Qualia — Qualia (singular quale, from the Latin for “what kind”) are the qualitative, subjective properties of conscious experience: the redness of red, the sharpness of pain, the aroma of coffee, the timbre of a musical note. They name what, within experience, is lived immediately and privately by the subject — its phenomenal character or felt quality. In its modern philosophical sense the term was introduced by C.I. Lewis in Mind and the World-Order (1929) and has become central to contemporary philosophy of mind. Qualia lie at the heart of what David Chalmers called the “hard problem of consciousness”: even if science explained every brain mechanism (the “easy problem”), it would remain open why and how such physical processes give rise to a subjective dimension, to the “what it is like” to be a given organism. Thomas Nagel pressed this point in his essay “What Is It Like to Be a Bat?” (1974): knowing all the bat’s physiology would still not tell us what it is like, from its own point of view, to perceive the world through echolocation. Frank Jackson reinforced the thesis with the knowledge argument — the scientist Mary, who knows everything about the physics of color but has never seen it — in “Epiphenomenal Qualia” (1982). Against this view, functionalists and physicalists such as Daniel Dennett deny that qualia are irreducible properties, treating them as illusions or as dispositions explicable in functional terms. The debate thus connects with neighboring notions such as phenomenal consciousness and the mind–body problem.
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