What If Your Red Is My Green? The Puzzle of Inner Experience
The World Inside Your Head
Look at a ripe strawberry in good light. Its red seems to shout at you. You know that color instantly. But here is a question that can keep you up at night: does the strawberry look exactly the same to the person standing next to you? Could her inner red be your inner green, and neither of you would ever guess? Philosophers call the “what it’s like” of an experience a quale (plural qualia). A quale is a property of your experience — the particular way vermilion looks, the sting of cold water, the smell of fresh bread. Your experience of that strawberry has a certain quale (Q_{R}). Your experience of a garden lime has a different one, (Q_{G}). These labels name the feel of seeing red and green, not the colors themselves.
The idea that we each have private inner qualities is not new. But it leads straight into one of philosophy’s most vivid thought experiments: the inverted spectrum. Imagine that everything in your friend’s world is color-swapped compared to yours, yet she uses all the same color words and behaves exactly as you do. If that were possible, it would shake our understanding of the mind, science, and what we can ever know about other people.
Locke’s Incredible Idea
In 1689 the English philosopher John Locke (1632–1704) gave the first clear description of such a swap. He asked us to picture two people, one whose mind works normally and another whose colour wiring is somehow rotated. When Invert looks at a violet, the inner “idea” she gets is exactly what Nonvert gets when he looks at a marigold — and the other way around. Yet both children learn to call the violet “blue” and the marigold “yellow,” because that’s what everyone around them does. Their behaviour, their choices, even their emotional reactions to colours, could stay perfectly matched. Locke thought this scenario was genuinely possible — and forever undetectable.
Later scientists noticed a catch. Human colour space is not a simple wheel; it’s lumpy. For instance, there are more distinguishable shades between blue and red than between green and yellow. Dark yellow turns into brown, while dark blue is still blue. Yellow is at its brightest when it’s very light, whereas blue shines at its brightest when it’s darker. If you tried to flip a normal person’s hues exactly 180°, these asymmetries would make the inversion detectable: Invert might call a muddy brownish shade “yellow” or say that the “blues” are lighter than the “yellows.” A perfect behaviourally invisible swap may be impossible for humans.
Still, philosophers found ways to keep the idea alive. Some imagined a person who sees only black, white, and grey — a simple colour world where inversion goes unnoticed. Others pictured creatures whose colour spaces were purpose-built with perfect symmetry. The point isn’t whether such beings exist; it’s that if we can even make sense of them, then inner experience isn’t pinned down by behaviour or function alone.
A Challenge to the Mind-Machine
Many philosophers who study the mind have held that mental states are nothing above and beyond what they do. This is functionalism: a pain just is the state that is caused by injury, makes you wince, and leads you to avoid the cause. A colour experience just is the state produced by light of a certain wavelength, connected to judgments and actions about coloured things. If functionalism is true, then two creatures who are perfect functional twins — identical in all their inputs, outputs, and internal links — must be in the same mental states.
The inverted spectrum seems to say otherwise. In the 1970s the philosophers Ned Block and Jerry Fodor argued that we can clearly imagine a situation in which Invert and Nonvert are functional duplicates but their colour qualia are swapped. When Nonvert sees a tomato, he has the quale for red. When Invert sees that same tomato, she has the quale for green. Yet everything they say and do is identical. If this scenario is possible, functionalism is false, because two functionally identical beings would differ mentally. A similar argument targets physicalism, the view that the mental is entirely physical. If Invert and Nonvert could be built of exactly the same physical stuff — down to the last atom — and still have different qualia, then qualia aren’t physical.
Block and Fodor pointed out that no one had ever spelled out a complete functionalist theory, and they thought the possibility of inverted qualia depended on conceptual coherence. But they insisted that the scenario is “conceptually coherent” — we can tell a clear story about it without contradiction. That alone, they thought, is trouble for theories that reduce the mind to function or matter. Others replied that being able to tell a coherent story doesn’t guarantee that the story is genuinely possible. After all, you can tell a seemingly coherent story about a perfectly round square, but that doesn’t mean such a shape could exist. The debate over whether inverted qualia are possible — or only imaginable — is still alive.
Can We Represent Our Way Out?
One popular approach to qualia is representationalism. On this view, your experience’s quale is entirely a matter of what the experience represents. For example, the quale of seeing red just is the property of being an experience that represents something as red. If two experiences both represent a tomato as red, they have the same colour quale — no matter what the wiring inside the brain feels like. This idea is attractive because it ties the mysterious feel of seeing red to something we might be able to explain scientifically: how the brain represents the world.
But a carefully constructed inverted-spectrum scenario makes life difficult for representationalism. Suppose Invert has lived with her swapped colours since birth, uses colour words like everyone else, and is just as good at picking out ripe fruit. It seems wrong to say that Invert is constantly misperceiving the world — that strawberries secretly look green to her. So we accept that the tomato looks red to both Invert and Nonvert. Yet by hypothesis their inner qualia are different. This would mean two experiences can represent the same colour but have different qualia, and representationalism would be false.
The representationalist can fight back. Some deny that the tomato really looks the same colour to both people — perhaps Invert is misperceiving, and our sense that this would be “unfair” is not a good argument. A more creative reply comes from the philosopher Sydney Shoemaker. He suggested that visual experiences don’t just represent ordinary colours; they also represent appearance properties — roughly, the way a colour presents itself to a particular perceiver. On this picture, the tomato looks red (the ordinary colour) to both Invert and Nonvert, but it also looks to have an appearance property (AP_{R}) to Nonvert and a different one (AP_{G}) to Invert. Since their experiences differ in what appearance properties they represent, representationalism can capture the difference without accusing Invert of error.
Another line of defence says the difference is not in what is represented but in the mode of presentation. Like thinking of the same actor as “Michael Caine” versus “Maurice Micklewhite” — same man, different way of grasping him — Invert and Nonvert could represent the very same property (redness) under different mental garb. This keeps the core of representationalism while acknowledging that qualia might outrun the simple objects represented. The debate is far from settled, and the inverted spectrum continues to be the key testing ground.
Why the Gap Matters
Even if representationalism wins, a deeper puzzle remains. Many philosophers think the inverted spectrum points to an explanatory gap — a chasm between physical facts and conscious experience. Imagine a future science that can describe every neuron, every chemical, every signal involved when you see red. Call that complete physical story (R). Now consider an inverted spectrum scenario in which (R) is exactly the same, but your quale is the one normally associated with green instead of red. If you can imagine that situation without contradiction, then the physical story does not fully explain why you have the quale you do.
This is not an argument that the inversion is actually possible — only that it seem imaginable, which is enough to make us wonder whether physical explanation alone can ever close the deal. The gap matters because it suggests that even the most perfect brain science might leave something out: the subjective what-it-is-like. This is why the inverted spectrum, dreamt up centuries ago, still drives research into consciousness today.
In your own life, you blithely assume that your friends’ inner worlds are like yours. When you both say the sky is blue, you believe you’re sharing the same quality. But the inverted spectrum shows that you cannot, even in principle, peek inside another person’s mind to check. It challenges the foundations of empathy, science, and self-knowledge. And it starts with a deceptively simple question: what if your red is my green?
Think about it
- If you and your best friend both call the sky “blue,” is there any way to prove you are having the same inner experience, or is it something you simply trust?
- Imagine a perfect robot copy of you that acts and talks exactly like you. Would it have qualia like you do, or would its “seeing red” be just a mechanical reaction with no feeling inside?
- If scientists could one day predict your experience of red from brain scans alone, would that explain why red looks the way it does, or would something still be missing?
