Aristotle's natural motion and modern inertial motion
Last week, in my history of science class, we finished the unit on ancient science and medicine. At the end of the unit, I asked my students the (ill-posed) question: so is any of this stuff we've been reading science or not? (We read Plato's Timaeus, Aristotle's Physics II and On the Heavens, Ptolemy's Almagest, and bits of Epicurus as well as Hippocratic writers.)
One sentiment that came to the fore was that the ancients were (on average) more willing to countenance teleological explanations in natural sciences than we are. I think this is definitely right on the whole. But I did want to ask a question about an example sometimes forwarded in defense of this claim. Aristotle says that part of what makes the element earth earth is its tendency to move towards the center of the universe (since Aristotle thought the planet Earth was at rest in the center of the universe). Air and fire move away from the center of the universe, and the celestial matter moves in a circle around the center. These 'natural motions' are taken to appeal to final causes in a way that modern science does not: water and earth have a 'goal' or 'end' (Greek telos), viz., the center of the universe. And (so the story goes) matter from the Early Modern period onwards, starting with Descartes at the latest, is not like that at all.
I, unfortunately, cannot make out a substantive difference here -- we can describe Aristotle and the moderns in the same terms: we can make Aristotle sound more modern, or make Newton et al. sound more teleological. In the modern dynamical picture, we have inertial motion: a body in motion will maintain that motion (speed and direction), unless acted upon by an outside force. If the telos of an Aristotelian hunk of earth is the center of the universe, the telos of a modern bit of matter is (something like) self-preservation. Its goal is resistance to change (of direction and speed).
Alternatively, we can characterize Aristotle as more modern: Aristotle is describing what happens to a bit of fire or water if it's just "left alone," i.e., what does a body do when it is free of any interference? Aristotle clearly disagrees with the moderns about what a body does when "left alone"... but Einstein disagreed with Newtonians over the same issue. In other words, we can think of the difference (on this topic) between Aristotle and the moderns as a disagreement over which trajectories are the ones bodies will follow when no external forces act upon them. Teleology doesn't appear here at all.
Of course, I could be overlooking something obvious in the Aristotelian text. Hopefully any real Aristotle scholars out there reading this will tell me if I have. If you want to check the text of On the Heavens for yourself, it’s online here.
Labels: Ancient history