Full Idea
We usually isolate the appropriate description of the essence of a particular thing by means of the differentiae which are peculiar to it.
Gist of Idea
We describe the essence of a particular thing by means of its differentiae
Source
Aristotle (Topics [c.331 BCE], 108b05)
Book Reference
Aristotle: 'Posterior Analytics and Topica', ed/tr. Tredennick,H/Forster,ES [Harvard 1960], p.327
A Reaction
I take this to be important for showing the definition is more than mere categorisation. A good definition homes in the particular, by gradually narrowing down the differentiae.