Imagine the two of us, arm in arm, looking at a sunset, where the horizon is fretted with golden fire and the deep blue night encroaches from the opposite side of the sky. "What beautiful colours", I say, and you agree.
And then, in the space of the following silence, I am struck by a worry. I can point at the sky and say it is blue, and you will concur. But are you really seeing that blue the way I am seeing it? Perhaps you have just learnt to call what you see "blue", but in actual experience you are seeing nothing like the vivid, rich, blue I see. You are an imposter, calling my blue by the same name as yours, but not really seeing it the way I do. Or, even worse, perhaps I am the one seeing a pale imitation blue, while you see a blue that is infinitely richer and more splendid than mine.
Now I admit that this worry lies in the realm of philosophy, not neuroscience. You might even ask me why I am worrying about this when we could be enjoying the glorious sunset. But when you think about it, it is not clear that I could ever have direct access to what it is like to be you, and you could never have direct access to what it is like to be me, or someone else, or something else, such as a bat. My worry seems more plausible when you consider colour blindness, which affects around 8% of men and half of one percent of women. Many people do not even realise they are colour blind. They live among the colour-seeing, getting by on the fact that there is usually some other difference between things of different colours that they can use to tell them apart, such as differences in shade or texture.
How green is my valley?
Our colour vision starts with the sensors in the back of the eye that turn light information into electrical signals in the brain – neuroscientists call them photoreceptors. We have a number of different kinds of these, and most people have three different photoreceptors for coloured light. These are sensitive to blues, greens and reds respectively, and the information is combined to allow us to perceive the full range of colours. Most colour blind men have a weakness in the photoreceptors for green, so they lose a corresponding sensitivity to the shades of green that this variety helps to distinguish.
At the other end of the scale, some people have a particularly heightened sensitivity to colour. Scientists call these people tetrachromats, meaning “four colours”, after the four – rather than three – colour photoreceptors they possess. Birds and reptiles are tetrachromatic, and this is what allows them to see into the infrared and ultraviolet spectra. Human tetrachromats cannot see beyond the normal visible light spectrum, but instead have an extra photoreceptor that is most sensitive to colour in the scale between red and green, making them more sensitive to all colours within the normal human range. To these individuals, it is the rest of us who are colour blind, as while most of us would be unable to easily distinguish an exact shade of summer-grass-green from Spanish-lime-green, to a tetrachromat it would seem obvious.
So yes, as we share this sunset, perhaps I am seeing something you cannot see, or you are seeing something I cannot see. If our colour vision is wired differently, the information going in could be more or less the same between us. But as you tell me this, with the sun sinking slowly below the horizon, you can sense that it has not really helped with my true worry. I am worried – and perhaps you are too – that although we both have the same machinery in our eyes and we are both able to see the green of the trees, the red of the sun and the blue of the sky, that when I say "blue", it creates an inner experience that differs from yours when you say "blue".