Qualia: The Redness of Red and the Limits of Science

Qualia are traditionally characterized by four properties, a list traced to Clarence Irving Lewis in 1929 and set out in this compact form by Daniel Dennett in 1988. They are said to be ineffable, meaning they cannot be fully communicated or captured in language: you cannot describe what red looks like to a congenitally blind person. Intrinsic, meaning non-relational: the redness of red is not defined by its relations to other things, it is simply what it is. Private, meaning directly accessible only to the subject having them: no one else can experience your qualia. And directly apprehensible, meaning immediately known by the subject: you know what your pain feels like simply by feeling it.

That list is not a settled description of a known thing. The characterizations are themselves debated, and the philosopher who gave them their tidiest modern statement did so in order to attack them: Dennett argues that each of the four can be challenged. Keep that in view for everything that follows. The arguments in the next three sections all assume that something answers to this description, and the argument in section six is that nothing does.

Human trichromatic color vision arises from three types of cone photoreceptor in the retina, sensitive to short, medium and long wavelengths, usually written S, M and L. Each type has its own distinct spectral absorption curve, so a given wavelength of light excites the three to different degrees, and the visual system uses the relative activation across the three to discriminate one wavelength from another. That signal is then processed onward through the visual system, including area V4. This is textbook vision science, and every step of it can be written down.

Frank Jackson's knowledge argument, published in 1982 and sharpened in 1986, is the most influential argument for the existence of qualia as non-physical. Mary is a brilliant scientist who has lived her entire life inside a black-and-white room. She has learned every physical fact about the physics, chemistry and neuroscience of color vision: wavelengths, cone responses, neural processing, the lot. Then Mary steps outside and sees a red tomato for the first time. The intuition Jackson is pumping is that she learns something new, namely what red looks like. Since she already knew every physical fact, the argument concludes that this new knowledge must be non-physical, knowledge of a quale, and therefore that physicalism, the claim that all facts are physical facts, is false.

Jackson himself later recanted. In Mind and Illusion, published in 2003 in the collection Minds and Persons edited by Anthony O'Hear, he abandoned the anti-physicalist conclusion of his own 1982 and 1986 argument. His revised view is that Mary gains a new ability, the ability to recognize and imagine red, rather than any new factual knowledge, which puts him with the physicalist-friendly ability hypothesis. This is not a footnote and it is not optional. Mary's Room is still routinely presented as a standing refutation of physicalism, and the man who built it stopped believing it did that.

The inverted spectrum is a thought experiment, not a finding. Could the quale you experience on seeing red be the one another person experiences on seeing green, with both of you calling the same physical stimulus red, behaving identically, and neither ever able to tell? If a spectrum inversion of that kind is genuinely conceivable while remaining undetectable in function and behavior, then qualia are not fixed by functional organization alone. Sydney Shoemaker developed the classic version in his 1982 paper The Inverted Spectrum; Ned Block's Inverted Earth, from 1990, is a related variant that runs the inversion through the environment rather than purely within one subject. No one has ever observed an inverted spectrum in anybody, and the argument does not require that anyone has.

Two kinds of variation are genuinely documented. Red-green color blindness affects roughly 8 percent of men and roughly 0.5 percent of women of Northern European descent, with red-green variants accounting for the large majority of all color-vision deficiency; the sex skew follows from X-linked inheritance, since men carry only one X chromosome and a single defective opsin gene is therefore sufficient. And at the other end, a genuine human tetrachromat has been individually confirmed: a woman referred to in the literature as cDa29, identified by the Newcastle University neuroscientist Gabriele Jordan and colleagues after a multi-decade search that concentrated on mothers of color-blind sons, was shown under controlled testing to reliably discriminate colors that trichromats cannot tell apart, consistent with four functional cone types rather than three. This is real evidence that human color experience differs between people at the level of physiology. It is not evidence that settles the inverted-spectrum question, which is about whether two people with identical physiology could still differ in the quality itself, and that question remains open.

Daniel Dennett argues that qualia, understood as intrinsic, private, ineffable, immediately apprehensible properties, do not exist. The case is made in Quining Qualia in 1988 and at length in Consciousness Explained in 1991. What exists, on this account, is information processing: a complete account can be given of discriminatory behavior, memory, verbal report and reaction without invoking any mysterious raw feel at any point. The intuition that there is something more, some felt residue left over once all of that is explained, is in Dennett's view a persistent cognitive illusion. The verb in his title is deliberate. To quine qualia is to eliminate the concept from philosophy altogether, following Quine's proposal to eliminate the analytic and synthetic distinction, and the target is not a detail of the definition but the whole idea.

Representationalist, or intentionalist, theories, associated with Michael Tye and Dretske, hold that qualia are identical with, or supervene on, the representational content of experience. When you see red, on this view, the quale just is that content: the experience represents the surface as having a certain reflectance property, and there is no further inner quality standing behind the representation. The attraction is that it naturalizes qualia, reducing them to information-bearing states of a kind physical science already deals with. The standing objection is that it moves the problem rather than solving it, because it does not explain why a representation should have any felt quality at all.

The answer to the qualia question decides, in advance, whether a machine could ever have an inner life. If qualia are real and non-functional, then no artificial system could have them however sophisticated it became, unless it possessed the right kind of physical substrate, a position in the neighborhood of biological naturalism. If instead qualia are functional or representational, then a sufficiently complex artificial system might have them, and consciousness would not care what it was made of. The question is empirically open, and at present it is untestable: nothing in the two scenarios differs in anything an outside observer could measure.

Panqualityism runs the other way and makes qualia basic. Some panpsychist philosophers, working in the tradition of Russellian monism, propose that qualia are fundamental features of all matter: what physics describes are the structural and relational properties of particles, and those are complemented by intrinsic qualitative properties that constitute the inside aspect of matter. On that picture the redness of red is not produced by the brain at some threshold of complexity; it is the sort of thing the world was made of all along. It is a real position in the literature and it is nowhere near established.

It is misleading to say that qualia have been scientifically detected. No scientific instrument has detected or measured a quale. Neuroscience identifies neural correlates of reported experience, which is a real and valuable achievement and an entirely different thing: it tells you which activity accompanies a report, not that the qualitative feel itself has been observed. The felt quality is not an observable in any current scientific framework, and a paper announcing a neural correlate has not found one.

It is inaccurate to say that the qualia debate is resolved, in either direction. The dispute between qualia realists, eliminativists, representationalists and the other positions on the field remains one of the most active and unsettled in philosophy of mind. No consensus exists. Anyone presenting the existence of qualia as an established fact, and anyone presenting their elimination as a completed piece of work, is ahead of the argument.