Qualia
The subjective, first-person character of experience — the redness of red, the painfulness of pain — and the central problem of contemporary philosophy of mind.
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.