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Full Idea
For general terms write 'if Fx then Fy' and vice versa, and 'if Fxz then Fyz'..... The conjunction of all these is coextensive with 'x=y' if any formula constructible from the vocabulary is; and we can adopt that conjunction as our version of identity.
Gist of Idea
We can paraphrase 'x=y' as a sequence of the form 'if Fx then Fy'
Source
Willard Quine (Word and Object [1960], §47)
Book Ref
Quine,Willard: 'Word and Object' [MIT 1969], p.230
A Reaction
[first half compressed] The main rival views of equality are this and Wiggins (1980:199). Quine concedes that his account implies a modest version of the identity of indiscernibles. Wiggins says identity statements need a sortal.