The word quale (or qualia) derives from the Latin for "quality." As used by C. I. Lewis (1929) and those following him, it refers to the qualities of phenomenal individuals, such as color patches, tastes, and sounds. In this sense the term means what George Berkeley meant by "sensible qualities," or what later philosophers meant by sensa or sense data. Since the demise of sense data theories, the term qualia has come to refer to the qualitative, or phenomenal, character of conscious, sensory states, so that it is mental states, not phenomenal individuals, that are the subjects of predication. Another expression for this aspect of mental life is the "raw feel" of experience, or "what it's like" to have certain sensory experiences. Qualia are part of the phenomenon of the subjectivity of consciousness, and pose one of the most difficult problems for a materialist solution to the mind-body problem...