How can we be sure that we perceive color the same way? In other words, how do I know that the red I see looks the same as the red that you see? We are taught from birth to identify red objects as red, but what if what someone calls red really looks green for example, yet they only call it red because that is what has been taught?

This is the classic skeptical worry about 'spectrum inversion'. Not allphilosophers would agree (surprise, surprise), but I am inclined to saythat you can't know for sure. And I agree with you that the fact thatwe agree about the name of the color doesn't help much. At the sametime, I think we may have good reason to believe that similar coloursare experienced similarly by different people, insofar as we have goodreason to believe that they go through similar neurophysiologicalprocesses when they are exposed to light of the same frequency.

This is a natural and important question to wonder about. It is also an old and distinguished one, dating back at least to John Locke. In its modern incarnation it's often called the problem of "spectrum inversion" or "qualia inversion". Two people might make all the same color discriminations, and use color language in all the same ways, so that outwardly (from a third-person perspective) we would have no reason to say that colors don't "look the same way" to them. But how can we be sure? Isn't it possible that a red object looks to me exactly the same way a green object looks to my functional twin, and vice versa?

There is currently a raging disagreement about this, and it leads directly into the fascinating and vexing mind-body problem. Much of the debate turns on what we should mean by "the way a color looks" or "the qualitative character" color experience. Some hold that the way things look or seem first-personally does not go beyond the way that a person reacts to, processes, and acts upon her environment. For such philosophers ("functionalists") if two people really are functionally the same--that is, they make all the same color discriminations and use color words in all the same ways--then things thereby look the same to them from the inside. Philosophers who hold this need somehow to explain away the intuition you expressed--that it's nevertheless possible for things to look differently to such twins. Other philosophers honor the intuition and hold, therefore, that the felt, phenomenal character of a color experience is not exhausted by the role that that color experience plays in a person's cognitive functioning. But then one promising materialist way that we might try to understand our mental states--as states that essentially play a certain cognitive role in us--can't capture everything. In fact, it leaves out an experience's most central aspect--the way it feels from the inside. And if this transcends cognitive function, then perhaps it's not physical at all.

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