The Refutation of All Heresies: Book VII. 7Substance, According to Aristotle; The Predicates.

Since, however, as I have stated, substance is threefold, viz., genus, species, (and) individual; and (since) we have set down animality as being the genus, and man the species, as being already distinct from the majority of animals, but notwithstanding still to be identified (with animals of his own kind), inasmuch as not being yet moulded into a species of realized substance,--(therefore it is, that) when I impart form under a name to a man derived from the genus, I style him Socrates or Diogenes, or some one of the many denominations (in use). And since (in this way, I repeat,) I comprehend under a name the man who constitutes a species that is generated from the genus, I denominate a substance of this description individual. For genus has been divided into species, and species into individual. But (as regards) the individual, since it has been comprehended under a name, it is not possible that, according to its own nature, it could be divided into anything else, as we have divided each of the fore-mentioned (genus and species).

Aristotle primarily, and especially, and preeminently entitles this--substance, inasmuch as it cannot either be predicated of any Subject, or exist in a Subject. He, however, predicates of the Subject, just as with the genus, what I said constituted animality, (and which is) predicated by means of a common name of all particular animals, such as ox, horse, and the rest that are placed under (this genus). For it is true to say that man is an animal, and horse an animal, and that ox is an animal, and each of the rest. Now the meaning of the expression "predicated of a Subject" is this, that inasmuch as it is one, it can be predicated in like manner of many (particulars), even though these happen to be diversified in species. For neither does horse nor ox differ from man so far forth as he is an animal, for the definition of animal is said to suit all animals alike. For what is an animal? If we define it, a general definition will comprehend all animals. For animal is an animated Substance, endued with Sensation. Such are ox, man, horse, and each of the rest (of the animal kingdom). But the meaning of the expression "in a Subject" is this, that what is inherent in anything, not as a part, it is impossible should exist separately from that in which it is. But this constitutes each of the accidents (resident) in Substance, and is what is termed Quality. Now, according to this, we say that certain persons are of such a quality; for instance, white, grey, black, just, unjust, temperate, and other (characteristics) similar to these. But it is impossible for any one of these to subsist itself by itself; but it must inhere in something else. If, however, neither animal which I predicate of all individual animals, nor accidents which are discoverable in all things of which they are nonessential qualities, can subsist themselves by themselves, and (yet if) individuals are formed out of these, (it follows, therefore, that) the triply divided Substance, which is not made up out of other things, consists of nonentities. If, then, what is primarily, and pre-eminently, and particularly denominated Substance consists of these, it derives existence from nonentities, according to Aristotle.