3. But perhaps we should rather speak of some single category, embracing Intellectual Substance, Matter, Form, and the Composite of Matter and Form. One might refer to the family of the Heraclids as a unity in the sense, not of a common element in all its members, but of a common origin: similarly, Intellectual Substance would be Substance in the first degree, the others being substances by derivation and in a lower degree.
But what is the objection to including everything in a single category, all else of which existence is predicated being derived from that one thing, Existence or Substance? Because, granted that things be no more than modifications of Substance, there is a distinct grading of substances themselves. Moreover, the single category does not put us in a position to build on Substance, or to grasp it in its very truth as the plausible source of the other substances.
Supposing we grant that all things known as substances are homogeneous as possessing something denied to the other genera, what precisely is this something, this individuality, this subject which is never a predicate, this thing not present in any thing as in a subject, this thing which does not owe its essential character to any other thing, as a quality takes character from a body and a quantity from a substance, as time is related to motion and motion to the moved?
The Second Substance is, it is true, a predicate. But predication in this case signifies a different relation from that just considered; it reveals the genus inherent in the subject and the subject’s essential character, whereas whiteness is predicated of a thing in the sense of being present in the thing.
The properties adduced may indeed be allowed to distinguish Substance from the other Existents. They afford a means of grouping substances together and calling them by a common name. They do not however establish the unity of a genus, and they do not bring to light the concept and the nature of Substance.
These considerations are sufficient for our purpose: let us now proceed to investigate the nature of Quantity.