Full Idea
The principle of Leibniz's Law marks off what is peculiar to identity and differentiates it in a way in which transitivity, symmetry and reflexivity (all shared by 'exact similarity, 'equality in pay', etc.) do not.
Clarification
Leibniz's Law here is the Indiscernibility of Identicals
Gist of Idea
Leibniz's Law (not transitivity, symmetry, reflexivity) marks what is peculiar to identity
Source
David Wiggins (Sameness and Substance [1980], 1.2)
Book Reference
Wiggins,David: 'Sameness and Substance' [Blackwell 1980], p.21