Full Idea
I take Aristotle to be arguing that no definition applies uniquely to an individual - it is always potentially applicable to a range of individuals.
Gist of Idea
Aristotle's definitions are not unique, but apply to a range of individuals
Source
report of Aristotle (Metaphysics [c.324 BCE], defs) by Charlotte Witt - Substance and Essence in Aristotle 4.2 n7
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.109
A Reaction
So much the worse for Aristotle, I think. Surely unique entities can be defined? No one thinks their pet dog is just any dog, even after the age and breed have been identified.
Related Idea
Idea 11380 Two things with the same primary being and essence are one thing [Aristotle]