The Inverted Spectrum

The scenario

You and I both stop at red lights. We both call ripe tomatoes red. We pass all standard color vision tests. We agree on every color discrimination, every color relationship: red is more similar to orange than to blue, and so on. Our color behavior is identical in every way that could be measured or tested.

But when you look at what we both call "red," your inner experience is what I would describe as green if I could somehow step inside your perspective. Your entire color spectrum is systematically inverted relative to mine.

There is no way to detect this from the outside. The inversion is perfect and consistent. The question is whether such a scenario is coherent, and if it is, what follows.

What it shows if coherent

If the inverted spectrum scenario is genuinely possible, it means that qualia, the felt qualities of experience, are not fixed by functional role. The functional role of your "red" experience is identical to mine. It is caused by the same wavelengths, it causes the same behavior, it stands in the same relationships to other color experiences. But the phenomenal character differs.

This is a direct challenge to functionalism. Functionalism holds that mental states are what they are because of their functional roles. If two states share all the same functional relationships and causal connections, they are the same mental state. The inverted spectrum, if coherent, shows that two states can be functionally identical while phenomenally different. Functionalism leaves qualia unexplained.

The scenario also supports the view that there is something about experience that is essentially private: not just inaccessible in practice, but inaccessible in principle, even with complete behavioral and functional information.

The coherence objection

Functionalists have a significant reply: the scenario may not be coherent at all.

Color experiences do not come in isolation. Your experience of red is what it is partly in virtue of how it relates to your experience of orange, pink, purple, and every other color. These relationships are not external add-ons to the experience; they may be partly constitutive of it.

If that's right, you can't simply invert the spectrum while holding all the functional relationships fixed. Changing which experience you call "red" would change the whole structure of relationships, and that would be detectable. A truly systematic inversion might not leave behavior unchanged after all.

Shoemaker took this objection seriously and tried to show the spectrum could be partially inverted in ways that preserved enough relational structure to remain undetectable. Whether he succeeded is still debated. The question of whether qualia can vary independently of function remains one of the genuinely open problems in philosophy of mind.