Full Idea
There are no free-floating possibilia. Every possibility is part of a world - exactly one world - and thus comes surrounded by worldmates, and fully equipped with extrinsic properties in virtue of its relations to them.
Gist of Idea
There are no free-floating possibilia; they have mates in a world, giving them extrinsic properties
Source
David Lewis (On the Plurality of Worlds [1986], 4.4)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.230
A Reaction
This is a key claim in the possible worlds understanding of modality, contrary to the more common sense and normal language claim that a possibility is an isolated thing.