jessicat@jessi_cata
To further elaborate on knowledge arguments for multiple realizability:
Suppose there is a planet with intelligent aliens. The aliens have two subtypes. One subtype processes the *functional* emotion of lust with ABC-fibers (some kind of neural circuit or similar). The other subtype processes functional lust on XYZ-fibers. The I/O behavior of ABC-fibers is practically indistinguishable from that of XYZ-fibers. (By "functional lust", I mainly mean the I/O behavior that would be expected from actually experiencing the emotion of lust, while staying agnostic on whether lusty experience actually occurs.)
We could imagine a situation where one of these aliens has never experienced functional lust, and also does not know if they have ABC-fibers or XYZ-fibers. They may now enter a situation (like reaching puberty) where they would, predictably-to-them, have functional lust. It would strongly seem to them that they really are experiencing lust.
This alien may also be uncertain about whether ABC-fibers lead to actual lust (not just functional lust), and similar for XYZ-fibers. The question is, when they experience lust (or at least it strongly seems to them that they do), what update do they make?
The functionalist answer is that they make no notable update. They already predicted they would have functional lust. There is no good candidate for a "further fact" they would learn about whether they "actually experience lust".
A possible non-functionalist answer would be that the alien, upon experiencing lust, learns: "If I have ABC-fibers, then ABC-fibers implement lusty experience; and if I have XYZ-fibers, then XYZ-fibers implement lusty experience". This is a possible further fact, according to the kind of alien who is not a functionalist.
We could imagine that the alien actually has ABC-fibers, and learns this after the fact. Then they know that ABC-fibers must be able to implement the emotion of lust, because they experienced lust and have ABC-fibers. But perhaps they don't know this about XYZ-fibers, despite the functional isomorphism; a "what is it like to be an alien with XYZ-fibers?" hard-problem question.
However, we could imagine there is another alien who started in a similar epistemic state, and who actually has XYZ-fibers. Through an analogous sequence of events, this second alien would learn that XYZ-fibers can implement lusty experience.
The situation looks pretty symmetric. Why couldn't they both realize this, and update that both ABC-fibers and XYZ-fibers can implement lust? But that's a functionalist conclusion, and it could have been gotten to without the empirical update of experiencing lust.
As an alternative, suppose the first (ABC) alien believes that they, having ABC-fibers, really experience lust, but aliens with XYZ-fibers do not really have that emotion, even if they have functional lust. The ABC alien believes that the XYZ alien, in an analogous epistemic position, makes a wrong update; the XYZ alien is subject to an illusion, where they make belief updates *as if* they experienced real lust, when they really only had functional lust. Perhaps the XYZ alien is simply wrong about their experience; they are subject to illusory quasi-lust.
But this raises the question of how the ABC-alien can know they really experienced lust, because their epistemic state is isomorphic. Sure, their beliefs and memories updated as if they really experienced lust, but functional lust was enough to ensure that. Like the XYZ-alien, the ABC-alien could (if functionalism is false) be subject to an illusion, where it seemed to them that they experienced lust, but this seeming was illusory, because they only had functional lust, not the actual experience of lust.
At this point it is possible for the non-functionalist to bite the bullet and accept that evidence one is experiencing an emotion is hard to come by, even if there is evidence of having the function of the emotion. But if they're accepting that level of introspective opacity of experience, why believe in qualia in the first place, rather than being an illusionist or eliminative physicalist? There is not much to motivate qualia realism in the first place aside from introspection.
There are also semantic responses relating to Kripkean secondary intensions and 2D semantics, which claim that the ABC alien can hold that they (and not the XYZ alien) experience real lust, and the XYZ alien can similarly hold that they (and not the ABC alien) experience real lust, and these beliefs are logically compatible, because "lust" picks out a different physical predicate when said by aliens of each subtype. (But this semantic complexity is both unnecessary and unintuitive, in my view)