Qualia

Qualia are the experiences of sensory input (as opposed to the describable facts of such input). (The singular is the two-syllable word quale.) In the classic example, a sighted person can see red, but cannot describe the experience of such a perception; the best he can do is draw an analogy (e.g., "red looks hot") or provide informational descriptions (e.g., "it's the color you see when light of such-and-such wavelength is directed at you."). Simpler still, consider the impossibility of ever describing the experience of seeing color to a blind person.

In philosophical terms, qualia are parts of experiental knowledge, i.e., that which can only be known through experience. The ancient Sufis summed up the idea in their parable about coffee: "He who tastes, knows; he who tastes not, knows not."

Implications

Philosphers and scientists alike have pondered qualia for a long time without resolution. AI researchers wonder whether machines that pass the Turing Test would experience qualia, and whether they would even need to do so. It is also possible that a sentient AI would admit to experiencing qualia, but, like us, would fail to describe qualia using only language.

Our failure to define qualia also makes people wonder if color (and sounds, etc.) are experienced differently by each person. For example, it would be impossible to tell if some people see colors inverted, since they would still call roses red and grass green.

There is a chapter called "Qualia Disqualified" in the book Consciousness Explained by philosopher Daniel Dennett. In that chapter, Dennett argued that the philosophical topic of qualia has become too convoluted and bizarre to be of any further use. In contrast, the biologist Gerald Edelman was willing to accept qualia and incorporate them into his brain-based theory of mind.

A commonly held idea concerning qualia is that experience in and of itself is a fundamental feature of reality and is thus, similar to space and time, irreducible in terms of explanation. Materialists logically downplay such a concept as a tendency by dualists to place anything they cannot explain into the metaphysical domain.