Epiphenomenalism is the claim that even if consciousness is real in the Hard Problem sense, there is no room for it to be causally efficacious. That is, we may really see red and feel pain in ways that are irreducible to the mindless unconscious interactions of our brains' neuroanatomical parts, but our consciousness is a helpless observer. The mindless unconscious parts still do their mindless unconscious work, including controlling our muscle movements and speech, while the consciousness stays trapped in the press box, experiencing it all, including the delusion that it itself is controlling anything.
The main argument for ephiphenomenalism is that since we know physics pretty well, and we are getting better all the time at neuroscience, sometime in the not too distant future we should be able to characterize all of our behavior (even the "behavior" of our mental processing, stripped of any considerations of subjective qualitative consciousness) strictly in terms of nuts and bolts neuronal processing without recourse to notions of consciousness. The physical world is causally closed. That is, every physical thing that ever happens has an understood physical cause. Therefore there is no way that some hitherto undiscovered mysterious force of consciousness could have any physical effect, including the effect of making my neurons fire, my muscles move, etc. If a perfectly accurate physical account can be given of every neuronal event that happens as I type this essay, or comment on the beauty of a sunset, and this account is given strictly in terms of ordinary physics, then it puts people who believe that the Hard Problem exists in an awkward position. Subjective consciousness, if it exists in the Hard Problem sense, would appear to be redundant, an extra, a loose thread hanging off the natural world, or it would violate the laws of physics.
So does consciousness just watch the processing, without influencing it at all? I know I am conscious in some way that can not be reduced to a functional description of the causal interactions of my micro-parts, and my consciousness certainly thinks that it is in control of my fingers as I type this. It thinks (or experiences) that when I write about how subjective consciousness feels, each word I write is dictated (or at least strongly influenced) by my actual, immediate perception of how subjective consciousness feels.
The epiphenomenalist would have us believe that this is not true, that there is no real contact between the physical body and brain on one hand and consciousness on the other, or at least only one-way contact. So while my consciousness has the perception of writing a sentence about consciousness, and that of commanding fingers to press certain keys on my keyboard, the completely unconscious mechanistic brain is really ordering the very same fingers to type out the very same sentence. Essentially, as far as our actions are concerned, including the ones we most closely associate with emotion and experience, we are zombies. We just happen to have a parasitic consciousness along for the ride, one which is deluded into thinking that it is calling the shots. For this to be the case, of course, the mechanistic processes would have to maintain absolutely perfect synchrony with my actual consciousness throughout my entire lifetime, or my consciousness would notice the discrepancy. It is as if, given a puppet dancing on a stage, we were told that the puppet is really doing the dancing by itself, but so well, in such perfect sync with the puppeteer pulling the strings, that the puppeteer never catches on.
There are some ideas, the old saying goes, that are so preposterous only a philosopher would take them seriously. No, there is no knock-down purely logical argument against epiphenomenalism, but as would-be scientists, we should feel comfortable discarding the more wildly implausible ideas, and epiphenomenalism is such an idea. Evolutionarily, why would nature have played such an elaborate trick on us? Why not just evolve us as zombies and have done with it?
In the epiphenomenalists' defense, it is sometimes suggested that there is nothing mysterious about the synchrony between puppeteer and puppet if some third party is actually controlling both of them. The fingers type a sentence about consciousness mechanistically, and the subjective consciousness says (and believes), "I meant to do that." Some volitional center could be controlling both our actions and our experience. In this case, we paint our thoughts with a much thinner coat of qualitative consciousness than we might otherwise think.
In our more generous moods, we might believe that the mechanistic zombie part of us is very complex, and it is worth its while to do some cognitive garbage collection and house cleaning, to investigate and thereby improve its internal mechanisms for absorbing, digesting, and applying information about the world and itself. Self-knowledge, even understood purely functionally, has definite behavioral advantages for a complex enough system. Perhaps our purely cognitive machinery has evolved to constantly self-evaluate, to second-guess all of its conclusions and perceptions. Might not such a system "notice" that at some low level of internal representation it could probe no further, that it could not get inside its seeing of red, for example? Might this impasse cause the system's attention to shift toward the problem? Could such a system's self-probing possibly end up being externally articulated, like Chalmers' book or these essays? Would the system ever come up with an idea like epiphenomenalism (after all, it was the mindless mechanistic neural processing which typed this very essay, completely unaided by my consciousness, according to the epiphenomenalist)? It is not immediately obvious that the answer to these questions is no.
It could be, then, that while there is a consciousness in the Hard Problem sense, it monitors unconscious cognitive processing, as if it had a lot of diagnostic probes alligator-clipped onto various stages and parts of this processing.
This almost makes epiphenomenalism respectable, but it is still pretty implausible. If the coupling of qualia to functional states and mechanisms is so very tight that every qualitative state is dictated by a functional state, to the extent that even my wondering about consciousness corresponds perfectly to some functional self-diagnostic probing, epiphenomenalism becomes a moot point. There are not, then, two distinct parts, a mindless functional part and a helpless (but deluded) conscious part; instead there is just one mechanism which has a qualitative aspect. We are aware of every decision we make, every action we perform as our own, because at a very fine-grained level our immediate conscious experience is of the very mechanism that is actually doing the driving. In this case, the question of whether consciousness has any effect degenerates into the problem of free will, and the answer depends on endless hours of late night discussions about how we define our terms and where we draw our lines. If my mind's functioning has two aspects, cognitive and experiential, can we even say that one aspect is "me" and not the other, or that one aspect is doing all the willful work and not the other? If you couple the two aspects (functional and experiential) closely enough to make epiphenomenalism remotely plausible, then you couple them too closely to say that one is efficacious and the other is not.
Epiphenomenalism does raise a serious challenge, though. If it is false, and qualitative phenomenal consciousness is really guiding my fingers now, as it seems to be, then this spooky mysterious thing called consciousness has macroscopic, observable effects in the real physical world. Where, then is the interface? Why haven't brain scientists noticed by now that certain neurons fire at certain times for no reason that they can explain with current physics?
If you accept the Hard Problem, and you believe that epiphenomenalism is false, then you are committed to the belief that current physics is wrong, or at least substantially incomplete in some sense that allows for an as-yet undiscovered force to have a physical effect. Somehow, large-scale, high-level consciousness is able to exert an influence on, for example, motor neurons, and make them do things that they simply would not do if they were only subject to ordinary physical laws without the influence of consciousness. This is a tall order.
When discovered, I suspect that it won't be so much a case of some single event happening that we can't explain, as it will a lot of events, each of which should be random according to accepted physical laws, but which happen in sync with each other, or in some pattern, which once recognized, will be undeniable. Any one of these events, when studied alone, will be seen to obey normal physical laws, but considered together, they will have a pattern and an organization that we can not account for with normal physical laws. I imagine that the influence exerted by consciousness on physical systems will ultimately be compatible with the laws of physics.
This, of course, is pure speculation, but it points us in a certain direction. If we are to take the Hard Problem seriously, and if we reject the notion of epiphenomenalism, we are placing our bets on some high-level, large scale process, structure, or field that has qualitative content and influences physical things through a loop-hole in physics.