Full Idea
Lewis thought that the property of being F was identical with the set of objects, in all possible worlds, which are F.
Gist of Idea
The property of being F is identical with the set of objects, in all possible worlds, which are F
Source
report of David Lewis (On the Plurality of Worlds [1986], §1.5) by Ross P. Cameron - Intrinsic and Extrinsic Properties
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.269
A Reaction
I can't make head or tail of a theory which says that a property is a set of objects. I'll show you a room full of objects and tell you they are a property. How are you going to work out what the property is? 'Being F' is a predicate, not a property!