Full Idea
The modal Platonist denies that knowledge always depends on a causal relation. The reality of possible worlds is an ontological requirement, to secure the truth-values of modal propositions.
Gist of Idea
Knowledge of possible worlds is not causal, but is an ontology entailed by semantics
Source
Stephen Read (Thinking About Logic [1995], Ch.2)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.52
A Reaction
[Reply to Idea 10982] This seems to be a case of deriving your metaphyics from your semantics, of which David Lewis seems to be guilty, and which strikes me as misguided.
Related Idea
Idea 10982 How can modal Platonists know the truth of a modal proposition? [Read]