Full Idea
Suppose I was trying to define you. I would say you are an animal that is lean or white or some such, all of which also apply to other things.
Gist of Idea
If I define you, I have to use terms which are all true of other things too
Source
Aristotle (Metaphysics [c.324 BCE], 1040a13)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.223
A Reaction
Commentators have taken this to mean that Aristotle is only interested in kinds and not individuals, but recent thinking says this is wrong. Universals prevent you from really getting at the thing you want to define. Definitions are limited.