Qualia: The Hard Core of Consciousness
Qualia (singular: quale) are the subjective, intrinsic, qualitative characteristics of conscious experience — what it is like to undergo that experience from the inside. The redness of red as you experience it; the particular painfulness of a headache; the smell of coffee; the taste of a strawberry; the felt texture of sandpaper. Qualia are what the experience is like to you, not what the experience is caused by or what information it carries.
The concept of qualia captures the intuition that there is an inner, subjective, first-person aspect of conscious experience that seems irreducibly different from anything that could be described in third-person, objective, physical terms. A complete neuroscientific account of color vision could specify every photoreceptor activation, every neural signal, every cortical processing stage, every behavioral output — and yet, it seems, still leave something out: what it is like to see red. This is the sense in which qualia appear to be explanatorily irreducible to physical or functional descriptions.
C.I. Lewis coined the term 'qualia' in its modern philosophical sense in 1929, but the underlying concept appears throughout the history of philosophy. Locke's 'ideas' in the Essay Concerning Human Understanding involve something like qualia; Berkeley's immaterialism is motivated by the irreducibility of experience; and James's discussions of the 'stream of consciousness' in Principles of Psychology (1890) are centrally concerned with the qualitative character of experience.
Qualia are characterized by several properties that make them philosophically troublesome for physicalism. Intrinsicness: the qualitative character of a sensation seems to be an intrinsic feature of the experience, not merely relational or functional. Privacy: only the experiencing subject has direct access to their qualia — others can infer but not directly observe. Ineffability: the full qualitative character of an experience seems difficult or impossible to fully convey in language to someone who has not had the experience. Immediacy: qualia are known directly, without inference or interpretation.