Full Idea
Two-dimensionalism says the necessity of a statement is constituted by the fact that the secondary intensions is a necessary proposition, and their a posteriori character is constituted by the fact that the associated primary intension is contingent.
Gist of Idea
Two-D says that a posteriori is primary and contingent, and the necessity is the secondary intension
Source
Robert C. Stalnaker (Conceptual truth and metaphysical necessity [2003], 2)
Book Reference
Stalnaker,Robert C.: 'Ways a World Might Be' [OUP 2003], p.205
A Reaction
This view is found in Sidelle 1989, and then formalised by Jackson and Chalmers. I like metaphysical necessity, but I have some sympathy with the approach. The question must always be 'where does this necessity derive from'?