Full Idea
Lewis has a preference for a nominalist conception of properties as classes of possible and actual concrete particulars.
Gist of Idea
Properties are classes of possible and actual concrete particulars
Source
report of David Lewis (New work for a theory of universals [1983]) by Kathrin Koslicki - The Structure of Objects II.3
Book Reference
Koslicki,Kathrin: 'The Structure of Objects' [OUP 2008], p.29
A Reaction
I'm sympathetic to nominalism, but still can't swallow the idea that a property like redness is nothing more than a collection of particulars, the red things. This class will include all sorts of non-red features.