Full Idea
Every man of common sense has a clear and distinct notion of identity. If you ask for a definition of identity, I confess I can give none. It is too simple a notion.
Gist of Idea
Identity is familiar to common sense, but very hard to define
Source
Thomas Reid (Essays on Intellectual Powers 3: Memory [1785], III.Ch 4)
Book Reference
'Personal Identity', ed/tr. Perry,John [University of California 1975], p.108
A Reaction
'Identical' seems to be a two-place predicate, but the only strict way two things can be identical is if there is actually just one thing. In which case just drop the word 'identity' (instead of defining it), and say there is just one thing here.