I’ve been studying Aristotle’s Physics with the aid of St. Thomas Aquinas’ commentary, and today I began on Chapter 8 of Book II. Aristotle has asserted that nature acts for an end—that acorns are for the sake of oak trees, that teeth are for the sake of chewing, and so forth. Many modern scientists deny this kind of final causality on evolutionary grounds. Teeth aren’t really for the purpose of chewing; it just happened, evolutionarily, that creatures with teeth chewed better than those without. It just worked out that way. (I over-simplify, of course.)

In Chapter 8, Aristotle describes the argument of those who deny that nature acts for an end. Here’s what he has to say; it’s unusually straightforward. First, he points out that the rain doesn’t fall to make the corn grow:

…the sky rains, not in order to make the corn grow, but of necessity…. What is drawn up must cool, and what has been cooled must become water and descend, the result of this being that the corn grows. Similarly if a man’s crop is spoiled on the threshing-floor, the rain did not fall for the sake of this—in order that the crop might be spoiled—but that result followed.

Got that? He’s described the evaporation and condensation cycle. Rain falls because that’s simply what water does. That’s what he means by “necessity”: water simply works that way. The Greeks didn’t have the same notion of physical laws that we do, but that’s what he’s talking about.

Well, if that’s true of water, then why not of, say, teeth?

Why then should it not be the same with the parts in nature, e.g., that our teeth should come up of necessity—the front teeth sharp, fitted for tearing, the molars broad and useful for grinding down down the food—since they did not arise for this end, but it was merely a coincident result…such things survived, being organized spontaneously in a fitting way; whereas those which grew otherwise perished and continue to perish, as Empedocles says his “man-faced ox-progeny” did.

In other words, why shouldn’t teeth grow simply because that’s the way matter works (organized, as we now know, in terms of DNA molecules), the successful arrangements being retained and the unsuccessful perishing.

Aristotle just summarized the basic notion of natural selection, and used it as an argument against final causality, much as the so-called “New Athiests” do. He disagrees, of course; he’s raised the argument to strike it down. I’m quite curiously to see how he does it.