To whom it may concern:
This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. If a discussion gets unwieldy, celebrate by turning it into a top-level post.
(After the critical success of part II, and the strong box office sales of part III in spite of mixed reviews, will part IV finally see the June Open Thread jump the shark?)
If a subjective experience is the same event, differently described, as a neural process, you can be wrong about whether you are having it. You can also be wrong about whether you and another being share the same or similar quale, especially if you infer such similarity solely from behavioral evidence.
Even aside from physical-side-of-the-same-coin considerations, a person can be mistaken about subjective experience. A tries the new soup at the restaurant and says "it tastes just like chicken". B says, "No, it tastes like turkey." A accepts the correction (and not just that it tastes like turkey to B). The plausibility of this scenario shows that we can be mistaken about qualia. Now, admittedly, that's a long way from being mistaken about whether one has qualia at all - but to rule that possibility in or out, we have to make some verbal choices clarifying what "qualia" will mean.
Roughly speaking, I see at least two alternatives for understanding "qualia". One would be to trot out a laundry list of human subjective feels: color sensations, pain, pleasure, tastes, etc., and then say "this kind of thing". That leaves the possibility of zombies wide open, since intelligent behavior is no guarantee of a particular familiar mental mechanism causing that behavior. (Compare: I see a car driving down the road, doing all the things an internal combustion engine-powered vehicle can do. That's no guarantee that internal combustion occurs within it.)
A second approach would be to define "qualia" by its role in the cognitive economy. Very roughly speaking, qualia are properties highly accessible to "executive function", which properties go beyond (are individuated more finely than by) their roles in representing, for the cognizer, the objective world. On this understanding of "qualia" zombies might be impossible - I'm not sure.