Full Idea
What sorts things into their proper genus and species are the definitions that express what they are.
Gist of Idea
The definitions expressing identity are used to sort things
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.92)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.92
A Reaction
This is straight from Aristotle, though Aristotle's view is a little more complex, I think. If the definitions 'express what they are', then definitions seem to specify the essence.