On a possible worlds account, 'Bachelors are unmarried' and 'Brothers are male siblings' express the same proposition.

The first had to do with the way propositions are individuated on a possible worlds account. The view that propositions are sets of possible worlds does not individuate propositions very finely. For example, consider any pair of sentences that express metaphysically necessary propositions, say ‘Bachelors are unmarried’ and ‘Brothers are male siblings’. Since these propositions are true in all possible worlds, each must be the set of all possible worlds. But there is only one such set. Thus there