Full Idea
We don't conceive it possible that two things of the same kind should exist in the same place and time...When, therefore, we demand whether any thing be the same or no, it refers to something that existed at a time and place, and was the same with itself.
Gist of Idea
Two things can't occupy one place and time, which leads us to the idea of self-identity
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.27.01)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.328
A Reaction
I find the notion of 'self-identity' puzzling. I've always taken it to be a logicians' idea, but Locke seems to arrive at it by looking for whatever is identical with some original object, and the floating relation having to hook back onto itself.