Full Idea
There is no means by which we might distinguish a possible world from what is true at it. ...Whereas our ability to separate a place, or a time, from its occupier is crucial to realism about places and times, as is a distance relation.
Gist of Idea
Unlike places and times, we cannot separate possible worlds from what is true at them
Source
Graeme Forbes (The Metaphysics of Modality [1985], 4.2)
Book Reference
Forbes,Graeme: 'The Metaphysics of Modality' [OUP 1985], p.78
A Reaction
He is objecting to Lewis's modal realism. I'm not fully convinced. It depends whether we are discussing real ontology or conceptual space. In the latter I see no difference between times and possible worlds. In ontology, a 'time' is weird.