Full Idea
The equation of a possible world with the way that the (actual) world might be is wrong: the way a distant world might be is not a way the world might be, but a way we might allow it to be given how some intervening world might be.
Gist of Idea
Possible worlds aren't how the world might be, but how a world might be, given some possibility
Source
Michael Dummett (Could There Be Unicorns? [1983], 8)
Book Reference
Dummett,Michael: 'The Seas of Language' [OUP 1993], p.347
A Reaction
The point here is that a system of possible worlds must include relative possibilities as well as actual possibilities. Dummett argues against S5 modal logic, which makes them all equal. Things impossible here might become possible. Nice.