more about seeing yellow

I was out looking at yellow things with Andy at our friends' Michele and Brent's place last night. We had a nice supper and unwound from a hectic week. When we got home, there was this email from my Dad:

I've always wanted to know why some people ask questions whose answers are inherently unknowable -- such as whether everyone sees colours the same. What experiment could possibly determine the answer to that question? Metaphysics begins with questions purportedly about the real world whose answers cannot possibly be determined by any experiment.

For your particular question, ask yourself how you learned to use the word "yellow". When your usage developed to the point that you and your parents used the word "yellow" for almost exactly the same things, the meaning of the word, for you, stabilized. So we'll agree that A, B, C, and D are yellow, and that P, Q, R, and S are not. In that sense we "see the same thing" but whether what is going on inside you, when you say "that's yellow" is the same as what is going on inside me, when I say the same thing, is I think completely unknowable. And as long as we can communicate efficiently, and not have you saying that things that I think are red are really yellow, it doesn't matter whether what's going on inside you and what's going on inside me are the same.

Let's put it this way. If your mother were to claim that what you see is that same as what I see when we both say that something is yellow, how could the two of us possibly prove her wrong? If she were to claim that what you see is different from what I see, how could the two of us possibly prove her wrong?

Dad, who read a lot of logical positivist philosophers a long time ago

question: can you prove either way?

mompoet - exactly.