One thing that particularly impressed me about Jeffrey Gray was his well-informed scepticism about colour. This must be a problem for people like you. According to you what people take to be colour qualia are actually just reality: the real qualities or properties of real objects in the real external world. If I can demonstrate that colours are not out there - that they're in your head, or in some Platonic world, or some phenomenal realm - you'll have to re-think, won't you? You realise, of course, that colour is not a property of physical objects themselves, but comes from the light they reflect or emit? You probably suppose, however, that each colour corresponds directly to the wavelength of the light in question. Not so. The actual wavelength varies tremendously according to the light falling on the object; what colour you see depends to a great extent, not on the wavelength of the light coming from the object you're actually looking at, but from a complicated comparison with the wavelengths coming from nearby objects. This helps to ensure that (within limits) perceived colour stays constant in spite of varying light conditions: but if you isolate a coloured surface from the kind of clues provided by comparison with other surfaces, a colour can't be assigned to it at all - it just looks white.

I understand perfectly well that the colour vision system is complex, and that the brain infers colours from the available evidence through unconscious and largely involuntary processes. So what? At the end of the day, colour is a property of objects - Gray himself concedes that colour vision is a fallible but practical guide to surface reflectance. Colour is an interesting property of real objects - if it were imaginary, as you seem to think, how come the colour of objects is so consistent and such a good guide to various physical and chemical properties? Colour vision is useful enough to have evolved separately several times in different species - if it wasn't telling us something real, how could it be so useful?

Yes, yes - colour corresponds with real properties. But colour itself is not in the external physical world. It's like those cases where a picture from a telescope or a microscope is given colours artificially to help show up the different substances. The colours are not real, but they do help you understand more about what you're seeing. It turns out that all colour is more or less like that -yes, it's helpful, but that doesn't mean it directly reflects the physical reality. After all, we only actually sense three different wavelengths. Our eyes have cells which respond with different degrees of enthusiasm to three different colours: all the others are inferred from the ratios between the three. A good thing, too, in a way, because otherwise colour photography would be a lot more difficult. As it is, we can give the appearance of millions of colours just by different mixtures of red, green and blue. If you think about it, though, this means that the way we see things is actually wrong. To us, a combination of red wavelengths and green wavelengths looks the same as pure yellow wavelengths - but it isn't the same. Compare sound, another wavelength phenomenon. If I play middle C on the piano and the E above it at the same time, it doesn't sound like D. But with vision, the wavelengths just get averaged out. Part of the reason, of course, is that your ears don't attempt to assign sounds to a precise location in space. They can afford to have one accurate sensor (a little hair-like thing) for lots of different wavelengths. But your eyes need to assign colour to a precise spot - so they can't crowd hundreds of different cells into that one small area of the retina. All the same, it is possible to do a bit better, and some animals detect more than three different wavelengths. Pigeons can do four, for example, so they actually see more colours than we do. Now how do you cope with that? According to you, colours are real: but in that case pigeons somehow have a different reality?

I don't claim that our senses are perfect; pigeons may see colour more accurately, certainly, but I absolutely deny that they see more colours, except in the trivial sense of being able to discriminate finer gradations. They see the same spectrum we do; just more clearly. Ask yourself this: if we only sense three wavelengths, why do we see more than three colours? Of course, there can be an indefinite number of different mixtures of the three, but I think it's obvious that the spectrum we see contains more than that - the full ROYGBIV seven. You rightly pont out that a mixture of red and green light looks the same as pure yellow light: but if we can only sense the red and the green, why don't they both look like mixtures? But they don't. They look like yellow: and yellow doesn't look at all like any mixture or combination. The spectrum is real, and our senses infer that reality and present it to us even though they can't detect it comprehensively.

That sounds borderline loony to me. What about people who are red-green colour-blind? If our senses somehow manage to infer this Platonically real spectrum in spite of inadequate evidence, surely they should be able to see the full spectrum, too? And as for ROYGBIV - you're actually going to defend the separate existence of indigo? Everyone knows that Newton stuck that one in just to make sure the number of colours was the mystical value seven.

Red-green colour-blind people do indeed see the full spectrum. You don't imagine that they look at a rainbow and find that the red or the green stripe is missing? They see both red and green - they just have difficulty telling which is which. I don't know about indigo particularly, but I think Newton was right about assigning seven colours to the spectrum. Perhaps this is another case, like gravity's action at a distance, where his mystical leanings allowed him to entertain ideas that orthodox scientists had an allergic reaction to. Consider the analogy with sound. Both are wave phenomena. Sound comes in octaves, each with seven distinct notes. When you get to the eighth note, it is a higher or lower version of the first one - C above or below middle C, say. Now the basis of the octave is a doubling, or halving of the frequency, and that's why the two notes sound the same. If a wave comes along and twitches one of those little hairs you were talking about eight times a second, it matches up with a wave which twitches it four times or sixteen times. The other notes on the scale come from slightly more complex ratios. The Greeks knew all about this. Now the visible spectrum is just the right length to provide seven notes out of an octave of light. And that's what we get.

So you maintain that the colours we see are the only possible colours? What about octarine?

What's that - an eight-legged citrus fruit?

Octarine is Terry's Pratchett's imaginary eighth colour. Obviously magic is involved in his case, but there has to be an octarine, doesn't there? The mere conceivability of another colour shows that the spectrum is not an absolute reality. It seems to me that, just as we can always encounter a completely new smell, there would always be scope for a new colour, if our eyes were able to develop new responses the way our nose presumably can. But I don't even need to rely on conceivability. Some insects can see ultraviolet light, for example, and some snakes can see infrared. They must assign to those wavelengths colours which we can't see, mustn't they?

Not at all. Look at the way the spectrum forms a closed circle. If we extended it downwards below red, we should simply get another, lower, violet. Now I grant you that the 'lowerness' would have to expressed in some way - possibly as 'warmth'. The colours of the visible spectrum are differentiated in terms of warmth, so perhaps the lower violet would appear distinctly warmer than the one we're used to (great scope for interior decorators...). I repeat, the spectrum is a reality. You can call it a mathematical reality if that helps, but it's real. If we saw colour the way we hear pitch, all this would be obvious. But the fact that we can't see colour harmonies or more than a single octave of colours means there's never been any scope for a genius to come along and produce a regularised interpretation of the spectrum, the way J.S.Bach did for the musical scale.

I'm simply amazed by all this. But at least it is nice to hear you indulging in speculative phenomenology.