Full Idea
The what-it-was-to-be-that-thing [to ti en einai] is a unity of a kind straight off, just as it is a being of a kind. And that is why none of these things has some other cause of their being a unity, any more than they do of their being a being of a kind.
Gist of Idea
An essence causes both its own unity and its kind
Source
Aristotle (Metaphysics [c.324 BCE], 1045b04)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.249
A Reaction
This seems to be the key importance of the notion of essence - it is what both bestows unity on things in the world (which is basic to ontology and epistemology), and what enables us to categorise things (basic to epistemology).