David asked:

How do I know that what I perceive as the color red is the same color that you perceive as being red?

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There are a couple of ways of approaching an answer to this question, depending on just how the
question was intended — as a question about knowledge, or as a question about perception.

You know that you are using the label "red" correctly if you have adequate evidentiary justification for
a belief that you are calling "red" the same suite of things in world that others call "red". This is the
process of learning what the symbol "red" means. And it is the process of learning the English
language.

To know more specifically that you are labelling as "red" the same things that I label as "red", you
need to compare your list of "red" things with my list of "red" things. If the two lists correspond
sufficiently, then we are both using the label in a similar manner. The match between our respective
lists of "red" things need not be exact. It merely needs to be sufficiently similar to avoid confusion in
most cases.

To know that you are perceiving the color red the same way that I do is a separate question. And you
may be surprised to find that it is largely irrelevant. It makes absolutely no difference to anyone (other
than a scientist curious about that specific aspect of perception) whether we each perceive red in the
same way or not. One of us could be color blind, or wearing color transposing glasses. Or it may be
natural for "red" things to appear differently to each perceiver. Makes no difference. All that matters is
that we each respectively call "red" (roughly) the same things in the world.

It is only if we find different things in the world that we each label as "red" that we can explore the
differences in how we perceive "red". And aside from those clear cases of color blindness, we will
almost always find that such differences in our respective lists of "red" things is due to a difference in
our vocabularies. My wife, for example, has a much richer vocabulary of "red-like" color names
(crimson, raspberry, candy cane, garnet, rose, wine, etc.). So I label as "red" many more things in the
world than she does. Does she perceive "red" differently? Doesn't make any difference. And barring
more scientific investigation with specific light frequencies, there is no way to tell.

Stuart Burns

This is actually still a hotly debated question. The short answer, in my very strong opinion, is that you
don't. But there are various caveats to this, aside from the several schools that will claim I'm simply
wrong. The first caveat is that if you mean something like: could I be seeing green when you see red,
and vice-versa, then you've got problems because of various asymmetries in the color absorption
patterns of our retinas and the way we process color in the cortex. If you look at some of Palmer's
work, you'll see detailed arguments to this effect (e.g.: Palmer, S. E. (1999). Vision science: photons
to phenomenology.Cambridge, MA, The MIT Press.). There are only certain color substitutions that
will preserve the symmetries in the "color wheel" of color relationships that humans see. So if you
stick to known colors you might be able to make a very restricted claim of this sort, but that's about
all.

But if you're asking whether when you see red you're seeing something that I cannotexperience at
all, because in the same circumstances Isee (i.e., have the conscious experience of) a completely
different color, call it "bred", then we're in another area entirely. Then all the human color relationships
can be preserved, and all the physical characteristics; the only difference is in our experiencesof
color, and mine and yours are simply different across the board. Then we're stuck (and note that I'm
still giving you myviewpoint on this), at this point in our knowledge of the brain and of consciousness.
This is an aspect of what's termed the "hard problem", i.e., how to knowwhat's going on experientially
for other people, to put it roughly. You can see that there will be no practical differences, as far as we
can tell (and zowie, if there was one, whoever found it would win the Nobel) between us. I'd still cross
at bgreen lights, while you'd cross at green lights; I'd drink borange juice while you'd drink orange
juice.

If you want to be absolutely blown away by all the debate around this question, go here:
http://ling.ucsc.edu/~chalmers/online.htmland look for stuff on the "hard problem".

Steven Ravett Brown