The Refutation of All Heresies: Book VII. 4Sketch of Aristotle's Philosophy.

Aristotle, then, makes a threefold division of substance. For one portion of it is a certain genus, and another a certain species, as that (philosopher) expresses it, and a third a certain individual. What is individual, however, (is so) not through any minuteness of body, but because by nature it cannot admit of any division whatsoever. The genus, on the other hand, is a sort of aggregate, made up of many and different germs. And from this genus, just as (from) a certain heap, all the species of existent things derive their distinctions. And the genus constitutes a competent cause for (the production of) all generated entities. In order, however, that the foregoing statement may be clear, I shall prove (my position) through an example. And by means of this it will be possible for us to retrace our steps over the entire speculation of the Peripatetic (sage).