A posteriori necessities in one dimension

Arguably, the proposition that Mark Twain is Samuel Clemens and the proposition that water is H2O are both a posteriori. Nevertheless, they both seem to be necessary. Ever since Davies and Humberstone (Philos Stud 38(1):1–31, 1980), it has been known that two-dimensional semantics can account for this fact. But two-dimensionalism isn’t the only theory on the market that purports to do so. In this paper, I will look at two alternatives, one by Scott Soames and one by Kathrin Glüer-Pagin and Peter Pagin, and argue that both of them fail. Regarding the former, I argue that the conceptually possible but metaphysically impossible worlds one is required to postulate are hard to conceive of on closer inspection. As for the latter, the proposal doesn’t work for certain modal sentences, and I show that it cannot be easily amended.