Full Idea
We cannot know what something is without knowing how it is marked off from other things. Identity is thus of a piece with ontology.
Gist of Idea
We know what things are by distinguishing them, so identity is part of ontology
Source
Willard Quine (Ontological Relativity [1968], p.55)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.55
A Reaction
Actually it is failure of identity which seems to raise questions of individuation. If I say 'this apple is [pause] identical to this apple', I don't see how that helps me to individuate apples.