 Philosophers have paid more attention to the examples given by Kripke than to other possible cases of the necessary a posteriori, and for that reason the discussion in the rest of this section will mostly focus on Kripke’s cases. Two strategies for explaining these examples can be distinguished. Dualists about metaphysical and epistemic modality (dualists, for short) hold that the phenomena reflect a deep and fundamental distinction between two kinds of modality. Monists, by contrast, believe that all the data can ultimately be explained by appeal to a single kind of modality. They may agree that there are cases in which a single sentence is, in some sense, both necessary and a posteriori, or both contingent and a priori. But they insist that there is no similar distinction at the level of worlds or propositions. Rather, the phenomenon arises because a single sentence can be associated with two different propositions, one that is necessary and another that is contingent.