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Qualia (singular, quale) is a philosophical term referring to the experiential properties of sensation; the "redness" of red, the "warmness" of warmth, the "garlickness" of the taste of garlic, and so on. By definition, it is impossible to know or understand qualia without direct experience of them, and as a result, they are inherently person-specific and uncommunicable. (One cannot be certain whether or not the "redness" that another sees is experientially identical—or even similar—to the "redness" that one sees, even when looking at the same object.)
 What the fuss is about
The term "qualia" originated in philosophy, although it has been picked up by some scientists (and rejected by some philosophers).
Qualia have become a major component in many modern formulations of the philosophy of mind because of their role in illustrating the difference between knowledge-by-description about, such as might be programmed into a computer, and knowledge-by-acquaintance of, which can only be obtained through direct experience. Computers, like p-zombies, are often assumed to lack qualia and therefore be in some way intrinsically different from humans at a fundamental cognitive level. Whether this is a distinction that actually makes a difference is, apparently, another matter.
Controversies about qualia include:
- Whether qualia exist at all.
- Whether "qualia" is a useful concept/word.
- Whether qualia have interesting epistemological properties, such as being irrefutable and/or ineffable.
- Whether qualia can have unaccountable variations and absences.
- Can computers have qualia.
- Whether qualia defy physical explanation/description.
The last dispute is particularly significant. Concern about the problem of qualia (and consciousness in general) is about the only motivation for philosophers to reject physicalism or naturalism that is not propelled by mysticism or religious dogma.
 The Knowledge Argument
The Knowledge Argument is a rational challenge to physicalism. The argument seeks to prove that qualia are real through a purely rational (i.e., not empirical) thought experiment. The fictitious story of Mary the Colour Scientist, who lives in a black-and-white world, is invoked to demonstrate the point. Supposedly Mary learns all physical facts about colour including how colour is processed by the cornea, optic nerve, and brain. Once Mary has all physical facts about colour, she is permitted to experience a red tomato for the first time. The question is whether Mary learns something new upon experiencing colour for the first time. If yes, then rationalists conclude that the physical description that Mary had of colour was inadequate and thus physicalism is false. Although the argument has traction among rationalists, empiricists generally do not find the argument persuasive.
 Objectivity and testability
The concept of qualia has been rejected by some philosophers and neuroscientists as not even wrong, or simply untestable and therefore useless. It also has scientific defenders. It is, after all, hard to dispute that one seems to have qualia, subjectively. One's pain hurts, and one's food has a taste, and so on. Neuroscientist V.S. Ramachandran and philosopher William Hirstein have suggested that one person's qualia could be rendered accessible to another once we have the ability to "connect" two brains via some sort of cable. Unfortunately, this technology is a long way off.
Daniel Dennett, like many in the cognitive sciences, characterizes qualia as a useless and unfalsifiable concept, saying that it must be possible to know whether a change in qualia occurs and whether there is a difference between having qualia and not having them. Simply ignoring any ineffable forms of qualia (or alternatively just redefining them as some measurable property a la Ned Block) is the de facto position in the cognitive sciences as something must have a measurable effect if you want to do an experiment on it, and consequently, not have your research grant tossed in the garbage bin. Once again, this problem could theoretically be solved if we could hook up two brains Ramachandran-style, but that's currently impossible. As a result, certain versions of qualia tend to be seen as fuzzy or speculative at best with harder versions such as the inverted spectrum tend to be written off as philosophical woo.
 Variant qualia
 Unaccountable variations
One way of dramatising the claim that we don't know other people's qualia is to suppose that, for all we know, their "red" is our "green" and vice-versa—they have "spectrum inversion". It is widely accepted that spectrum inversion would be undetectable, because the individual would just learn that to call the colour of tomatoes, "red, even though he "really" sees them as green, and so on.
Some do not accept the undetectability claim, however. For instance, in I Am a Strange Loop, Douglas Hofstadter makes the argument against the "inverted spectrum" from a physiological angle, using sound as one of his examples. To detect a sound, the vibrations in the air must strike the eardrum, which activates a specific spot on the cochlea. The fact that the experience of sound is correlated with a measurable physical phenomenon (the vibration and which part of the cochlea reacts to it) refutes the idea that qualia can be inverted in a neurotypical person. Very high-pitched frequencies, for one, tend to cause people physical pain. If inverted qualia were true, we should expect to see a similar reaction from some people when a very low note is played, which doesn't happen. Owen Flanagan has similarly argued that the general tendency to class reddish colours as "warm" and bluish colours as "cool" would give the game away.
In the same spirit that we can imagine others to have different qualia, we can imagine them to have no qualia. Philosophers call human-like entities that lack consciousness or qualia zombies. However, whereas movie zombies give away their demented status by shambling and grunting incoherently, philosophical zombies, p-zombies are exact behavioural, functional or physical duplicates. Some philosophers such as David Chalmers think the mere imaginability of p-zombies disproves materialism, as the result of an armchair argument. Others beg to differ.
