Inverted spectrum

Inverted spectrum is the hypothetical 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.

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. 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: