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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".

Behind blue eyes

My worry about your inner perception of the colour blue is a facet of the basic isolation that is part of the human condition. Even if we think we can really know other people, we cannot be certain of that knowledge. Historically, psychologists have adopted a stance called behaviourism, which acts as if questions about inner experience are irrelevant. This approach states that if you call my blue "blue", and you can always tell it from red, and if we both know it is the correct colour for the sky, my eyes and the Smurfs, then who cares what the inner experience is?

There is a lot of mileage in this perspective, but maybe there is also some wisdom in trying to convince ourselves that the difference between our inner experiences is real, and does matter – and, in fact, that some difference is inevitable. We use common words, and use them to refer to shared experiences, but nobody can see the same sunset, merely because perception is a property of the person, not of the sunset. Because there is something that it is like to be you, and your “you-ness” is unique, we are certainly seeing different things when we talk about looking at something blue, if only because the act of seeing incorporates feelings and memories, as well as the raw light information arriving at our eyes.

In any case, the sun has set and we walk away. You might be seeing a richer blue in the sunset than me, but you will not have the same memories of the other sunsets I have seen and the people I have watched them with. We could get our vision tested and find out who was better at perceiving colours, but we would never know what it was like to be the other person seeing a particular colour. As long as we can both say that it is a beautiful sunset, we can agree and be secure in the knowledge that I see my blue, and you see your blue, and although we may not see the exact same thing, we have shared it. And that sharing is itself unique to you and me, because no two other people in the world have the same two minds.

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