Suppose that over the next hour, you are consciously and constantly perceptually aware of a red wall, while I am consciously aware of nothing red—I am in a room where everything is blue—except that for exactly one nanosecond, half way through the hour, I have induced in me your intrinsic mental state, followed by a restoration of my previous state. Thus, if I was consciosu of something red during that nanosecond, I don't remember it afterwards.
But was I ever conscious of something red? I doubt it. A mental state that short just "doesn't register". Plausibly, given how humans are constituted, there just is no such thing as a pain that's only a nanosecond long, and there is no such thing as a consciousness of red that's only a nanosecond long.
But this yields the following argument, where we stipulate that a state is "near-instantaneous" provided that it has a temporal dimension of at most a nanosecond:
- All our intrinsic states are reducible to our near-instantaneous intrinsic states. (Premise)
- Humans have no near-instantaneous intrinsic state of being conscious of red. (Premise, justified by the thought-experiment)
- Being conscious of red is an intrinsic state of us. (Premise)
- So, a human's being conscious of red is reducible to near-instantaneous states that are not themselves consciousnesses of red.
If this argument is right, then qualia are reducible. It does not, however, follow that they are reducible to physical states. It could, instead, be the case that conscious states are reducible to fundamental mental states that are not conscious states. The hard problem of mind is the problem of intentionality, perhaps, not the problem of consciousness (a sentiment I think I have heard from more than one person).
But all that said, I am not sure of premise (1) in the argument above. And I am not completely sure that the thought experiment succeeds. Maybe one can say that one is near-instantaneously aware of red but one forgets it right away. I don't know.