Over at The Splintered Mind, Teed Rockwell has put up a brief defense of an argument that zombies are impossible. The argument is this:
(1) Zombies are possible if and only if subjective experiences are epiphenomenal.
(2) Subjective experiences are epiphenomenal if and only if we have direct awareness of them.
(3) There is no such thing as direct awareness.
Ad (1). This is not true. The closest truth in this vicinity is:
(1`) If it is possible that subjective experiences are epiphenomenal, then zombies are possible.
I can't see why subjective experiences would have to be epiphenomenal in the actual world in order for zombies to be exist in some other world. Why couldn't it be the case that qualia have some functional role in our cognitive architecture in the actual world, but the non-qualitative aspects of cognition subserve that functional role in another world? Perhaps the idea is that properties are individuated by the causal powers they grant when instantiated, so that P = Q only if, necessarily, Px causes p iff Qx causes p. Then a property is actually epiphenomenal iff necessarily epiphenomenal. This is implausible to me, though. Surely some property bestows some causal powers in the actual world that it does not in other worlds. But if that is the case, then qualia could be properties of this sort. They could bestow functional cognitive properties on conscious thinkers in the actual world that they do not in other worlds. Anyway, all this is OK for Rockwell's argument so far, since we only need something of the strength of (1`) for the argument to go through, if (2) and (3) are true.
The real problem is with (2). I suppose what would make (2) plausible is an argument to the effect of:
(4) We are either directly aware of a property's being instantiated, or we infer it.
(5) We infer some fact p on the basis of q only if p causes q.
(6) The instantiations of epiphenomenal properties do not cause anything else.
Then, from (3), we get Rockwell's desired conclusion. As things stand, I lean towards saying that epiphenomenalism is actually false, but might have been true. By (1`), this immediately gets us that zombies are possible. But let's assume that epiphenomenalism is true, and even that (1) is true. In this sort of position, my beef has always been with (5). I always thought that, if epiphenomenalism is true, then we brought qualia into our ontology to explain the justification we have for believing self-ascriptions of certain mental states. The qualitative character of the state of, say, seeing an apple from ten feet away in decent, neutral light is supposed to be what explains how I generally know, or am justified in believing, that I see an apple when I do. This is not to say that the instantiation of a certain quale causes me to believe that I see the apple. Rather, the instantiation of the quale (perhaps given some other conditions about my proper mental functioning) is what provides that belief with its positive epistemic status. But it would be strange to say that the conferral of that epistemic status is a causal matter. I want to say that the instantiation of the quale does not causally affect my belief's epistemic status because the epistemic status of a belief is not the sort of thing that can be caused to be one way or another. I do infer the existence of qualia, in general, but not from their causal roles. I infer them to explain the epistemic status that I confer on mental state self-ascriptions. So qualia are a counterexample to (5) because they are an instance in which inference to the best explanation is not inference from cause to effect.