Full Idea
If we replace modal words like 'possible' with quantification across worlds, clearly the notion of 'world' must exclude impossible worlds, otherwise 'possibly p' will be true if 'p' holds in an impossible world.
Clarification
'Quantification' asserts what a proposition is referring to
Gist of Idea
If 'possible' is explained as quantification across worlds, there must be possible worlds
Source
Colin McGinn (Logical Properties [2000], Ch.4)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.70
A Reaction
The point here, of course, is that the question is being begged of what 'possible' and 'impossible' actually mean.