 Two-dimensional semantics has also been used for more ambitious philosophical purposes. Influential theorists like David Lewis, Frank Jackson and David Chalmers argue that a generalized 2D semantic framework can be used to isolate an apriori aspect of meaning. Roughly, the idea is that speakers always have apriori access to the truth-conditions associated with their own sentences. On the face of it, this apriority claim seems to conflict with the observation that certain necessary truths, such as ‘water = H2O’, can be known only on the basis of empirical inquiry. But proponents of generalized 2D semantics argue that the 2D framework undercuts this objection, by showing how such aposteriori necessities are consistent with apriori access to truth-conditions. The positive reasons to accept generalized 2D semantics, however, are bound up with larger (and partly disjoint) explanatory projects. As a consequence, debates over the merits of generalized 2D semantics touch on broader controversies about apriority, modality, semantic theory and philosophical methodology.