Seeing Green

Evelyn's birthday passed a few months ago. We were at a bar with her friends and I somehow got into a discussion with some guy about the nature of seeing green.

I'm sure the thought has run though many people's heads in some form or another. It basically goes like this: what if your green is not my green? Ignoring colour-blindness, when I look at the grass I register it as "green" and so do you. We are both, of course, able to verbalize this, but there's no way to tell if we're really seeing the same "green". What if "green" to you is really what I consider to be "red"? There'd be no way of knowing - you'd still call it "green", it's still the colour we both associate with grass, etc.

I remember having these sorts of thoughts when I was younger, but it was only at U of T that I discovered that this is actually quite a famous thought experiment. It's called the Inverted Spectrum and it dates back to John Locke.

Why is this way of thinking so prevalent? I suspect it's because it matches the way most people think they experience colours. When we see a patch of grass, there's an overwhelming impression that the "green" we see is "out there" in some fashion. The green is "in the grass", so to speak, and the only thing we have to do is react to it. There is a separateness to the thing we are reacting to and the reaction itself, and hence it's not hard to imagine a situation where, even though two people's reactions might be the same, the thing they are reacting to might be fundamentally different - as different as, say, red and green.

Note that the "thing we are reacting to" isn't meant to be identified directly with the grass or the particular wavelength of light corresponding to the colour green. The idea is that two people see the same grass, get the same wavelength of light in their eyes, and are both able to identify the grass as green (their reaction is the same) but that, somehow, one person is really seeing red, while the other person is really seeing green.

The term we are looking for here, as I found out in University, is "qualia". One of my professors at U of T used to describe qualia as "the greenness of green". It's green - but separate from the wavelength of light that represents green, separate from our reaction to green, etc. Plato would probably have a field day with this.

On the one hand, I know what people mean, more or less, when they explain this. On the other hand, the more I try and pin this idea down, the more it doesn't make any sense to me. Where exactly is this "greenness" supposed to reside? I mean, try and think about this. You have two people. They both see a patch of green. A certain wavelength of light enters their eyes. They both say "That's green!". They are both telling the truth. What exactly would it mean for one person to really be seeing red? Where would the difference reside? It can't reside in the physics or the psychology. So...where is it? It really boils down to this; if you believe that two otherwise normal people can see different colours when they look at a patch of grass, you have to believe that there is something more to concept of green than the physics and the psychology - more than the wavelength and the reaction.

We can put it a slightly different way. Do you think it's possible for two people to be physically and behaviourally identical, down to the very atom, and yet still have fundamentally different experiences when they see the same a patch of grass? I think you'd have to be willing to concede at least the theoretical possibility if you believe in qualia as I described it above

• if you believe that two people can look at the same patch of grass and both recognize it as green and yet be experiencing two different colours.

It just seems non-sensical to me, but then again, I would say that, coming from an engineering background. What am I missing?