Full Idea
It is not a good idea to think of possible worlds as sets of propositions, and at the same time to think of propositions as sets of possible worlds.
Gist of Idea
You can't define worlds as sets of propositions, and then define propositions using worlds
Source
Nathan Salmon (The Logic of What Might Have Been [1989], I n3)
Book Reference
Salmon,Nathan: 'Metaphysics, Mathematics and Meaning' [OUP 2005], p.131
A Reaction
Salmon favours thinking of worlds as sets of propositions, and hence rejects the account of propositions as sets of worlds. He favours the 'Russellian' view of propositions, which seem to me to be the same as 'facts'.