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Inverted spectrum

Inverted spectrum is the apparent possibility of two people sharing their colour vocabulary and discriminations, although the colours one sees — their qualia — are systematically different from the colours the other person sees.

Inverted qualia

The argument dates back to John Locke^[1]. It invites us to imagine that we wake up one morning, and find that for some unknown reason all the colors in the world have been inverted. Furthermore, we discover that no physical changes have occurred in our brains or bodies that would explain this phenomenon. Supporters of the existence of qualia argue that, since we can imagine this happening without contradiction, it follows that we are imagining a change in a property that determines the way things look to us, but that has no physical basis. In more detail:

Metaphysical identity holds of necessity
If something is possibly false, it is not necessary
It is conceivable that qualia could have a different relationship to physical brain-states
If it is conceivable, then it is possible
Since it is possible for qualia to have a different relationship with physical brain-states, they cannot be identical to brain states (by 1).
Therefore, qualia are non-physical.

The argument thus claims that if we find the inverted spectrum plausible, we must admit that qualia exist (and are non-physical). Some philosophers find it absurd that an "armchair argument" can prove something to exist, and the detailed argument does involve many assumptions about conceivability and possibility, which are open to criticism. Perhaps it is not possible for a given brain state to produce anything other than a given quale in our universe, and that is all that matters.

The idea that an inverted spectrum would be undetectable is also open to criticism on more scientific grounds:^[2]^[3]

Inverted spectrum arguments have applications to behavioralism, physicalism, representationalism, functionalism, skepticism and the hard problem of consciousness.

In his book I Am A Strange Loop, Douglas Hofstadter argues that the inverted spectrum argument entails a form of solipsism in which people can have no idea about what goes on in the minds of others?contrary to the central theme of his work. He presents several variants to demonstrate the absurdity of this idea: the "inverted political spectrum", in which one person's concept of liberty is identical to another's concept of imprisonment; an inverted "sonic spectrum" in which low musical notes sound like "high" ones and vice versa (which he claims is impossible because low sounds can be felt physically as vibrations); and a version in which random, complex qualia such as riding a roller coaster or opening presents are reversed, so that everyone perceives the world in radically different, unknowable ways.