Full Idea
Perhaps the main job of a theory of universals is to give an account of resemblance.
Gist of Idea
Universals are meant to give an account of resemblance
Source
David Lewis (Against Structural Universals [1986], 'Why believe')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.82
A Reaction
This invites the quick reply, popular with some nominalists, of taking resemblance as primitive, and hence beyond explanation.