Full Idea
Lewis's knowledge of what possible worlds there are and of other general truths about worlds is posterior, not prior, to his knowledge of what things are possible and what things are impossible.
Gist of Idea
Lewis can't know possible worlds without first knowing what is possible or impossible
Source
comment on David Lewis (Possible Worlds [1973]) by William Lycan - The Trouble with Possible Worlds 07
Book Reference
'The Possible and the Actual', ed/tr. Loux,Michael J. [Cornell 1979], p.295
A Reaction
This elementary objection seems to me to destroy any attempt to explain modality in terms of possible worlds. It is a semantics for modal statements, but that doesn't make it an ontology. To assess possibilities, study actuality.