Categories is the intro text to Aristotle’s Metaphysics, or so some essay from Plato.Stanford.Edu said. Good enough for me. It is short and clear.
Some things are predicable of a subject but never in a subject. By “being present in a subject” Aristotle means “incapable of existence apart from a subject” (2, 1a).
Substance is that which is neither predicable of a subject nor present in a subject.
- Primary: The individual man or horse.
- Secondary: the species man; the genus animal.
Key point: everything except primary substances is either predicable of a primary substance or present in a primary substance. The proposition “the man is an animal” is necessarily true, but not the reverse. Further, the species is to the genus as subject is to predicate.
A primary substance has no contrary, for what can be the contrary to an individual man? Yet, while remaining numerically one it can admit contrary qualities.