Qualia (singular: "quale", pronounced KWAHL-ay) are most simply defined as the properties of sensory experiences by virtue of which there is something it is like to have them. These properties are, by definition, epistemically unknowable in the absence of direct experience of them; as a result, they are also incommunicable. The existence or lack of these properties is a hotly debated topic in contemporary philosophy of mind.

Qualia have played a major role in contemporary philosophy of mind, largely because they are often seen as being a de facto refutation of physicalism. There is some debate over the precise definition of qualia, as various philosophers emphasize or deny the existence of certain properties.

Definition of qualia

Daniel Dennett identifies four properties which are commonly ascribed to qualia; that is, qualia are:

1. ineffable; that is, they cannot be communicated, or apprehended by any other means than direct experience.
   
2. intrinsic; that is, they are non-relational properties, which do not change depending on the experience's relation to other things.
   
3. private; that is, all interpersonal comparisons of qualia are systematically impossible.
   
4. directly or immediately apprehensible in consciousness; that is, to experience a quale is to know one experiences a quale, and to know all there is to know about that quale.

It is important to note that qualia do not have the status of observed properties, which we are sure exist but might be wrong about; rather, the concept of qualia is first and foremost dependent on its definition, and the existence of qualia is predicated on the existence of properties which fit that definition. Thus if we were to discover that there is such a property as "what it is like to have a certain experience", but that this property was in fact knowable to others, it would not be a quale. Thus the contemporary debate over whether or not qualia exist is largely centered on whether or not experiences do in fact have properties that fit this definition. At the present time, there is little consensus over whether or not this is indeed the case.

If qualia exist, a normally-sighted person who is able to see red is unable to describe the experience of such a perception in such a way that a listener who has never experienced color will be able to know everything there is to know about that experience. Though it is possible to make an analogy, such as "red looks hot", or to provide a description of the conditions under which the experience occurs, such as "it's the color you see when light of such-and-such wavelength is directed at you," supporters of qualia contend that such a description is incapable of providing a complete description of the experience.

C. I. Lewis, in his book Mind and the World Order published in 1929, was the first to use the term "qualia" in its generally agreed modern sense. (His original definition was that qualia are the "recognizable qualitative characters of the given.")

There is an ancient Sufi parable about coffee which nicely expresses the concept: "He who tastes, knows; he who tastes not, knows not."

Arguments for qualia

Arguments for qualia generally come in the form of thought experiments which are designed to lead one to the conclusion that qualia exist. For example, the inverted spectrum thought experiment invites us to imagine that we wake up one morning, and find that for some unknown reason all the colors in the world have been reversed. Furthermore, we discover that no physical changes have occurred in our brains or bodies that would explain this phenomenon. Supporters of qualia argue that since we can imagine this happening without contradiction, it follows that we are imagining a change in a property which determines the way things look to us, but which has no physical basis. The argument thus claims that if we find the inverted spectrum plausible, we must admit that qualia exist.

The knowledge argument

In Frank Jackson's "Epiphenomenal Qualia" (Jackson 1982), Jackson offers what he calls the "Knowledge Argument" for qualia. The clearest example of this argument runs as follows:

Mary the color scientist knows all the physical facts about color, including every physical fact about the experience of color in other people, from the behavior a particular color is likely to elicit to the specific sequence of neurological firings that register that a color has been seen. However, she has been confined from birth to a room that is black and white, and is only allowed to observe the outside world through a black and white monitor. When she is allowed to leave the room, it must be admitted that she learns something about the color red the first time she sees it—specifically, she learns what it is like to see that color.
This thought experiment has two purposes. First, it is intended to show that qualia—the properties of experiences which determine what it is like to have those experiences—exist. If we agree with the thought experiment, we believe that Mary gains something after she leaves the room—that she acquires knowledge of a particular thing that she did not possess before. That knowledge, Jackson argues, is knowledge of the quale that corresponds to the experience of seeing red, and it must thus be conceded that qualia are real properties, since there is a difference between a person who has access to a particular quale and one who does not.

The second purpose of this argument is to refute the physicalist account of the mind. Specifically, the Knowledge Argument is an attack on the physicalist claim about the completeness of physical truths. The challenge the Knowledge Argument poses to physicalism runs as follows:

1. Before her release, Mary was in possession of all the physical information about color experiences of other people.
2. After her release, Mary learns something about the color experiences of other people.
3. Therefore, Before her release, Mary was not in possession of all the information about other people’s color experiences, even though she was in possession of all the physical information.
4. There are truths about other people’s color experience which are not physical.
5. Physicalism is false.

Finally, Jackson argues that qualia are epiphenomenal: that is, that they are causally inefficacious with respect to the physical world. Jackson does not give a positive justification for this claim—rather, he seems to assert it simply because it defends qualia against the classic problem of dualism. Our natural assumption would be that qualia must be causally efficacious in the physical world—however, if qualia are to be non-physical properties (which they must be in order to constitute an argument against physicalism), it is almost impossible to imagine how they could have a causal effect on the physical world. By redefining qualia as epiphenomenal, Jackson is thus able to protect them from the demand of playing a causal role.

Arguments against qualia

Daniel Dennett

In his paper "Quining Qualia" and his book Consciousness Explained, Daniel Dennett offers an argument against qualia which attempts to show that the above definition breaks down when we try to make a practical application of it. In a series of thought experiments which he calls "intuition pumps", he brings qualia into the world of neurosurgery, clinical psychology, and psychological experimentation. His argument attempts to show that once the concept of qualia is so imported, it turns out that we can either make no use of it in the situation in question, or that the questions posed by the introduction of qualia are unanswerable precisely because of the special properties of qualia.

In Dennett's updated version of the "inverted spectrum" thought experiment, "the neurosurgical prank", you again awake to find that your qualia have been inverted—grass appears blue, the sky appears green, etc. According to the classical account, you should be immediately aware that something has gone horribly wrong. Dennett argues, however, that it is impossible to know whether the diabolical neurosurgeons have indeed inverted your qualia (by tampering with your optic nerve, say), or have simply inverted your connection to memories of past qualia. Since both operations would produce the same result, you would have no means on your own to tell which operation has actually been conducted, and you are thus in the odd position of not knowing whether there has been a change in your "immediately apprehensible" qualia.

Dennett's argument revolves around the central objection that, for qualia to be taken seriously as a component of experience—for them to even make sense as a discrete concept—it must be possible to show that

a) it is possible to know that a change in qualia has occurred, as opposed to a change in something else;
or that

b) there is a difference between having a change in qualia and not having one.
Dennett attempts to show that we cannot satisfy (a) either through introspection or through observation, and that qualia's very definition undermines its chances of satisfying (b).

Dennett also has his own response to the "Mary the color scientist" thought experiment.

He argues that Mary would not, in fact, learn something new if she stepped out of her black and white room to see the color red. Dennett asserts that if she already truly knew "everything about color", that knowledge would include a deep understanding of why and how human neurology causes us to sense the "qualia" of color. Mary would therefore already know exactly what to expect of seeing red, before ever leaving the room. Dennett argues that although we cannot conceive of such a deep knowledge, if a premise of the thought experiment is that Mary knows all there is to know about color, we cannot assume that we can fathom or even describe such knowledge -- or that such knowledge doesn't exist.

Hence, as a true physicalist, Dennett leaves open the possibility that scientists may one day understand exactly how the brain creates the illusion of qualia, a finding which would destroy the plausibility of qualia as a real entity.