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Full Idea
Existence and unity are two other ideas that are suggested to the understanding, by every object without, and every idea within. ..And whatever we can consider as one thing, whether a real being, or idea, suggests to the understanding the idea of unity.
Gist of Idea
Every external object or internal idea suggests to us the idea of unity
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.07.07)
Book Ref
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.131
A Reaction
It seems to me blatantly obvious that there is a close tie between this fact of metaphysics or psychology (or both) and the notion of a 'unit' in mathematics. Without this faculty of 'identifying' things, there would be no numbers or counting.