Full Idea
If the genus simpliciter does not exist over and above the specific forms constitutive of it, alternatively if it exists just as matter, it is evident what the definition is, the account derived just from the differentiae.
Gist of Idea
If the genus is just its constitutive forms (or matter), then the definition is the account of the differentiae
Source
Aristotle (Metaphysics [c.324 BCE], 1038a05-)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.213
A Reaction
This is how I understand an Aristotelian definition - as a lengthy and fine-grained account of the details which pick out some individual within the main genus which constitutes it.