Full Idea
A what-it-was to-be-that-thing only belongs to those things for whom an account just is a definition.
Gist of Idea
Things have an essence if their explanation is a definition
Source
Aristotle (Metaphysics [c.324 BCE], 1030a06)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.179
A Reaction
That seems to be that 'to ti en einai' (aka essence) only has a 'logos' if it has a 'horismos'. It seems that having a definition as its account is a necessary condition for an essence, but not sufficient. It looks to me as if essence must be explanatory.