Inverted Spectrum

Suppose persons A and B have different color sensations while gazing at a ripe red tomato: A perceives “redness,” but B has the sensation that A has when A is looking at a purple grape. A and B both can visually detect ripe tomatoes and both learned to use language the same way; only their subjective sensations differ. If A and B have systematically swapped sensations (red-purple, orange-blue, etc.), then they are said to have an inverted qualia spectrum relative to each other. Thus, the inverted spectrum (IS) possibility is that, given the same wavelength light stimulus, two people might systematically have different color sensations (qualia) yet behave the same.

The IS received its classic statement in John Locke's 1690 An Essay Concerning Human Understanding. ...