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Full Idea
Forbes has two principles of identity, which we can call the No Bare Identities Principle (identities hold in virtue of other facts), and the No Extrinsic Determination Principle (that only intrinsic facts of a thing establish identity).
Gist of Idea
Identities must hold because of other facts, which must be instrinsic
Source
report of Graeme Forbes (The Metaphysics of Modality [1985], 127-8) by Penelope Mackie - How Things Might Have Been 2.7
Book Ref
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.31
A Reaction
The job of the philosopher is to prise apart the real identities of things from the way in which we conceive of identities. I take these principles to apply to real identities, not conceptual identities.