Full Idea
If any two items have a single substance [ousia, primary being] and a single what-it-is-to-be-that-thing [to ti en einai, essence], then they are themselves a single thing.
Gist of Idea
Two things with the same primary being and essence are one thing
Source
Aristotle (Metaphysics [c.324 BCE], 1038b14)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.216
A Reaction
[alternative translations by Vasilis Politis] This isn't quite the identity of indiscernibles, because it allows superficial identity along with deep difference (H2O and XYZ, for example, or jadeite and nephrite).
Related Idea
Idea 12081 Aristotle's definitions are not unique, but apply to a range of individuals [Aristotle, by Witt]