Full Idea
If I see simultaneously two shades of green, I can see that they resemble each other, and I see that they resemble each other more than they resemble a shade of red; in this way I become acquainted with the universal 'resemblance'.
Gist of Idea
I learn the universal 'resemblance' by seeing two shades of green, and their contrast with red
Source
Bertrand Russell (Problems of Philosophy [1912], Ch.10)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.59
A Reaction
This is strikingly different from the account of Hume, who seemed to regard resemblance as a fairly mechanical, computer-like activity of the brain, whereas Russell (an empiricist) responds by inclining towards Platonism. Hume sounds better here.