Full Idea
Aristotle holds that substances are distinct from other beings by virtue of their high degree of unity.
Gist of Idea
Substance is distinct being because of its unity
Source
report of Aristotle (Metaphysics [c.324 BCE], ousia) by Charlotte Witt - Substance and Essence in Aristotle 4
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.101
A Reaction
It seems to me that the notion of 'substance' (translating 'ousia' thus) can't mean anything more than 'being with identity'. Then 'essence' is offered as that which bestows the identity on the being.