Full Idea
A possible world can't be defined (by Stalnaker and Plantinga) as a way the world might have been, because a possible world is possibly the world, yet no way the world might have been is possibly the world.
Gist of Idea
The actual world is a possible world, so we can't define possible worlds as 'what might have been'
Source
Kit Fine (The Problem of Possibilia [2003], 2)
Book Reference
'The Oxford Handbook of Metaphysics', ed/tr. Loux,M /Zimmerman,D [OUP 2005], p.163
A Reaction
His point is that any definition of a possible world must cover the actual world, because that is one of them. 'Might have been' is not applicable to the actual world. It seems a fairly important starting point for discussion of possible worlds.