Full Idea
In addition to the difference of time or of place there must always be an internal principle of distinction: although there can be many things of the same kind, it is still the case that none of them are ever exactly alike.
Gist of Idea
No two things are quite the same, so there must be an internal principle of distinction
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.27)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.230
A Reaction
This rests on Leibniz's unusual view that all things (even electrons) are qualitatively distinct. Personally I disagree with that, but agree with the idea. Things have time and place because they have identity, not the other way around.