Category: consciousness
Associated thinker(s): John Locke, Sydney Shoemaker, Ned Block
Original publication or approximate period: Locke, Essay II.xxxii.15; modern functionalist versions developed in the twentieth century
Difficulty: Intermediate
Themes: qualia, function, perception
Related experiments: marys-room, zombies
Kind: thought experiment
Research checked: 2026-10-30
Two people reliably call ripe tomatoes red and grass green. They make the same discriminations and use color words in the same circumstances. Yet imagine that the experience one has when looking at a tomato resembles the experience the other has when looking at grass. For the strongest philosophical versions, all relevant functional and behavioral relations must be preserved despite this experiential difference.
Could experiences differ while every relevant functional role remains the same?
The case tests whether the character of experience is exhausted by what a mental state does. Specifying a genuine inversion is difficult because color experiences have structured relations involving similarity, saturation, brightness, and memory.
Imagine trying to specify the inversion in a diagram. If red becomes green, what happens to orange, purple, perceived warmth, and judgments of similarity? If those relations change, the pair may no longer be functional duplicates. If they remain fixed, explain exactly what has changed and why the description remains coherent. This exercise does not prove inversion impossible; it identifies the work required to make it philosophically decisive. The stronger the sameness claim about function and information processing, the more carefully the alleged phenomenal difference must be described.
Qualia-based objections argue that functional roles leave phenomenal character underdetermined. Functionalists may deny that a complete role-preserving inversion is coherent or maintain that alleged phenomenal differences with no consequences have not been adequately specified. Representational approaches examine whether changes in experienced color require changes in represented properties.
Ordinary color blindness does not instantiate the stipulated case because discrimination and behavior differ. Swapping a few color labels is also insufficient. The asymmetries of actual color space make a simple red-green swap a poor automatic model of complete inversion.
Intrapersonal inversion asks whether one person’s experience could change while functional organization stays fixed. Interpersonal versions compare different people. Some inversions concern represented colors rather than intrinsic qualia and therefore support different arguments.
Historical association: Locke, Essay II.xxxii.15; modern functionalist versions developed in the twentieth century. The sources above support the scenario and its intellectual context. Where an original work was not retrieved in full, its account is mediated by the named academic reference; no claim of full primary-source verification is made. The explanation and reflection questions here are original editorial prose.