What on earth are “qualia,” and what’s so problematic about having them inverted? Daniel Dennett famously said that qualia is “an unfamiliar term for something that could not be more familiar to each of us: the ways things seem to us” (think of colors, or sounds, or taste). He also added that qualia is “one of philosophy's most virulent memes,” and although I don’t buy into the whole memetics spiel, I must admit that he has a point.
The problem of inverted qualia goes back to John Locke, who asked us to imagine a situation in which we wake up one day and — without any physical change having occurred in the world or in our brain — we suddenly perceive colors in a different way: what used to be red now gives the sensation formerly known as green (and vice versa). Ok, one might say, cute little thought experiment, but who cares? We are supposed to care because the inverted qualia argument allegedly shows that secondary qualities (like colors), and particularly first person “phenomenological” experiences of said qualities, do not depend on a particular physical substrate in the brain, i.e. they have no physical basis.
What? Well, here is the actual formal argument, as far as it goes:
Premise 1: If X is possibly false, then X is not necessary.
P2: It is conceivable that the relationship between qualia and physical states of the brain be different from what it actually is.
P3: What is conceivable is possible.
Conclusion 1: Qualia are therefore not identical with brain states.
C2: Also, qualia are not physical.
Got that? That’s the beauty of analytic philosophy: its arguments can be expressed in a formal fashion, which is meant to make as clear as possible what one’s premises and conclusions are, so that others can fairly examine them and either accept them or knock them down one by one. (For comparison, try doing the same with anything by Derrida or Foucault, good luck.)
With the case in question, we could of course attack any or all premises. I am going to let P1 stand, because it does actually tell us that if something is logically possible then it is physically possible, and I do believe that the set of physical possibilities is nested within the set of logical ones (though one could of course argue that that depends on which type of logic one is using, etc.).
P2 is tricky: yes, it is conceivable that the relationship between qualia and physical states of the brain be different from what it actually is, all one has to imagine is different physical properties of light, or different chemicals perceiving light falling on our retinas, or a different type of signal transduction in the brain. But the crucial part of the inverted qualia argument is not just that the relationship between qualia and physical states could be different, it is that qualia could be inverted with no physical change at all with respect to the way things are at the moment. That, I maintain, is impossible. In other words, we certainly could have brains wired in a way so that what to other animals looks red would look green to us, but that can only be accomplished by a physical change in the way the brain works (indeed, we do have empirical examples of something like this: the bewildering phenomenon of synesthesia).
P3, as appealing as it superficially is, is also highly debatable. I can conceive, for instance, of a universe with different physical laws, like a different gravitational constant. But that doesn’t guarantee that such a universe is possible: there may be very good reasons, unknown to modern physicists, why such a universe could actually not come into existence. This is a fascinating area of inquiry, concerned with the relationship between logical and physical possibility. But it’s treacherous territory, and if I were a non-physicalist, I wouldn’t stake too much on it. (This is, of course, why I don’t buy David Chalmers’ silly arguments about zombiesand the hard problem of consciousness.)
What about the conclusions, then? Obviously, all we need to do is to refute one of the three premises and we are done, the conclusions no longer follow. Still, I’ll probably buy into C1, if we modify it thus: qualia are not necessarily identical with the particular brain states we happen to have. Different brain states could generate the same qualia, depending on the complex pathways connecting the physical objects in the external world, their perceivable properties, and the evolutionary history and physical makeup of our own perceptual systems.
C2, on the other hand, I think is simply daft: qualia are not physical? Really? So why do we need physical objects, physical eyes, physical neurons, and so on, to perceive them? Alter any of the above, and our perception of qualia changes, a really strong reason to believe that qualia are in fact physical. (Similarly, the minimally reasonable position about consciousness is what some philosophers refer to as the “no ectoplasm clause”: however consciousness works, it’s grounded in a functional physical brain; take the brain away, you’ve got no more consciousness.)
So, whatever disagreement Kripke and McGinn are still having about inverted qualia, I doubt it matters in the long run: secondary qualities are better and better explained by neurobiology and cognitive science, and philosophers should make use of such explanations to inform the many interesting debates still open in philosophy of mind.