Qualia - The subjective or qualitative properties of experiences. What it feels like, experientially, to see a red rose is different from what it feels like to see a yellow rose. Likewise for hearing a musical note played by a piano and hearing the same musical note played by a tuba. The qualia of these experiences are what give each of them its characteristic “feel” and also what distinguish them from one another. Qualia have traditionally been thought to be intrinsic qualities of experience that are directly available to introspection. However, some philosophers offer theories of qualia that deny one or both of those features.

The term “qualia” (singular: quale and pronounced “kwol-ay”) was introduced into the philosophical literature in its contemporary sense in 1929 by C. I. Lewis in a discussion of sense-data theory. As Lewis used the term, qualia were properties of sense-data themselves. In contemporary usage, the term has been broadened to refer more generally to properties of experience. Paradigm examples of experiences with qualia are perceptual experiences (including nonveridical perceptual experiences like hallucinations) and bodily sensations (such as pain, hunger, and itching). Emotions (like anger, envy, or fear) and moods (like euphoria, ennui, or anxiety) are also usually taken to have qualitative aspects.

Qualia are often referred to as the phenomenal properties of experience, and experiences that have qualia are referred to as being phenomenally conscious. Phenomenal consciousness is often contrasted with intentionality (that is, the representational aspects of mental states). Some mental states—for example, perceptual experiences—clearly have both phenomenal and intentional aspects. My visual experience of a peach on the kitchen counter represents the peach and also has an experiential feel. Less clear is whether all phenomenal states also have intentional aspects and whether all intentional states also have phenomenal aspects. Is there really something that it is like to have the belief—even the occurrent belief—that there is a peach on the counter? What could be the representational content of the experience of an orgasm? Along these lines, the nature of the relationship between phenomenal consciousness and intentionality has recently generated considerable philosophical discussion. Some philosophers think that phenomenal consciousness reduces to intentional content, while others think that the reductive relationship goes in the other direction. Still other philosophers deny both claims.

From the standpoint of introspection, the existence of qualia seems indisputable. It has, however, proved remarkably difficult to accommodate qualia within a physicalist account of the mind. Many philosophers have argued that qualia cannot be identified with or reduced to anything physical, and that any attempted explanation of the world in solely physicalist terms would leave qualia out. Thus, over the last several decades, qualia have been the source of considerable controversy in philosophy of mind.