[mikeash]

It seems clear to me that qualia must affect physical reality. If it didn't, we wouldn't be talking about it. If it was just a byproduct with causality going only in one direction, then we'd never talk about it, because the behavior of a physical system with that byproduct would be identical to that same physical system without it. There has to be a causal chain from this discussion back to qualia, unless the discussion happened by coincidence, which is extremely unlikely.

I don't think this tells us anything about what qualia is or whether it is or isn't a material process, but I don't think it's tenable to say that it either doesn't exist entirely, or exists but doesn't affect anything.

[mikhael]

i'll admit that that's a convincing argument, but there are ways in which apparent causality can be shown to be illusory. one thought experiment i remember from school involves someone watching a movie in which one character punches another character, and the punched character falls backward. suppose the watcher knows absolutely nothing about how movies are recorded, or re-played, and sees only the lifelike images; then there would be clear, apparent causality of the second character falling over as an effect of being punched by the first. in reality, the only causality is that set up by the mechanics of the movie projector.

[mikeash]

Well, it's a probabilistic argument. While it's possible that it's just a coincidence that we both have qualia and discuss qualia, it's an extremely unlikely coincidence.

To twist your movie analogy beyond all use, it's like trying on some clothes in a dressing room, then watching a movie with a scene that features you trying on those exact same clothes in that exact same dressing room in the exact same way you tried them on. It's possible that the filmmaker just happened to capture the exact same scene by chance, but it's vastly more likely that he was secretly recording you.

[mikhael]

i would argue that we can't accurately infer any likelihood without knowing what the entire probability space is. in the dressing-room example, we are assuming there are not many, many dressing rooms that look similar, and many, many people that look just like us, and many different sets of the same clothes. but that is just an assumption. we have no idea about the probability space of different material universes and how qualia is embedded in them.