Full Idea
Imaginability is a poor criterion of possibility. We can imagine the impossible provided we do not imagine it in perfect detail and all the time.
Gist of Idea
The impossible can be imagined as long as it is a bit vague
Source
David Lewis (On the Plurality of Worlds [1986], 1.8)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.90
A Reaction
In general I agree, but Williamson nicely opposes this view. The fact is that we derive most of our understanding of what is possible from imagination. We just have to realise that we can get it wrong, and so must attend to detail.