Two remarkable moments from Aristotle’s Physics

After having a good experience teaching Plato’s Timaeus, I was dreading the next step in my Shimer Great Books seminar on great texts in cosmology: Aristotle’s Physics. Aristotle never seems to teach well, not simply because of his dry writing style, but because of his often confusing presentation — throwing out multiple ideas, some tied to specific names but most not, before gradually whittling the possibilities down to his own preferred solution. Undergraduate readers tend to find that approach uniquely impenetrable. It has been more fun teaching the Physics this year than I expected, though, in part because of how close it is to everyday experience — and also, perhaps, because my years of hard experience have helped me to give them reading strategies.

In any case, I want to highlight two remarkable passages from Aristotle that, to me, vindicate his stature as a real scientific thinker. The first comes amid his discussion of whether there is purpose in nature. His emphasis on purpose and teleology is the biggest single difference between his view and that of modern science, and it can be easy to claim that he simply assumes or postulates that there is purpose in the world with no proof. Yet he does argue for it (starting at 198b16, following Robin Waterfield’s Oxford World’s Classics translation):

What is wrong with the idea that nature does not act purposively and does not do things because they are better?…. Take teeth, for instance: what is wrong with the idea that the front teeth necessarily come through sharp and suitable for biting, and the back teeth flat and good for crushing food? Why should there be purpose behind this? Why should it not just be an accident? And the same question could be asked about any other part of the body which seems to have some purpose. So where every part turned out to be just as it would have been if it had had some purpose, the creatures survived because, spontaneously, they happened to be put together in a useful way. But everything else has been destroyed and continues to be destroyed….

I don’t know what to call this other than an anticipation of the theory of natural selection. Nor is it a dismissive account, because he actually gives it a lot more space than he gives to many of the “wrong” views he discusses. Nevertheless, he concludes, “But it is impossible for this to be the way things are. The point is that the things mentioned turned out as they do either always or usually, and so does every other natural object, whereas no chance or spontaneous event does” (b32ff). The apparent purposefulness of the parts of the body can’t be the result of chance because they are too regular and reliable. From our perspective, the dichotomy is false — but to get there, you need the idea of individual traits changing at random, in a way that is both beneficial and heritable, and there is nothing in Aristotle’s experience (nor that of most pre-modern non-world-travellers) that would suggest such a thing is possible. So he anticipates our view and rightly rejects it for lack of evidence.

Later, when refuting the existence of a void, Aristotle makes the argument — very unconvincing from our perspective — that the void would not allow for the kind of structured motion that he sees in the world (e.g., earth moves downward toward the center, while fire moves up). If everything existed in an undifferentiated void, those kinds of regularities would not exist and, in fact, there would be no particular reason for anything to move: “anything in a void is bound to be at rest, since there is nowhere for it to move to more or less than anywhere else” (214b28ff). Further, he claims — again, very unconvincingly from a modern viewpoint and also for his medieval commentators — that motion can only occur through contact, i.e., movement (other than the “natural” movement toward or away from the center, which is already impossible in a void) requires a push. But, if we were to go so far as to suppose motion in a void were possible, “it would be impossible to explain why something which has been set in motion should stop anywhere: why should it stop here rather than there? Either it never moves or it has to go on and on moving forever, unless something stronger than it impedes it” (215a19ff). So he seems to be saying that, in a system where things moved in a void, objects at rest would tend to stay at rest and objects in motion would tend to stay in motion, unless an outside force intervenes. That sounds a lot like Newton, doesn’t it? But he dismisses the possibility because, crucially, that’s not what happens. Objects tend to go up or down according to their “natural” motions, then rest there. Again, he anticipates the modern scientific view, but he dismisses it for lack of evidence — and in this case, I can hardly blame him, since getting direct empirical evidence of pure Newtonian motion requires travelling to outer space.

This, I think, is the real source of Aristotle’s authority in the pre-modern world — he really has thought through all the plausible options, and in most cases, he has chosen the option that fits best with the overwhelming evidence of experience. It’s just that the evidence he has access to is “merely” that of an earth-bound creature with a lifespan that is relatively short compared to deep evolutionary time. Teaching Aristotle in a way that gets students to take him seriously therefore requires the surprisingly difficult work of actually getting them to think systematically about what happens in their everyday world, rather than the abstract theories they memorize in a typical science class. Heidegger says that Aristotle was the last Greek philosopher “with eyes to see” — reading Aristotle together, we learn to reopen our eyes.