Full Idea
Any definition must presuppose the notion of identity precisely because a definition affirms the identity of two concepts.
Gist of Idea
Definitions identify two concepts, so they presuppose identity
Source
Colin McGinn (Logical Properties [2000], Ch.1)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.8
A Reaction
McGinn is arguing that identity is fundamental to thought, and this seems persuasive. It may be, though, that while identities are inescapable, definitions are impossible.