Type-B materialists (to use David Chalmers's jargon) claim that though zombies are conceivable, they are not metaphysically possible. This article calls this position regarding the relation between metaphysical and epistemic modality “modal autonomism,” as opposed to the “modal rationalism” endorsed by David Chalmers and Frank Jackson, who insist on a deep link between the two forms of modality. This article argues that the defense of modal rationalism presented in Chalmers and Jackson (2001) begs the question against the type-B materialist/modal autonomist.
The argument proceeds as follows. Modal rationalists claim that for all nonphenomenal macro properties, the appropriate supervenience conditional is both necessary and a priori. Hence, type-B materialists must engage in special pleading when they claim that the relevant supervenience conditional for phenomenal properties, expressing the supervenience of the phenomenal on the physical, is necessary but not a priori. However, what Chalmers and Jackson demonstrate, if anything, is that the conditional that includes all the microphysical plus the phenomenal in the antecedent, and nonphenomenal macro facts (such as facts about water and other natural kinds, among other things) in the consequent, is a priori. The question arises why, since facts about water and the like do not metaphysically supervene on the phenomenal facts, is it appropriate to include the phenomenal facts in the antecedent of the relevant supervenience conditional.
This article argues for the following claims: First, that it's crucial to the general semantic framework Chalmers and Jackson defend that they do include the phenomenal facts in the supervenience conditional; without them, the conditional would not be a priori. Second, that the only way to argue from the a priori character of these conditionals to the applicability of modal rationalism to the nonphenomenal cases is to rely either on modal rationalism itself or on the denial of type-B materialism. Obviously, in the context of this argument, either way would beg the question.