Certainly, a _scientific theory_ is judged based on its predictive power. This is why I mentioned my opinion, earlier, that science is "the art of useful fiction." The goal isn't to achieve truth but to predict.
Philosophy, on the other hand, is concerned with what is _really_ true, not predictions.
Certainly, the experience of qualia has no effect demonstrable to an outside observer. That doesn't make it any more or less real. I experience, therefore it must exist. I merely can't prove it to you. The great thing is that you can prove it to yourself. All you have to do is pause for a moment and rationally examine what you feel.
If your interested in an attempt to explain it from a materialist perspective, you may wish to look at Roger Penrose _The Emperor's New Mind_. He argues that consciousness is intricately tied to QM. Or at least I think he does... It's difficult reading and I haven't read the whole thing yet!
It's on Google Books, here: http://books.google.ca/books?id=oI0grArWHUMC&dq=The+Empe...
The first chapter provides an excellent explanation of qualia.
That is the anti-realist position, but it could be true in the sense that it corresponds to the actual nature of reality, if you are a realist.
> You have only one data point: your own consciousness.
Yep, and that is the data point that physicalism doesn't seem to be able to account for.
your second assertion doesn't make any sense either. "account" would imply that pzombies would be different from "normal" normal people I guess?
Yes, p-zombies are different from normal people (by definition).
The problem is precisely one of predictions. Physicalism doesn't predict the existence of conscious experience. (And if you really think its necessary to give an argument for the existence of these experiences other than "we all have them and know that we have them," I am not sure what to say -- you are simply in denial.)
It is absolutely necessary to give an argument for the existence of these experiences.
I know that you believe you have them, and that I believe I have them, but that doesn't mean that they have any tangible reality, it just means that our brains store things in a way that they will respond "yes, I have conscious experience."
Alternatively, should I accept as real everything that a person believes they experience? A schizophrenic's voices are real? A memory of a past trauma that is, in fact, a false memory, proves that the past trauma is real? This is a very dangerous path to walk, accepting the validity of introspection as a means of investigating the world...
It does in the case of experiences. I can't even conceive of what it would mean to be mistaken about the very fact that you are having experiences. It would be a very different sort of mistake from that of being mistaken about the _object_ of an experience (thinking that you are hearing voices when there aren't any, etc. etc.)
Also, your use of the word "tangible" is very odd in this context. Almost by definition, nothing could be more tangible than an experience.
>[I know that] I believe I have them
So, do you believe that you have them or not?
That huge dataset still won't tell you what it's like to see red, though. It will just tell you what happens in the brain when someone sees red.
>It really doesn't, though, unless you already ascribe something mystical and aphysical to this "subjective experience," begging the question.
You don't have to do anything of the sort. You just note that there's nothing in the physical description which corresponds to the subjective quality of your experience. To make it more concrete, there's nothing that explains why seeing red isn't like seeing blue and vice versa.