Full Idea
A simple substance is its own essence.
Gist of Idea
A simple substance is its own essence
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.103)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.103
A Reaction
Aquinas takes complex substances to have their essences in various ways, but this thought is the basis of all essence. Presumably the Greek word 'ousia' names the key ingredient.