A correspondent writes:
Consider the brain of a human looking at three colored balls, which are red, blue, and green respectively. Then a trustworthy logically omniscient agent comes along and truthfully says the following:
1. Brains like this brain, faced with balls like these in the absence of logically omniscient intervenors, engage in information processing that results in inner z-representations to the effect that the red, blue and green balls are inducing different qualia. This typically results in utterances like "when I see these three different balls they look phenomenally red, blue, and green."
2. Across possible non-zombie worlds, in 10% there are indeed different qualia or phenomenal colors for each of the balls, while in 90% the red and blue balls have the same phenomenal color, which is distinct from the phenomenal color of the green ball.
In this case, and using your account, wouldn't the brain maximize the likelihood that its z-beliefs would result in true beliefs by z-believing that it sees only two phenomenal colors rather than three?
Sure (assuming that the measure of a priori credibility over these worlds mirrors the 10/90 split). But it's like a case where an angel tells you that experiences like yours are most likely to be non-veridical because an 'evil demon' or BIV world is objectively more probable than true Earth. If that claim really is true, then sure, I guess you should disbelieve your senses in such a case. But it's highly implausible.
The thought experiment raises a more interesting issue though: might there be constraints on the bridging laws that convert physical configurations into qualia? Although you can have any physical arrangement without corresponding qualia (cognition without mentality), it's not so clear that you can have any old qualia without corresponding computations (mentality without cognition). That is, we might doubt whether 'raw phenomenal feel' can hold entirely independently from the rest of our mental economy. Instead, it may be that in order to see two balls as the same colour, one must also be disposed to judge that they are the same colour, etc.
On this picture, then, you can either have a zombie world with no qualia at all, or else qualia that correspond appropriately to physical structures (which may still allow for some variation, e.g. colour inversion). But there's no sense to be made of free-floating qualia that are thoroughly out of sync with the physical computations that are going on in the world.
(I don't have any arguments for this view. I'm not even sure whether it's well-motivated. I just thought I'd throw the idea out there and see what others think...)