato, Phaedo 99bc)

Thus, it is argued, those who attempt to explain nature in terms of nature alone are forced to deny the ultimate binding Good in the universe, and hope that they will someday discover a "stronger and more immortal Atlas" to hold their universe together.

Similarly, Aristotle argued that it is error to attempt to reduce all things to mere necessity, because such thinking neglects the purpose, order, and final cause that causes the apparent necessity. He wrote:

"Democritus, however, neglecting the final cause, reduces to necessity all the operations of nature. Now they are necessary, it is true, but yet they are for a final cause and for the sake of what is best in each case. Thus nothing prevents the teeth from being formed and being shed in this way; but it is not on account of these causes but on account of the end; these are causes in the sense of being the moving and efficient instruments and the material. …to say that necessity is the cause is much as if we should think that the water has been drawn off from a dropsical patient on account of the lancet alone, not on account of health, for the sake of which the lancet made the incision." Aristotle, Generation of Animals V.8, 789a8-b15

Extrinsic and intrinsic finality

Teleology depends on the concept of a final cause or purpose inherent in all beings. There are two types of such causes, intrinsic finality and extrinsic finality.

• Extrinsic finality consists of a being realizing a purpose outside the being realizing it, for the utility and welfare of other beings. For instance, minerals are "designed" to be used by plants which are in turn "designed" to be used by animals.
• Intrinsic finality consists of a being realizing a purpose by means of a natural tendency directed toward the perfection of its own nature. In essence, it is what is "good for" a being. For example, physical masses obey universal gravitational tendencies that did not evolve, but are simply a cosmic "given." Similarly, life is intended to behave in certain ways so as to preserve itself from death, disease, and pain.

Over-emphasizing extrinsic finality is often criticized as leading to the childish and anthropomorphic attribution of every event to God's will, and mere superstition. For instance, "If I hadn't been at the store today, I wouldn't have found that $100 on the ground. God must have intended for me to go to the store so I would find that money." Such abuses were criticized by Francis Bacon ("De Dignitate et Augmentis Scientiarum," III, iv), Descartes ("Principia Philosophiæ", I, 28; III, 2, 3; "Meditationes", III, IV), and Spinoza (Ethica, I, prop. 36 app.).

Intrinsic finality, while more subtle, provides the basis for the teleological argument for the existence of God, and its modern counterpart, intelligent design. Proponents of teleology argue that it resolves a fundamental defect in philosophical naturalism. They argue that naturalism focuses exclusively on the immediate causes and mechanisms of events, and forgets to look for the reason for their synthesis. Thus, it is argued, if we take a clock apart, we discover in it nothing but springs, wheels, pivots, levers etc. But having explained the mechanism which causes the revolutions of the hands on the dial, is it reasonable to say that the clock was not made to keep time?

Teleology and modern philosophy

"Philosophical naturalism", see W.V.O. Quine as a proponent for instance, considers teleological explanations to be invalid. Naturalism posits that nothing beyond measurable phenomena actually exists, and therefore the argument that some unknown and/or unknownable rational being is responsible is invalid. Other philosophers, such as Allan Sandage (who, of course, is not a philosopher), Antony Flew and Alvin Plantinga, argue that teleology and other "arguments from design" are perfectly valid.

Charles Darwin's theories of evolution, which hold that species develop by natural selection, reduced the influence of traditional teleological arguments (though Darwin himself was criticized by some, accused of being a teleologist). Such arguments were still advanced by many during the resurgence of creationist sentiment in the early 1980s.

However, through Darwin's theory of evolution, purpose received a new justification in the context of survival of the fittest. For example: The tiger has claws so it can hunt and kill its prey. Even though this sounds like a teleogical argument it can be explained in terms of evolution: Having claws provided the tiger with an advantage in the selection process, hence the reason it survived. This justifies the slightly incorrect use of the notion of purpose in the theory of evolution. Because now, the seemingly teleological argument has a scientific justification, this argumentation is called teleonomy as opposed to teleology - like astronomy stands in contrast to astrology.

In recent years, some scientists have advocated an "Anthropic Principle" which explains the values of certain physical constants etc. as being the ones necessary for the existence of the human beings who make the relevant observations. Critics have suggested that some forms of this argument have a fatal tinge of teleology.

An illegitimate teleology may occur when one speculates, without sufficient proof, that X causes Y. See logical fallacy.