Full Idea
Class Nominalists substitute classes or sets for properties or kinds, so that being white is just being a member of the set of white things; relations are treated as ordered sets.
Gist of Idea
'Class Nominalism' says that properties or kinds are merely membership of a set (e.g. of white things)
Source
David M. Armstrong (Universals [1995], p.503)
Book Reference
'A Companion to Metaphysics', ed/tr. Kim,Jaegwon/Sosa,Ernest [Blackwell 1995], p.503
A Reaction
This immediately seems wrong, because it invites the question of why something is a member of a set (unless membership is arbitrary and whimsical - which it usually isn't).