Full Idea
It is the identification of properties with sets that rules out, for Lewis, an actualist account of possible worlds.
Gist of Idea
Lewis rejects actualism because he identifies properties with sets
Source
report of David Lewis (On the Plurality of Worlds [1986]) by Robert C. Stalnaker - Mere Possibilities 1.1
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.7
A Reaction
I suppose the sets which are the properties have to include all the possible red things as well as the actual one. This escapes the renate/cordate problem.