I already wrote this in a previous post, but after reading a post at Philosophy Et.Cetera – Why do you think you are conscious?, let me just repeat in short the issue which seems to me is devastating for epiphenomenalism.
The issue is, that epiphenomenalism would tell us that there is nothing contradictory in the idea of zombie-world which is same as ours, just that in it humans lack consciousness.
However, if you think of it, in that zombie world, the twin-zombie-Chalmers, have written the same books and papers, that he wrote in this world. And same goes for Richard, and all other epiphenomenalists.
Now, in this world, we have a good account of why they wrote those things, and why they say those things. We talk about beliefs, and intuitions, and thinking, awareness of qualia, and so on. So, in this world, we have psychological account of why those things are written. And really, any anti-physicalist would want to say that they are saying those things because they mean them.
However, those things don’t exist in the zombie world, so they can’t be used in the explanation. Things like intentions, beliefs, qualia, awareness, and so on, are not supposed to be there. And yet, still, by some wonder, the same books, the same papers, are still written.
Maybe this doesn’t go against the idea that the zombie world is conceivable, for sure it is possible that by random chance zombies get to write the things Chalmers and others wrote, but it seems to me, makes the idea very implausible.
So, the question for epiphenomenalists is – do they have any better explanation of why zombie-Chalmers ended up writing those books and papers, or is the mere chance (which I guess they will agree is very implausible?) the only thing on which they rely?