 Accountable variations
On the other hand, people whose sensory organs or brains work in non-standard would uncontroversially be expected to have non-standard qualia (which is to say that the only point of controversy is the existence of qualia at all). Colour blind individuals probably have different colour qualia to normally sighted individuals, for instance. People with synaesthesia may associate tastes or colours with numbers, for another example. V.S. Ramchandran has conducted experiments with a colourblind synaesthete who reports his own synaesthetic colours as having a weird, "Martian" quality.
 Standardised and unstandardised
Standardising things by color as in traffic lights or color-coded folders would be a complete mess in a world where qualia could be inverted or radically different from each other. However, difference in qualia is apparent in cases like color-blindness, which is caused by a genetic defect that leads to certain cone cells in the retina. Research on these types of disorders affecting perception suggests that differences in qualia can be predicted in some way as self-reports and introspection change in a predictable fashion when sensory organs are changed or damaged. However, there are also the facts that every brain is unique and that we do not experience sensory input directly, but the input is "reconstructed" and interpreted by the brain. Thus, some level of individual difference is expected in subjective experiences, but they will fall within a limited range or distribution. Radical changes in perception are usually the work of damage to the sensory organs or brain or hallucinations. Discussion of the concept of qualia itself tends to be limited to pure philosophy due to the somewhat problematic nature of the term. What might be called qualia is often simply referred to in terms of "introspection," "subjective experience," or sometimes "what-is-it-likeness" (in reference to Thomas Nagel's essay "What is it Like to be a Bat?") in psychology. The existence of subjective experience is sometimes taken to be an argument against scientific psychology or reductionism in psychology, however, the two can easily be reconciled.
 Qualia and computers
There is, even among naturalists, a widespread apprehension that robots, computers and artificial intelligences would lack human-style emotions, and perhaps even sensation. It is certainly a staple of science fiction, where androids are by default emotionless, unless of course fitted with an emotion chip. Star Trek's Mr Data is also astonished by "real" touch, when a piece of human skin is grafted onto him by the Borg Queen — whatever tactile sensors he has being fitted with lacking that special something.
Perhaps the origin of the idea lies in the fact that there is no obvious way to program in emotional feelings and sensory qualia. Or perhaps it is because computers and robots are perceived as running on "pure logic", which is seen as a separate faculty to emotion and sensation in humans. A counteragument is that humans are made of matter, and robots are made of matter, and no magic pixie dust is responsible for sensation and emotion in humans.
There are some researchers who think they can programme "artificial qualia" into an AI, but they caution that: "[w]e are not interested in the problem of establishing whether robots can have real phenomenal experiences or not.
 What qualia are for
V. S. Ramachandran believes that qualia have a purpose: if an entity has a complex mind with a lot of different things going on, and if it has the ability to manipulate concepts, then it is in danger of paying too much attention to the wrong things, of getting lost in its thoughts. It needs some things to stand out, to be more urgent or compelling than others. Pain is obviously compelling. Colour is like mental highlighter — in fact, synaesthetes can use mental highlighting to perform some tasks more effectively. On a related theme, Richard L Gregory has hypothesised that present experience is more vivid than recalled experience because we need to pay attention to the present. If we had perfectly vivid recollection of past experiences, we might get confused and start reacting to it as though it were happening. However, others argue that these postulations misrepresent qualia, since they explain only the evolution of attention and focus in brain activity, rather than emergence of qualia itself.
- Entry for qualia at the Stanford Encyclopedia of Philosophy
- Entry for inverted qualia
- Knowledge Argument at the Stanford Encyclopedia of Philosophy
- ↑ 1.0 1.1 Ramachandran, V.S. and Hirstein, W. (1997), "Three laws of qualia; What neurology tells us about the biological functions of consciousness. Journal of Consciousness Studies, 4:5–6, 429–57.
- ↑ Dennett, Daniel C. (1988) Quining Qualia. In: Marcel, A. & Bisiach, E. (eds.) Consciousness in Modern Science, Oxford University Press.
- ↑ Ned Block. Qualia: What Is It Like To Have an Experience? In R. Gregory (ed.) Oxford Companion to the Mind
- ↑ See Chapter 23, "Killing a Couple of Sacred Cows," pp. 333-342 in I Am a Strange Loop
- ↑ 5.0 5.1 Owen Flanagan. (1992) Consciousness Reconsidered.
- ↑ David Chalmers. The Two-Dimensional Argument against Materialism. In Brian P. McLaughlin & Sven Walter (eds.), Oxford Handbook to the Philosophy of Mind. Oxford University Press (2009)
- ↑ V.S. Ramachandran and E.M. Hubbard. The Phenomenology of Synaesthesia. Journal of Consciousness Studies, 10, No. 8, 2003, pp. 49–57
- ↑ Color-Blindness, WebMD
- ↑ Thomas Nagel. What is it like to be a bat? The Philosophical Review LXXXIII, 4 (October 1974): 435-50.
- ↑ Qualia and Computers, Teed Rockwell, Cognitive Questions
- ↑ In Search of the Computational Correlates of Artificial Qualia
- ↑ Verification of Synaesthesia
- ↑ Qualia Flag the Present
- ↑ If Qualia Evolved
- ↑ Epiphenomenal Qualia
- ↑ Qualia