Modality and the Mind-Body Problem: Reply to Goff and Papineau, Lee, and Levine

I am grateful to Philip Goff and David Papineau, Geoff Lee, and Joe Levine for their commentaries on The Character of Consciousness. All of the articles in this symposium focus on the first half of the book, especially section III on the metaphysics of consciousness. Each of them addresses my arguments against materialism and offers interesting responses to them. The main focus is on the two-dimensional modal argument in chapter 6 and on related considerations about necessity and apriority. For background, recall that anti-materialist arguments such as the conceivability and knowledge arguments start from an epistemic gap between the microphysical truths P and a phenomenal truth Q, and from there infer an ontological gap between them. For example one can first argue that the material conditional ‘If P, then Q’ is not a priori, and from there argue that it is not necessary, so that materialism is false. In response, type-A materialists deny that there is an epistemic gap, typically holding that there is an a priori entailment from P to Q. By contrast, type-B materialists typically allow that there is an epistemic gap but deny that there is an ontological gap, typically holding that ‘If P, then Q’ is necessary even though it is not a priori. Type-B materialists typically take inspiration from Kripke’s argument that truths such as ‘Water is H2O’ and ‘Hesperus is Phosphorus’ are necessary but not a priori. If one can make the case that ‘If P, then Q’ is also an a posteriori necessity, then this will strike at the heart of the antimaterialist arguments above. In response, the two-dimensional argument in effect makes a case that the materialist needs a much stronger sort of a posteriori necessity than one finds in the Kripke cases, a sort that there is good reason to reject. According to a two-dimensional analysis, the standard Kripkean a posteriori necessities are weak a posteriori necessities in that their primary intensions are false at some centered metaphysically possible world: for example, the primary intension of ‘water is H2O’ is false at a Twin Earth centered world (at which the watery stuff around the center is XYZ). Less technically, we