Full Idea
Aristotle's notion of form or essence is meant to explain why there is an individual substance there at all, not what features constitute the identity of a given individual substance within a domain of individual substances.
Gist of Idea
Aristotle's essence explains the existence of an individual substance, not its properties
Source
report of Aristotle (Metaphysics [c.324 BCE], hylom) by Charlotte Witt - Substance and Essence in Aristotle 4.4
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.126
A Reaction
I begin to think that the notion of 'essence' is extremely useful in aiding our grasp of reality, but the notion of 'substance' is not. We can just talk of 'identity', without implying some stuff that constitutes that identity. Essence is powers.