Full Idea
A what-it-was-to-be-that-thing only belongs to those things for whom an account just is a definition.
Clarification
An 'account' will be 'logos'
Gist of Idea
Essence only belongs to things whose account is a definition
Source
Aristotle (Metaphysics [c.324 BCE], 1030a02)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.179
A Reaction
Kit Fine likes the idea of essential identity being pinned down by the act of definition. It strikes me that a unique definition may amount to a great deal less than a proper account.