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Full Idea
Possible worlds might be concrete (Lewis), or abstract (Stalnaker), or structured universals (Forrest), or collections of sentences (Jeffrey), or mere combinations of properties and relations (Armstrong).
Gist of Idea
Possible worlds could be concrete, abstract, universals, sentences, or properties
Source
Frank Jackson (From Metaphysics to Ethics [1998], Ch.1)
Book Ref
Jackson,Frank: 'From Metaphysics to Ethics' [OUP 2000], p.11
A Reaction
A helpful summary. I don't like concrete, or collections of sentences. Whatever they are, they had better be 'possible', so not any old collection or idea will do.