Full Idea
The hypothesized modality of 'strong' metaphysical necessity says there are fewer metaphysically possible worlds than there are logically possible worlds, and the a posteriori necessities can stem from factors independent of the semantics of terms.
Gist of Idea
Strong metaphysical necessity allows fewer possible worlds than logical necessity
Source
David J.Chalmers (The Conscious Mind [1996], 2.4.2)
Book Reference
Chalmers,David J.: 'The Conscious Mind' [OUP 1997], p.137
A Reaction
Chalmers sets this up in order to reject it. He notes that it involves a big gap between conceivability and possibility. If a world is logically possible but metaphysically impossible, then it is impossible, surely?