Here's how the physicist David Deutsch (2011) describes qualia:

Consider the following thought experiment. You are a biochemist with the misfortune to have been born with a genetic defect that disables the blue receptors in your retinas. Consequently you have a form of colour blindness in which you are able to see only red and green, and mixtures of the two such as yellow, but anything purely blue also looks to you like one of those mixtures. Then you discover a cure that will cause your blue receptors to start working. Before administering the cure to yourself, you can confidently make certain predictions about what will happen if it works. One of them is that, when you hold up a blue card as a test, you will see a colour that you have never seen before. You can predict that you will call it 'blue', because you already know what the colour of the card is called (and can already check which colour it is with a spectrophotometer). You can also predict that when you first see a clear daytime sky after being cured you will experience a similar quale to that of seeing the blue card. But there is one thing that neither you nor anyone else could predict about the outcome of this experiment, and that is: what blue will look like. Qualia are currently neither describable nor predictable — a unique property that should make them deeply problematic to anyone with a scientific world view (though, in the event, it seems to be mainly philosophers who worry about it).

It is now sometimes possible for those born deaf (or blind) to hear (or see) for the first time in adulthood.

Have philosophers studied these experiences? How have these experiences changed (or not changed) philosophers' understanding of qualia?

Is it for example true that those who were born deaf/blind but then were able to hear/see for the first time were completely unable to describe or predict before the fact what hearing or seeing is like?

  • Does this answer your question? The main reasons for "qualia" being a problem?
    – user14511
    Jul 1, 2020 at 7:34
  • 1
    Did philosophers look at such cases? Yes, e.g. Block discusses blindsight in Function of Consciousness. Does it have much effect on the qualia debate? No, because the qualia issue is by definition unaffected by any empirical findings. Philosophers most interested in such findings are physicalists, whose tendency is to explain qualia away, and while the Molyneux question about cured blindness is much discussed, it is tangential to qualia.
    – Conifold
    Jul 2, 2020 at 6:21

2 Answers 2


Restoration of sight in adults is important. Detailed NMR studies before and after restoration, though these cases are rare in the developed world. Groups of people in the developing world only able to access now standard procedures as adults like cataract removals. These seem to indicate limitations on neuroplasticity in adults, once the visual cortex has been linked to others through key development ages.

The idea of Mary The Colour Scientist walking out of her black-and-white room is not quite realisable yet, though gene-therapy techniques for colourblindness disorders look to be close. Tetrachromatism has been proven in at least one case, someone who can see four primary colours. And aphakia is interesting, seeing UV light where the lense has been removed, with Claude Monet being an especially interesting example, given his powers of capturing his impressions.

Human echolocation is a well studied area. Arguably that's part of understanding a bit of what it's like to be a bat.

Helen Keller is an interesting case, blind and deaf from 19 months old, she vividly described her first understanding of a word at age seven.

Visual processing is reckoned to take up 30% of our brains, touch 8%, hearing 3%. So I'd say it's a mistake to assume there's a switch that can flipped for a person to see blue for the first time, & their vision becomes like someone who has always seen it - maybe with training to follow, maybe never.

Oliver Sacks illustrates some of the most extreme cases of altered perception, usually resulting from brain injuries, in his book The Man Who Mistook His Wife For A Hat. The modern terminology 'neurodiversity' aims to expand our idea of the range of normal healthy functioning of brains, and prompt more support and adaptations to allow that, especially around common conditions like autism spectrum conditions, and dyslexia. Human variety increases what we can understand experientially together I would contend, looking at cases like Monet & Einstein (Einstein & Newton are thought very likely to gave been on the autism spectrum).

Have these cases changed any philosophers understanding of qualia? Probably not, though I'd say they should.

I see the frequent inability to have productive debate between 'pro' & 'anti' qualia camps stems from a confusion. There are non-linguistic animals which are not social and don't encounter their progenitors, which still have experiences of the world. But conceptual, abstracting consciousness, required for anything we call language whether body language or an emotional reaction or spoken, that requires neural architecture built by intersubjectivity, mental projection into the situation of others. In this way language cannot be private, you 'speak' to other beings as though they were you, and in the way and to the depth of extent that they are like you. The complexity of concepts alters experiences themselves, like having more colour words allows faster colour identification. And is dramatically deepened by collective practices, like structured play & story-telling. In particular, developed into the language of science practice, it allows careful inference from things like NMR & in future neural implants like Neuralink, about how our perceptions vary - a kind of expansion of intersubjectivity, rather then a revealing of subjectivity to have been a mirage. If a language is meaningful it can be translated, and all true knowledge fits in to that web of translatable languages somewhere, into possible experiences of a subject. When we understand dolphin languages it will be because we understand what it is like to be a dolphin, and can share with them what it is like to be human.


People born blind as a result of the retina non functioning proper still have the possibility to see all colors. There are blind people who see the direction of motion of an object even if they see no light at all. If the brain regions where the perception of color takes place is intact there is no reason why it can't be activated. The fact that it can't be activated from your retina doen't take away that fact.

It's different when the color giving regioms (in your brain) are not there. Maybe other brain parts can take over but that is highly questionable. You can't even reme mber a color if this part of your brain is taken away. Let alone know what the color looks like. Unless the color is part of the memory. But that does away with the region where the color is activated so even then it's not possible.

Blind people who can't use their retina anymore (but could before( can still dream in colors so being used to colors is a condition to see these qualia. People born blind are not used to these qualia but there is no reason why they should't be able to see them (if the blindness is due to a genetic defect causing a defect of the retina). The same holds for colorblind peoe.

  • Please provide references for claims.
    – CriglCragl
    Jun 28, 2021 at 12:33

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