What if your red is another person's green?
John Locke proposed a baffling possibility: two people could look at a ripe tomato and have completely inverted visual experiences. One sees what you call red; the other experiences the internal sensation you call green. Because both grew up calling that sensation "red," they agree on stoplights, color charts, and paintings. No behavioral test or brain scan can definitively prove whether your private qualia match anyone else's.
Locke and the Origins of the Inverted Spectrum
In his 1690 treatise 'An Essay Concerning Human Understanding', the English philosopher John Locke introduced a thought experiment that would become a cornerstone of the philosophy of mind. Locke asked what would happen if a person's eyes were altered such that the colors they experienced internally were systematically inverted relative to everyone else. For instance, looking at a violet might produce in that person the internal sensation that others associate with a marigold, and vice versa.
Locke noted that this alteration would likely remain entirely undetected. Because this person learned language in the same public environment as everyone else, they would still apply the word 'violet' to the flower, describe marigolds as yellow or orange, and agree on which objects match in shade. Their speech, choices, and reactions would align perfectly with the surrounding community.
This scenario, now widely referred to as the inverted spectrum, highlights a fundamental separation between two aspects of perception: our public, shared descriptions of the world and the private, subjective qualities of our conscious experience.
Understanding Qualia and Subjective Experience
To understand why the inverted spectrum matters to philosophers, one must examine the concept of 'qualia'. Qualia is the philosophical term for the subjective, qualitative properties of conscious experiences—the raw 'what it is like' to undergo a sensory state. The sharp ache of a headache, the scent of fresh pine, and the vivid redness of a ripe strawberry are all examples of qualia.