"Purple Haze: The Puzzle of Consciousness" by Joseph Levine, "Phenomenal Consciousness: A Naturalistic Theory" by Peter Carruthers, and "The Nature of Consciousness" by Mark Rowlands

The Vienna Circle was a group of scientifically-minded philosophers, many physicists by training, who in the 1920s and 30s developed the cluster of philosophical doctrines known as Logical Positivism. Among the Circle’s most distinguished members were Rudolf Carnap and Herbert Feigl, each of whom emigrated to America during the Nazi era. It is said that Feigl, the author of an important 1958 monograph defending a materialist approach to the mind-body problem, once gave a visiting lecture on the problem of consciousness at UCLA, where Carnap was teaching. Feigl argued that although there were good reasons for believing that the mind is fundamentally physical, the physical explanation of the ‘qualia’ of sensory experience – the ineffable sensory qualities involved in, say, smelling coffee – was still a mystery to science. Now the story becomes apocryphal. Carnap is supposed to have interrupted, ‘But Feigl, there is something missing from your lecture. Science is beginning to explain qualia in terms of the alpha factor!’. We can imagine Feigl somewhat alarmed by this interjection from the great Carnap: ‘But Carnap, please tell me: what is the alpha factor?’. ‘Well, Feigl’ Carnap replied ‘if you tell me what qualia are, I’ll tell you what the alpha factor is’.