Full Idea
It isn't possible to have two particulars that are similar in all respects - for example two eggs - for it is necessary that some things can be said about one of them that cannot be said about the other, else they could be substituted for one another.
Gist of Idea
Two eggs can't be identical, because the same truths can't apply to both of them
Source
Gottfried Leibniz (works [1690]), quoted by David Wiggins - Sameness and Substance 2.2
Book Reference
Wiggins,David: 'Sameness and Substance' [Blackwell 1980], p.56
A Reaction
[from a 'fragment' for which Wiggins gives a reference] This quotation doesn't rest the distinctness of the eggs on some intrinsic difference, but on the fact that we can say different things about the two eggs.