Plato and Aristotle take opposed positions on the question of where the “principle” of a thing, that which makes it what it is, is located. For Plato, such a principle is a form separate from the thing, in which the thing “participates.” For Aristotle, the principle shapes and organizes the thing from within, as its essence.
So why, if they are so opposed, do Plato and Aristotle, when pushed, turn into one another?
For Plato, forms are things and so must have their principles outside them. Republic 509b tells us that the ultimate principle of all of them, which makes each form what it is, is the form of the good. And this form has no principle beyond itself: it is “sovereign” over the entire intelligible order. Plato finesses this conclusion by claiming that the form of the good has no determinate nature, and so needs no further determining principle, but the fact remains: to understand the form of the good we must look to it, not beyond it; to that extent, Plato has become an Aristotelian.
For Aristotle, the principle of a thing is to be found within the thing; but what about the set of all material things, the cosmos itself? It is not only a thing, but an ordered thing, with earth at the center and the sphere of the fixed stars at the edge. Where does this unifying order come from?
It cannot come from anything within the cosmos, for everything within the cosmos contains matter, which means it can change. Anything which can change at all will eventually change, incrementally, into something else—at which time the thing it originally was will cease to exist. The world for Aristotle had no beginning in time; so if the principle of the cosmos were within it, like an essence, the cosmic order would have ceased to exist by now (see Phaedo 72b-d for a related argument).
So in Metaphysics XI, Aristotle concludes that the ordering principle of the cosmos is outside the cosmos: the immaterial, purely rational, and so consummately ordered, prime mover. The cosmos, we learn, “loves” the prime mover, but is unable to unite with it; so it imitates it by ordering itself. Whence this “love” (ἔρως), and what is it? We are not told; as Platonic did with participation (#8), Aristotle owes us a theory and gives us a word.
All of which was laid bare by Kant, most spectacularly in his “Third Antinomy.” The thesis of that antinomy states that everything has a cause; and since a cause for Kant at least partially precedes its effect in time, nothing can cause itself. The causes of a thing, the principles which make it what it is, are thus outside the thing, as with Plato.
The antithesis states, by contrast, that some things—free actions—are not caused by anything preceding them in time. To find the principle of a free act, you must look at the act itself, where you will find the atemporal moral law. So the antithesis is, broadly, Aristotelian .
Each of these two contradictory statements, Kant tells us, can be proven. What he doesn’t tell us is that his proofs are reductio’s: The proof of the thesis assumes the antithesis and demonstrates its falsity; the proof of the antithesis does the reverse.
So if you choose the (Platonic) thesis, you will eventually be forced into the (Aristotelian) antithesis, to avoid an infinite regress of causes according to which the form of the good would have a principle outside it, and that principle would also have a principle outside it, and so on…(This argument was known to Aristotle, who mysteriously called it the Third Man.)
If you choose the (Aristotelian) antithesis, you will be forced to adopt the (Platonic) thesis, on pain of locating the eternal order of the cosmos within the cosmos itself, where it cannot be because the cosmos contains matter. Matter brings change, and since anything which can change at all will eventually change into something else, the cosmic order would not be eternal.
These are not arguments (Kant’s “proofs” are notoriously bad) so much as ingrained tendencies. When we look at them that way, we see that Kant has sketched the structure of a metaphysical barn, around which Platonists and Aristotelians had been chasing each other for centuries—right up to the third antinomy itself, which abruptly stops the chase.
It does this by claiming that the thesis belongs, not to reality, but to one faculty of the mind (the understanding) while the antithesis belongs to another faculty, reason. Kant’s solution is not only abrupt, but more than a bit ad hoc (in spite of the hundreds of pages of argument intended to establish it). It also, as we will see, has other, and severe, problems—problems so severe as to impeach Kant’s entire “transcendental philosophy.”
But we also see, already, that if Kant had written nothing other than the “Third Antinomy,” he would have been the most important philosopher since Aristotle. Because it was he who uncovered, buried in the texts of the history of philosophy, an ancient and worrisome structure: the metaphysical barn.