Full Idea
The simplest plan for properties is to take a property just as the set of all its instances.
Gist of Idea
It would be easiest to take a property as the set of its instances
Source
David Lewis (On the Plurality of Worlds [1986], 1.5)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.50
A Reaction
I find this a weird and counterintuitive proposal. I suppose if you think maths has been reduced to set theory, you might want to reduce everything else, and then we can all go home. I thought things were in sets because of their properties.