Full Idea
In Categories a primary substance has ontological priority, where other things depend on its existence, ..but in Metaphysics he emphasizes conceptual priority, where the primary is what is understood through itself (a definable unity).
Gist of Idea
Is a primary substance a foundation of existence, or the last stage of understanding?
Source
report of Aristotle (Metaphysics [c.324 BCE], book) by Mary Louise Gill - Aristotle on Substance Intro
Book Reference
Gill,Mary Louise: 'Aristotle on Substance: Paradox of Unity' [Princeton 1989], p.3
A Reaction
Interesting for my view of essence as rooted in explanation. It is the Metaphysics version that appeals to me. A metaphysics is constructed from our modes of understanding. 'Concavity' is his example of a primary unity.