¹ Levine, Joseph, ‘Materialism and Qualia: The Explanatory Gap’ in Pacific Philosophical Quarterly 64 (1983), 354–61CrossRefGoogle Scholar.

² Hardin, C.L., ‘Qualia and Materialism: Closing the Explanatory Gap’, Philosophy and Phenomenological Research 48 (1987), 281–98CrossRefGoogle Scholar; Color for Philosophers: Unweaving the Rainbow (expanded edn) (Indianapolis, IN: Hackett, 1988)Google Scholar; ‘Reinverting the Spectrum’, in Byrne, Alex and Hilbert, David R. (eds), Readings on Color, Vol. 1: The Philosophy of Color, (Cambridge MA: MIT Press, (1997), 289–301Google Scholar; Clark, Austen, Sensory Qualities, (Oxford: Clarendon Press, 1993)Google Scholar; ‘I am Joe's explanatory gap’, at http://selfpace.uconn.edu/paper/PGAP.HTM (1994).

³ The notion of ‘mixture’ used here is purely phenomenal, and should not be confused with what happens when differently coloured lights or pigments are physically combined: such combinations are often surprising, and precisely because they do not correspond to purely phenomenal mixtures. It is likewise important not to confuse ‘unique hues’ with ‘primary colours’ (either additive or subtractive). These distinctions are unobvious, and many people – notably Brentano – claim that green actually looks like a mixture of yellow and blue; and I once taught a class where everyone insisted that they could see a unique purple which looked neither reddish nor bluish. Introspective reports are notoriously unreliable; however, more precise psychophysical experiments have largely stabilized the discussion. See, for example, Hurvich, Leo M., ‘Chromatic and Achromatic Response Functions’, in Byrne, Alex & Hilbert, David R. (eds), Readings on Color, Vol. 2: The Science of Color. Cambridge MA: MIT Press (1997)Google Scholar.

⁴ Except in extraordinary, laboratory-induced circumstances. On this, see Crane, H. and Piantanida, T.P., ‘On Seeing Reddish Green and Yellowish Blue’, Science 221 (1983), 1078–80CrossRefGoogle ScholarPubMed; Billock, V.A., Gleason, G.A. and Tsou, B.H., ‘Perception of forbidden colors in retinally stabilized equiluminant images: an indication of softwired cortical color opponency?’, Journal of the Optical Society of America A 18 (2001), 2398–403CrossRefGoogle ScholarPubMed; and Suarez, Juan and Nida-Rümelin, Martine, ‘Reddish Green: A Challenge for Modal Claims about Phenomenal Structure’, Philosophy and Phenomenological Research 78 (2009), 346–91Google Scholar. The circumstances involve the use of filling-in mechanisms, which occur within the visual cortex itself, and therefore, arguably, do not undermine the opponent process theory, which concerns only retino-cortical channels. That unusual phenomenology should result from unusual stimuli (and brain processes), if anything, rather tends to support the view that hue phenomenology is explicable in physiological terms; so these phenomena, although remarkable, are not relevantly embarrassing.

⁵ Kaiser, Peter K. surveys some results here in ‘Physiological Response to Color: A Critical Review’, Color Research and Application 9 (1984), 29–36CrossRefGoogle Scholar. There is, alas, not much evidence of the sort of neural links that we require, though it is sufficient, for philosophical purposes, to ask what would follow if there were. A further complication is that the warm/cool distinction is also connected to achromatic light/dark: on this, see Berlin, B. and Kay, P., Basic Color Terms: Their Universality and Evolution, (Berkeley CA: University of California Press, 1969)Google Scholar.

⁶ There are many results that support this, some of them directly psychophysical, some of them anthropological: the variations in colour vocabulary between different cultures has, since Berlin & Kay (1969), proved to be a very fertile research tool in this area: see, for example, Hardin, C.L. and Maffi, Luisa (eds), Color Categories in Thought and Language, (Cambridge: Cambridge University Press, 1997)CrossRefGoogle Scholar for a useful survey.

⁷ For more on hue asymmetries, see Palmer, Stephen E.et al., ‘Color, Consciousness and the Isomorphism Constraint’ (plus commentaries), Behavioral and Brain Sciences 22 (1999), 923–89CrossRefGoogle ScholarPubMed.

