Full Idea
Any realist theory of possible worlds will be circular in its attempt to illuminate modality, for there has to be some criterion of what counts as a possible world.
Gist of Idea
Realism about possible worlds is circular, since it needs a criterion of 'possible'
Source
David S. Oderberg (Real Essentialism [2007], 1.1)
Book Reference
Oderberg,David S.: 'Real Essentialism' [Routledge 2009], p.2
A Reaction
Seems right. At the very least, if we are going to rule out contradictory worlds as impossible (and is there a more obvious criterion?), we already need to understand 'impossible' in order to state that rule.