Full Idea
Leibniz's Law, the indiscernibility of identicals, is a truism which should not be confused with the more controversial identity of indiscernibles, which depends on the possibility of perfectly replicated universes.
Gist of Idea
The Indiscernibility of Identicals is a truism; but the Identity of Indiscernibles depends on possible identical worlds
Source
Georges Rey (Contemporary Philosophy of Mind [1997], 2.4)
Book Reference
Rey,Georges: 'Contemporary Philosophy of Mind' [Blackwell 1997], p.49