According to epiphenomenalism, qualia—the raw experiential feels—are causally inert. In particular, it seems that my beliefs about qualia are not caused by the qualia, but by the neural correlates of the qualia. But this would lead to the absurd possibility that I might take myself to have exactly the sensory experience I now do—the visual experience as of a computer screen, the auditory experience as of keys tapping and fans running, the tactile experience as of my left leg tucked under me—while having no sensory experiences whatsoever. Moreover, it seems to open up the way for an odd sceptical hypothesis: maybe I am wrong in thinking I am conscious, but actually I am a total zombie!
Maybe the "I am a total zombie" hypothesis isn't an option. For maybe my occurrent beliefs are essentially conscious. Perhaps an occurrent belief is partly constituted by a content-providing neural state and the right as-of-believing quale. So without the qualia, I wouldn't have the beliefs, and in particular I wouldn't believe that I am conscious. So I couldn't be wrong in thinking occurrently I am conscious. Alright, so while the "I am a total zombie" hypothesis can be ruled out, the hypothesis that I am a partial zombie, that I have no sensory qualia but only the as-of-believing qualia, still around, and seems almost as problematic.
Maybe, though, the occurrent belief that I am having a visual experience as of a computer screen is partly constituted not by, or not just by, an as-of-believing quale, but by the qualia of the visual experience. If so, then I can't have the occurrent belief that I am having a visual experience while being a visual zombie.
If we take the above solution, though, we run the danger of violating the platitude that our beliefs cause our actions. For if my occurrent beliefs are partly constituted by qualia, and qualia are causally inefficacious, then it seems that it is not the beliefs but their causally efficacious neural constituents that cause the actions.
I am not sure how much weight to put on this objection to epiphenomenalism. After all, if my car's headlights blind a driver, then my car blinded the driver, even if only derivatively. There is no problem with overdetermination when one of the overdeterminers is derivative from the other. It is, perhaps, a little troubling that our occurrent beliefs only derivatively cause our actions, but that might in fact be just right. For it could be that an occurrent belief is partly constituted by a non-occurrent belief and something—maybe the as-of-believing quale—that makes it occurrent. And then it could be that the associated non-occurrent belief is what causes the action—after all, non-occurrent beliefs certainly do affect our actions.
So the "Might I be a zombie?" objection has fallen. But there is still an objection in the vicinity. My memory of having had experiences is not caused by these experiences. And that is wrong: a memory of A must be caused by A (at least in the derivative kind of way in which even absences are said to cause—I can, after all, remember an absence).