The problem of spectrum inversion has often been raised as an objection to functionalism, as well as other materialist theories about consciousness. Briefly, the problem of spectrum inversion is as follows: even if there are two sensations that perform the same functional role in two different people it is still conceivable that these experiences will “feel” different. Thus it is argued that the functional role of an experience doesn’t explain why colors “feel” the same to different people, and hence that functionalism is missing something.
The obvious problem with this argument is that it begs the question against functionalism. It is part of the functionalist theory that the way an experience “feels” to us is determined by its functional role. Thus by assumption two experiences have the same functional role must “feel” the same. Of course it could be argued that this is a bad assumption, but to do so would require an argument that shows that accepting the assumption leads us to contradiction or absurdity, which spectrum inversion is not.
That response however is only open for a specific kind of functionalist theory, so perhaps the spectrum inversion argument is really a problem for other theories that don’t identify the “feel” of an experience so closely with its functional role. Let me then argue against the other assumption behind the spectrum inversion argument, that the experience of seeing a color feels the same to everyone. Do we have a reason to think that this claim is true? The only information we have about how other people experience color is based on how they describe their experience of it to us. However, someone’s description of their “feel” of color can’t capture the feeling itself, only its functional role (specifically what other experiences it is like, what associations it has for the person, ect). This is certainly evidence that our experience of color has roughly the same functional role as that experience in other people, but if we accept the possibility of spectrum inversion we are forced to admit that we really have no evidence from which to conclude that people experience color in the same way.
Let me run that by you again: if spectrum inversion is not a possibility then materialist theories are safe; if spectrum inversion is a possibility then as far as we know people really do experience the same color in different ways, and thus materialist theories are safe. Either way spectrum inversion is not actually a problem with materialism; it is more of a problem with how we describe our experiences to one another.
You can also see last time’s post for more thoughts on experience.