2. But are we really obliged to posit the existence of such genera?
Take Substance, for Substance must certainly be our starting-point: what are the grounds for regarding Substance as one single genus?
It has been remarked that Substance cannot be a single entity common to both the Intellectual and the Sensible worlds. We may add that such community would entail the existence of something prior to Intellectual and Sensible Substances alike, something distinct from both as predicated of both; and this prior would be neither body nor unembodied; for it were one or the other, body would be unembodied, or the unembodied would be the body.
This conclusion must not however prevent our seeking in the actual substance of the Sensible world an element held in common by Matter, by Form and by their Composite, all of which are designated as substances, though it is not maintained that they are Substance in an equal degree; Form is usually held to be Substance in a higher degree than Matter, and rightly so, in spite of those who would have Matter to be the more truly real.
There is further the distinction drawn between what are known as First and Second Substances. But what is their common basis, seeing that the First are the source from which the Second derive their right to be called substances?
But, in sum, it is impossible to define Substance: determine its property, and still you have not attained to its essence. Even the definition, "That which, numerically one and the same, is receptive of contraries," will hardly be applicable to all substances alike.