Full Idea
A speaker must be able to define a subject generically, and then to divide it into its various specific kinds until he reaches the limits of divisibility.
Gist of Idea
A speaker should be able to divide a subject, right down to the limits of divisibility
Source
Plato (Phaedrus [c.368 BCE], 277b)
Book Reference
Plato: 'Phaedrus and Letters VII and VIII', ed/tr. Hamilton,Walter [Penguin 1973], p.100