Qualia

Qualia (singular: quale) name the qualitative, subjective character of conscious experience: the redness of red as you see it, the painfulness of pain as you feel it, the bitterness of coffee on your tongue. The term entered widespread philosophical use through C. I. Lewis in the 1920s, but the contemporary debate is shaped by Thomas Nagel's What Is It Like to Be a Bat? (1974), Frank Jackson's knowledge argument (1982), and David Chalmers's formulation of the hard problem of consciousness (1995).

The core puzzle is whether qualia can be reduced to or explained by the physical and functional properties of the brain. Functionalists and physicalists argue that they can; property dualists like Chalmers argue that the explanatory gap between physical description and phenomenal experience is genuine and irreducible. The debate is live, with no broadly accepted resolution.

The contemporary debate organizes itself around several canonical thought experiments. Nagel's bat asks what it is like to be a creature with echolocation — and argues that even complete physical knowledge of the bat would leave the question unanswered. Jackson's Mary is a brilliant scientist who knows everything physical about color but has spent her life in a black-and-white room; on emerging, she allegedly learns something new. Chalmers's zombies are creatures physically identical to us but lacking phenomenal experience.

Responses divide into eliminativist, reductive, and non-reductive camps. Dennett argues that qualia as commonly conceived do not exist and that the appearance otherwise is a cognitive illusion. Reductive physicalists argue that qualia are identical to physical or functional properties, with the explanatory gap a matter of cognitive access rather than metaphysics. Non-reductive views — Chalmers's property dualism, Russellian monism, biological naturalism — accept the gap as genuine and look for accommodations short of full reduction.