 Finally, the distinction between primary and secondary conceivability and possibility pertains to Chalmers’ version of  Two-Dimensional Semantics.  His Epistemic Two-Dimensional Semantics (E2-D) proposes a way to address the challenge posed by Kripkean a posteriori necessities, namely the apparent problem that a posteriori impossibilities like “water is not H2O” seem conceivable. In response, Chalmers holds that “water is not H2O” is not secondarily conceivable, though it is still primarily conceivable. What exactly does this mean?