2-dimensionalism: WDTM?

``I...outline a two-dimensional intensional framework for handling a posteriori necessity. Every concept has two intensions, a primary intension and a secondary intension. The primary intension delivers a concept's referent in a centered world when the world is considered as actual (i.e., considered as an epistemic possibility); the secondary intension delivers a concept's referent in a world when it is considered as counterfactual. (The primary intension of ``water" picks out roughly the ``watery stuff" in a world; the secondary intension picks out H2O.) A statement is a priori when it has a necessary primary intension; a statement is necessary when it has a necessary secondary intension but a contingent primary intension. A priori necessities (such as ``water is H2O" have a necessary secondary intension but a contingent primary intension. A statement is conceivable (or logically possible) when its primary intension is true in some world; a statement is possible (metaphysically possible) when its secondary intension is true in some world. So the Kripkean gap between conceivability and possibility is explained at the level of statements, without appealing to a distinction between conceivable and possible worlds. The class of worlds in question is always that of the ideally conceivable (or logically possible) worlds. "

--From Chalmers's precis in Philosophy and Phenomenological Research