Serious Metaphysics and A Priori Entailment
Serious Metaphysics, in Jackson's sense, tries to identify a limited set of truths (i.e. true sentences) that entail (i.e. strictly imply) all truths. So what about
*) Everything is just as it actually is?
((p)(p <-> actually p), or (x)(F)(Fx <-> actually Fx))
(*) is true. It entails all other truths: whenever S is true, then so is "necessarily, if (*) then S". And it is fairly simple and economic: for instance, it doesn't contain macrophysical or phenomenal terms. Still, it's not serious metaphysics. What's wrong?
The most obvious problem is perhaps that (*) doesn't tell us anything about our world: it is a priori. Hence we can't infer from (*) that e.g. water covers 60% of the earth, even though it entails that. What we want from fundamental truths, it seems to me, is that they allow us to infer -- without any further empirical information -- all other truths. At least in principle.
If that is right, then type-B materialism, saying that all truths supervene upon, but are not deducible from, the physical truths, is not engaged in serious metaphysics.
Well, whether we call it "serious metaphysics" or not, isn't it still an interesting claim that all truths are necessitated by physical truths? Yes, but (*) shows that the claim is only interesting to the extent that physical truths don't contain certain kinds of rigid expressions: if "is just as it actually is" counts as a physical predicate, the claim is trivial. If physical truths don't contain any rigid expressions, the claim gets interesting, but probably collapses into the a priori entailment thesis. A mixed case is what Chalmers calls "panprotopsychism": Suppose some physical expression, say "spin-up", rigidly denotes whatever property occupies the spin-up role, which happens to be a kind of phenomenal or protophenomenal property, say pain. Then truths about spin-up states necessitate truths about pain, but we can't infer the latter from the former. This is still an interesting claim, but it doesn't seem to be properly classified as materialism. Why not? Because on this view, more than just the physical truths are needed to deduce all other truths.