Aristotle's conception of natural bodies is teleological.

He thought thought that natural bodies have natures, that a nature is the organization of the material so that there exists an object that behaves in the ways characteristic of the kind, and that these behaviors in the object are for the sake of an end.

Anaxagoras and Socrates

Aristotle's teleological conception of natural bodies will be a little clearer once it is set in the proper historical context.

In the Phaedo, in the famous passage in which the character sets out his intellectual autobiography, Socrates says that he hoped to learn from Anaxagoras but was disappointed in this. He says that Anaxagoras promised to explain things in terms of νοῦς ("intellect," "mind," or "reason") and thus to explain how things are for the best. Yet, despite this promise, Socrates says that Anaxagoras omits νοῦς from his explanations.

To explain why he was so disappointed, Socrates gives an analogy. He says that to explain why he sits in jail, it is necessary to say that he sits there because he decided that this was the best course of action. Anaxagoras, according to Socrates, does not provide this kind of explanation. He talks about the need for νοῦς but omits it from his explanations. His explanation, stated in terms of the analogy, is that Socrates sits in jail because his bones and sinews are arranged in a certain way. Socrates thinks that this answer is clearly unsatisfactory. The problem is that it mentions parts of the body but omits Socrates and his reasons from the explanation for why he sits in jail.

"My glorious hope, my friend, was quickly snatched away from me. As I went on with my reading I saw that the man made no use of intelligence (νῷ), and did not assign any real causes (αἰτίας) for the ordering of things, but mentioned as causes air and ether and water and many other absurdities. And it seemed to me it was very much as if one should say that Socrates does with intelligence whatever he does, and then in trying to tell the causes of everything I do, to say that the reason that I am sitting here is because my body consists of bones and sinews, because the bones are hard and are separated by joints, that the sinews are such as to contract and relax, that they surround the bones along with flesh and skin which hold them together, .... He would mention other such causes for my talking to you: sounds and air and hearing, and a thousand other such things, but he would neglect to mention the true causes, that, after the Athenians decided it was better to condemn me, for this reason it seemed (δέδοκται) best to me to sit here and more right (δικαιότερον) to remain and endure whatever penalty they ordered" (Phaedo 98b-e; cf. Timaeus 46d).

The "For Something" is in Nature

Aristotle, in his teleological conception of nature, generalizes the point Socrates makes in the intellectual autobiography passage in the Phaedo. According to Aristotle, something that happens is the behavior of a natural body (as opposed to something that happens by coincidence) only if the behavior in the body is for the sake of an end.

An example helps to clarify Aristotle's view. Consider the chemical reactions that happen in digestion. These chemical reactions could happen in glassware in some laboratory, but it can seem implausible to think that these reactions are an instance of digestion. The reason is that digestion is something that occurs in natural bodies. It seems that whatever happens in the glassware, it is not digestion because no natural body is digesting anything. Living bodies digest their food, and, according to Aristotle, this happens in them for the sake of an end.

"There is nothing in Aristotle to prevent us from saying that this process can be understood and explained in terms of the appropriate kind of chemistry. And yet to understand the process this way is not to understand it as the natural process it is. Presumably the same process, as described in material terms, could be reproduced artificially. But if it were, it would not be a case of digestion. And this not because it lacked some details or some mysterious quality the natural process has, but because it, as a whole and its details, would have to be explained differently. What makes the digestion of food the process it is, and hence is essential to our understanding of it, is that it is the exercise of the capacity or ability of this kind of organism to digest food. And similarly for the other life functions. In each case there is a material description in terms of material parts of the organism and of what happens to them in terms of their properties. But in each case the process is the natural or physical process it is, rather than a materially equivalent but formally different process only, because it is the exercise of an ability the organism has in virtue of its form or soul" (Michael Frede, "On Aristotle's Conception of the Soul," 150).

Aristotle's point is not that all natural bodies have minds and act for reasons. He does not think that digestion is a behavior a living body chooses as a matter of practical reasoning and deliberation. Aristotle does not believe that teleological behavior requires deliberation. He thinks that the "for something" is in things that do not deliberate.

   "This is the difficulty: why should we suppose nature acts for something (ἕνεκά του) and because it is better? Why should not everything be like the rain? Zeus does not drop the rain to make the corn may grow. It comes of necessity. For the rising vapour must needs be condensed into water by the cold, and must then descend, and incidentally (συμβαίνει), when this happens, the corn grows. Similarly also, if someone’s corn on the threshing floor is ruined it does not rain for the sake (ἕνεκα) of this, so that the corn is ruined, but the result is incidental (συμβέβηκεν) to the raining. What, then, is to stop the parts of nature from being like this--the front teeth of necessity growing sharp and suitable for biting, and the back teeth broad and serviceable for chewing the food, not coming to be for this, but by coincidence (συμπεσεῖν)? And similarly for the other parts in which the for something (τὸ ἕνεκά του) seems to be present. So that when all things turned out just as they had come to be for something, then the things, suitably constituted as an automatic outcome (αὐτομάτου), survived; when not, they died, and die, as Empedocles says of the man-headed calves.
   This difficulty, or something like it, is the account which might give us pause. It is impossible, however, that this should be how things are. The things mentioned, and all the things which are due to nature, come to be as they do always or for the most part, and nothing which is the outcome of luck or an automatic outcome (ἀπὸ τύχης καὶ τοῦ αὐτομάτου) does that. We do not think that it is the outcome of luck (τύχης) or coincidence (συμπτώματος) that there is a lot of rain in winter, but only if there is a lot of rain in August; nor that there are heatwaves in August, but only if there is a heatwave in winter. If, then, things seem to be either a coincidental outcome (συμπτώματος) or for something (ἕνεκά του), and the things we are discussing cannot neither be a coincidental nor a automatic outcome (μήτε ἀπὸ συμπτώματος μήτ᾿ ἀπὸ ταὐτομάτου), they must be for something (ἕνεκά του). But all such things are due to nature, as the authors of the views under discussion themselves admit. The for something (τὸ ἕνεκά του), then, is present in things which are and come to be due to nature" (Physics II.8.198b).

Aristotle's Answer to Socrates

Aristotle's teleology provides him with the beginnings of a solution to a problem that seems to confound Plato in the Phaedo.

Socrates hoped to learn from Anaxagoras that things happen as they do because νοῦς, in arranging things, arranges them so that things happen for the best. Aristotle can provide the sort of solution Socrates hoped to learn from Anaxagoras if he can work out a way in which natural bodies exercise their ways of functioning for the sake of having the best possible existence.

To work out this conception of natural bodies and their specific behaviors, Aristotle argues for the existence of the "unmovable first mover" (πρῶτον κινοῦν ἀκίνητον).

How the unmovable first mover is the teleological cause for the sake of which other things change is not easy to understand in detail. The idea is perhaps most straightforward in the case of development of reason in human beings, so is is helpful at this point to turn to Aristotle's discussion of the "soul" (ψυχή) in general and the form it takes in human beings.