Full Idea
Basing content on possible worlds that result in truth leaves no room for thoughts about genuine impossibilities, since there are not possible worlds whose actuality would make an 'impossible thought' true.
Gist of Idea
Truth conditions in possible worlds can't handle statements about impossibilities
Source
David Papineau (Thinking about Consciousness [2002], 3.7)
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.88
A Reaction
Negative existentials like 'no rabbits in this room' and 'no snakes in this room' seem to have the same truth conditions as well. I suppose the sentences must be translated into a logical form which suits the theory, with negation stuck on the end.