 Chalmers has responded in several places (1996, 131–134; 1999, 476–7; 2010, 141–205). His most detailed version of the conceivability argument (2010) uses the framework of two-dimensional semantics. This enables him to distinguish two kinds of possibility and two corresponding kinds of conceivability. In the ‘primary’ sense conceivability entails possibility; for example it is conceivable that water should have been a substance chemically different from H2O. In the other, ‘secondary’ sense, it is neither conceivable nor possible that water should have been chemically different. The difficulty for the conceivability argument can be expressed by saying that even if zombie worlds are primarily conceivable and therefore primarily possible, it does not follow that they are also secondarily possible. And a posteriori physicalists will typically deny that it follows, on the ground that only the secondary possibility of zombie worlds would entail the falsity of physicalism. At this point Chalmers in effect presents his opponents with a dilemma, which is (crudely summarizing his conclusions) that either the primary conceivability of zombies does after all entail their secondary possibility, in which case the conceivability argument works and materialism is false; or else what he calls ‘Russellian monism’, briefly considered at Section 5.3 below, is true. (See also Jackson 1998; and for discussions, Brueckner 2002; Loar 1999; Hill and McLaughlin 1999; Perry 2001, 169–208; Piccinini 2017; Sebastián 2017; Shoemaker 1999; Soames 2005; Yablo 1999.) 