Full Idea
Primary intension picks out a referent in a world considered as actual; secondary considers it as counterfactual. ...(62) We can think of the primary and secondary intensions as the a priori and a posteriori aspects of meaning, respectively.
Gist of Idea
Primary and secondary intensions are the a priori (actual) and a posteriori (counterfactual) aspects of meaning
Source
David J.Chalmers (The Conscious Mind [1996], 1.2.4)
Book Reference
Chalmers,David J.: 'The Conscious Mind' [OUP 1997], p.60
A Reaction
Primary intension is a priori because, it seems, it is stipulative ('water' means 'the watery stuff'), whereas the secondary intension (in counterfactual worlds) is empirical ('water' is used to refer to H2O/XYZ). We get internalism and externalism.