 It is easy to see how this hylomorphic analysis explains the unity of a substantial material particular, since neither the matter nor the form of such a particular is by itself a single material individual, and it is only when they are taken together that they constitute such an individual. But the question Aristotle is trying to answer is this: “why on earth is something one when the account of it is what we call a defnition?” (Ζ.12, 1037b11). Since proper definables are universals, it remains to be seen how the proposed solution applies to them. After all, universals are not material objects, and so it is not clear how they can be viewed as hylomorphic compounds. But Aristotle has at his disposal a concept that can fill this bill perfectly, viz., the concept of intelligible matter (hulê noêtê). (The main purpose of intelligible matter is to provide something quasi-material for pure geometrical objects that are not realized in bronze or stone, for example, to be made of.) So we surmise that it is for this reason that Aristotle goes on (1045a33) to introduce matter into the current context. If this is so, we may conclude that the material component in the definition of a species is intelligible matter. Elsewhere, he explicitly describes genus as matter: “the genus is the matter of what it is said to be the genus of” (Ι.8, 1058a23). So a species too, although it is not itself a material object, can be considered a hylomorphic compound. Its matter is its genus, which is only potentially the species defined; its differentia is the form that actualizes the matter. The genus does not actually exist independently of its species any more than bronze exists apart from all form. The genus animal, for example, is just that which is potentially some specific kind of animal or other. Aristotle concludes (1045b17–21) that “the ultimate matter and the form (morphê) are one and the same, the one potentially, the other actively … and what potentially is and what actively is are in a way one.”