Speaking of the obvious, I think there's a rather easy example available to dispense with the weighty implications attributed to qualia - in terms of both what qualia might imply, and what we might disagree on when speaking of qualia. The Alert Reader will have guessed by now that this easy example is the sense of taste. And speaking of abrupt transitions, it's time for a biology lesson! We begin with the Wikipedia article, because it is clear and concise, and I doubt my ability to abstain from plagiarizing it anyhow:
Taste (or, more formally, gustation) is a form of direct chemoreception and is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons. In humans and many other vertebrate animals the sense of taste partners with the less direct sense of smell, in the brain's perception of flavor.I want to make it clear here that we are talking about chemoreception, the sensing of chemicals. Not flavors. Flavors are the product of your brain, and they do not inhere in chemicals, but are rather the result of complex interactions between your neurology and what you put in your face. The quick & dirty breakdown is that your taste buds detect the molecular shapes of the things you eat (such as sugars, proteins, or alkaloids) or the presence of dissolved ions (acids are sour, salts are salty); then, nerve impulses are transmitted to your brain to tell it what's going on; then, you react to whatever it is that just happened.
This is what makes things like artificial sweeteners possible. You see, certain chemicals which do not occur in nature commonly (or at all) can trick your tongue into thinking that something is there which might not actually be there, just because they're similarly shaped in this or that way to the sort of thing that abounded in our evolutionary history. Case in point: aspartame. While the body still metabolizes it, aspartame is experienced by the body to be about two-hundred times sweeter than sucrose (though of a somewhat different taste, depending on who you ask), making its caloric content negligible for about the same amount of sweetness. Models of how sweetness works have grown in sophistication over time, the current model describing some eight reception sites and culminating in the development of lugduname, which is estimated to be some two-hundred-thousand times sweeter than sucrose.
Just in case we're not clear yet, let me say this in a different way: your taste buds are shape detectors and ion detectors which react with certain chemicals to send nervous impulses to your brain. The "taste" of this or that thing is the product of what your tongue tells your brain it is shaped like (or its pH value/alkali ion content). Seriously. Isn't that fucking weird?! And even weirder, there are substances called sweetness modifiers which temporarily but fundamentally alter the way sweetness is perceived, such as lactisole (which inhibits the taste of sweetness) and miraculin (which makes sour things taste sweet instead). I mean, whoah!
We were talking about qualia earlier, and now it's time to get back to that. I think that taste is perhaps the most vivid demonstration of the crowbar separation (thank you, Eddie Izzard) between reality and how we perceive it. Once again: shapes can be delicious, WTF. With touch, it's pretty easy to imagine sensations of pressure, wetness, or roughness being at least somewhat related to what's going on (with the possible exception of temperature - I mean, c'mon, rapidly vibrating molecules equals hot? Who comes up with this shit?). Same with perceptions of the visible wavelengths of light, or the audible wavelengths of sound. But taste? Give me a break! There just has to be some subjectivity going on here, because the "what it's like" is nothing like the "what it is"!
This also makes some of the debate about qualia seem rather silly. I'll briefly discuss two examples, first the analogical relationship between color-blindness and supertasting, then the inverted spectrum argument. I'll close with a quick Take That! directed at proponents of intelligent design, and a little treat for sticking it out through this interdisciplinary ramble.
First off, when we consider whether qualia are "the same" among different people, of course they're not. Nobody ever said they were, but when you consider that the colorblind see rainbows in a completely different way than most other people do, and that some people experience taste way more intensely than most other people do, then I think it becomes trivially easy to imagine that no two people experience the world in the same way. Even Epictetus was able to point out that two people can have wildly differing experiences of the same situation, and so it is not necessarily the world itself that causes us joy or suffering, but our attitudes toward it - what I would add is that a person may be just as powerless to change his or attitudes as to change his or her taste buds. Sometimes this can happen for no apparent reason, as with my early disgust and later love for mushrooms. Sometimes it happens due to experience, as with my aversion to light beer. Sometimes it doesn't happen no matter how hard you try: I will never be able to enjoy a resin spurge sandwich, because resiniferatoxin is so much more potent than capsaicin, it's not even hot any more (you just go into anaphylaxis and die).
With that in mind, the inverted spectrum argument is even easier to de-fang. For those who didn't click through (and I throw potholes all over the place, so I really don't blame you), the argument goes as follows:
1. Metaphysical identity holds of necessityNow, is it just me, or is the question simply beggared in premise three? I mean, if qualia could have a different relationship to one and the same physical brain-states, then of course they're not physical! Fuckin' duh! But this is exactly what is at issue. We see over and over again in the literature (Just look! Like, anywhere!) that when you change the sensory apparatus, you change the qualia. To a person who is relatively up on the neuroscience, while taste and sight vary greatly across the spectrum of human experience, the conceivability of two possible qualia X & Y for the same brain state Z is somewhere on the order of that for two possible diameters A & B for the same circle C. It's just that no two physiologies are perfectly identical (not even the same physiology at different points in time), so of course subjective experiences vary.
2. If something is possibly false, it is not necessary
3. It is conceivable that qualia could have a different relationship to physical brain-states
4. If it is conceivable, then it is possible
5. Since it is possible for qualia to have a different relationship with physical brain-states, they cannot be identical to brain states (by 1).
6. Therefore, qualia are non-physical.
As for the IDiots, well, I'm just going to come out and say that ethylene glycol tastes sweet and is very toxic. So that's a poison detection failure right there. But let's not forget that to supertasters, a wide variety of totally healthy foods taste absolutely repulsive; and to non-tasters, a wide variety of potentially fatal substances taste perfectly fine. Oh, and how about the fact that the sense of taste is based primarily on superficial characteristics of molecular compounds, and is therefore rather easy to hack?