The inverted spectrum is the concept of two people sharing their color vocabulary and discriminations, although the colors one sees—one's qualia—are systematically different from the colors the other person sees.
Overview
The concept dates back to John Locke. 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, i.e. swapped to the hue on the opposite side of a color wheel. Furthermore, we discover that no physical changes have occurred in our brains or bodies that would explain this phenomenon. Supporters of the hypothesis of qualia as non-physical entities 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:
Since it is possible for qualia to have a different relationship with physical brain-states, they cannot be identical to brain states.
Therefore, qualia are non-physical.
The argument thus states that if we find the inverted spectrum plausible, we must admit that qualia exist. 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 question, however, can arise how these critical philosopher's, using the same armchair technique that they are criticizing, refute the robust argumentation of the Inverted spectrum experiment? C. L. Hardin criticizes the idea that an inverted spectrum would be undetectable on scientific grounds: Paul Churchland using the Hurvich–Jameson opponent process criticizes the inverted spectrum on scientific grounds: Inverted spectrum arguments have applications to behavioralism, physicalism, representationalism, functionalism, skepticism and the hard problem of consciousness. Vincent Conitzer has illustrated the possibility of an inverted spectrum using the following scenario. Someone is observing a simulated reality through a VR headset, and is not aware that he is wearing a headset or that what he is observing is a simulation. Conitzer argues that the code responsible for the physics of the simulation can be separated from the code that determines in which colors the simulation is displayed by the headset. So, it would be possible to change that additional code, inverting the displayed colors, without changing anything about the simulated physics. Hence, which colors are displayed is a further fact over and above the facts of the simulated physics. 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 ; 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.
