Full Idea
For Aristotle the essences of individual substances are individual rather than universal, ...since nothing universal can be a substance, nor can it be a principle or cause of a substance.
Gist of Idea
Individual essences are not universals, since those can't be substances, or cause them
Source
report of Aristotle (Metaphysics [c.324 BCE], ess) by Charlotte Witt - Substance and Essence in Aristotle Intro
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.3
A Reaction
This is the second of Witt's three theses which she offers in opposition to the orthodox interpretation of Aristotle, and again I think she is right.