 Aristotle recognized that most ordinary, material things are composed of multiple substances, although he thought that some of them could be composed of a single, pure substance. Thus, he needed to give a criterion of purity that would individuate a single substance. His criterion was that pure substances are homoeomerous: they are composed of like parts at every level. “[I]f combination has taken place, the compound must be uniform—any part of such a compound is the same as the whole, just as any part of water is water” (De Generatione et Corruptione, henceforth DG, I.10, 328a10ff).[1] So when we encounter diamond in rock, oil in water, or smoke in air, Aristotelian chemistry tells us that there is more than one substance present.