Full Idea
If a 'world' is understood as a set of sentences, then possibility may be understood as consistency, ...but this seems circular, in that 'consistency' of sentences cannot adequately be defined save in terms of possibility.
Gist of Idea
If 'worlds' are sentences, and possibility their consistency, consistency may rely on possibility
Source
William Lycan (The Trouble with Possible Worlds [1979], 09)
Book Reference
'The Possible and the Actual', ed/tr. Loux,Michael J. [Cornell 1979], p.302
A Reaction
[Carnap and Hintikka propose the view, Lewis 'Counterfactuals' p.85 objects] Worlds as sentences is not, of course, the same as worlds as propositions. There is a lot of circularity around in 'possible' worlds.