Teleology and final causes: does nature do nothing in vain?

Aristotle, answered from the texts and cited to the page.

The position in brief

Nature does nothing in vain. The four causes — material, formal, efficient, and final — are required to give a complete account of any natural being.¹ The final cause (the telos, the for-the-sake-of-which) is primary in nature because the other causes are organized to serve it: the material is selected for the sake of the form, the form is what the developing being is moving toward, the efficient cause initiates the process that the final cause specifies as its endpoint.

The eye exists for the sake of seeing; the stomach for the sake of digesting; the leaf for the sake of photosynthesis (in the language of later science). To explain natural phenomena without final causes is to leave the most important question — why this and not otherwise — unanswered. The defense of teleology against the materialism of Empedocles and Democritus runs through Parts of Animals I.1 and is the founding methodological statement of systematic biology. Physis — nature — is itself an internal principle of motion and rest oriented toward an end; this is what distinguishes natural beings from artifacts.²³⁴

Sources

The ancient Nature-philosophers however took the opposite view. The reason of this is that they did not see that the causes were numerous, but only saw the material and efficient and did not distinguish even these, while they made no inquiry at all into the formal and final causes.
Generation of Animals, p. 77
For as yet no definite rule has been laid down in this matter. So also there is a like uncertainty as to another point now to be mentioned.
On the Parts of Animals
For germ or seed is both the seed of the organism from which it came, of the horse, for instance, from which it was derived, and the seed of the organism that will eventually arise from it, of the mule, for example, which is developed from the seed of the horse. The same seed then is the seed both of the horse and of the mule, though in different ways as here set forth.
On the Parts of Animals
E.g. why does one take a walk after supper? For the sake of one's health. Why does a house exist? For the preservation of one's goods. The end in view is in the one case health, in the other preservation. To ask the reason why one must walk after supper is precisely to ask to what end one must do it. Let C be walking after supper, B the non-regurgitation of food, A health.
Posterior Analytics

Where the argument is made

Physics II (the four causes, with final causation as primary in nature)
Parts of Animals I (the methodological introduction to the biological works, with the famous defense of teleology against the materialism of the Presocratics)
Generation of Animals
Metaphysics V (the discussion of cause)
De Anima II (the soul as the form and final cause of the living body)

How the argument was received

Teleology in physics was rejected by the early modern scientific revolution (Bacon, Descartes, Galileo, Newton) in favor of efficient-causal mechanism. Teleology in biology survived longer but was apparently displaced by Darwin's Origin of Species (1859), which seemed to give a mechanistic explanation for the appearance of design. The contemporary situation is more contested: defenders of Aristotelian teleology in biology (Allan Gotthelf, James Lennox, Marjorie Grene) argue that biological functions are genuine teleological facts not reducible to selection-history, and the teleosemantic tradition in philosophy of mind (Millikan, Dretske) has revived teleological notions in a naturalist framework. The doctrine remains foundational for Aristotelian metaphysics and ethics even where it has been rejected in physics.