⁸ Levine, Joseph, ‘Cool Red: A Reply to Hardin’, Philosophical Psychology 4 (1991), 27–40CrossRefGoogle Scholar.

⁹ Since supersaturated yellow is virtually unimaginable to us, it can hardly resemble green as much as ordinary yellow does! Of course, this fact need not automatically carry over for diagonally-inverted percipients; but the point is that arguments about numbers of perceptible differences are clearly far less conclusive here than they are often thought to be. The possibility of a supersaturated yellow is discussed briefly by Hardin (1988), 140. ‘Diagonal inversion’ is examined more thoroughly in a PowerPoint presentation on my website, where digitally inverted photographs are used to illustrate the phenomenon. See http://www.lancs.ac.uk/fass/doc_library/ippp/unwin_ppt_why_do_colours_07.pdf

¹⁰ Chalmers, David, The Conscious Mind: In Search of a Fundamental Theory, (Oxford: Oxford University Press, 1996), 99–100Google Scholar.

¹¹ See Hardin (1988), 129.

¹² Other difficulties also include the sheer suggestibility of naïve experimental subjects. But there is no reason why more sophisticated experiments should not be made, perhaps with properly trained psychophysicists as subjects.

¹³ It is particularly instructive to see how a single note of a square wave (which sounds roughly like a clarinet) can be made to sound like a chord formed from pure sine waves by removing and replacing harmonics. This helps to show why (undetectable) inversions of timbre are much harder to envisage than inversions of hue.

¹⁴ Nagel, Thomas, ‘Panpsychism’, in Mortal Questions, (Cambridge: Cambridge University Press, 1979), 181–95Google Scholar.

¹⁵ ‘For example, from a map of the United States it is easy to measure inter-city distances. Multidimensional scaling proceeds in the reverse direction: given a table of inter-city distances, it reconstructs the map’ (Clark (1993), 210).

¹⁶ See also Chalmers (1996), 235.

¹⁷ Block, , Ned, and Stalnaker, Robert, ‘Conceptual analysis, dualism and the explanatory gap’, The Philosophical Review 108 (1999), 1–46CrossRefGoogle Scholar; Chalmers, David and Jackson, Frank ‘Conceptual analysis and reductive explanation’, The Philosophical Review 110 (2001), 315–60CrossRefGoogle Scholar. See also Carruthers, David, ‘Reductive Explanation and the Explanatory Gap’, Canadian Journal of Philosophy 34 (2004), 153–74CrossRefGoogle Scholar; Marras, Ausonio ‘Consciousness and Reduction’, British Journal for the Philosophy of Science 56 (2005) 335–361CrossRefGoogle Scholar; Campbell, Neil, ‘Why We Should Lower Our Expectations about the Explanatory Gap’, Theoria 75 (2009), 34–51CrossRefGoogle Scholar; and Morris, Kevin ‘Does Functional Reduction Need Bridge Laws? A Response to Marras’, British Journal for the Philosophy of Science, 60 (2009), 647–657CrossRefGoogle Scholar.

¹⁸ However, Horst, Steven has usefully challenged many assumptions here, in Beyond Reduction: Philosophy of Mind and Post-Reductionist Philosophy of Science, (Oxford: Oxford University Press, 2007)CrossRefGoogle Scholar.

¹⁹ Though see David Cole (2000), ‘Inverted Spectrum Arguments’, at http://www.d.umn.edu/~dcole/inverted_spectrum.htm

²⁰ Levels of luminance provide a prediction of levels of perceived brightness only in certain narrowly defined circumstances, and even there the correlation is logarithmic, not linear.

²¹ For a similar reason, black/white inversion (as with a monochrome photographic negative) is not as readily conceivable as it might appear.

²² Chalmers (1996), 213–8.

²³ Sacks, Oliver, The Man Who Mistook His Wife for a Hat, (London: Picador, 1986), 88Google Scholar. See also x–xi. His main regret is that clinical descriptions of neurological disorders too often seem to depersonalize the patient. However, the idea may be generalized into a broader critique of Cartesianism and its successors.

²⁴ See also Campbell (2009).

²⁵ Earlier versions of this paper were read at research seminars at Lancaster University and the University of Central Lancashire, and at a Royal Institute of Philosophy seminar at the University of Bradford; and I am grateful for the many useful comments made.