Thought-experiment August: (1) What Mary—a future neurobiologist—knows

Welcome to the end of summer, thought-experiments bash! For the month of August we will be exploring classic thought-experiments from science and philosophy! So put on your thinking caps. It’s Gedankenexperiment August!

What Mary Knows:

Philosopher Frank Jackson first conceived this thought-experiment in order to explore some of the strange difficulties in constructing a science of consciousness. Assume a flat ontology—the universe is material and only material processes occur. Mary is a future neurobiologist. She knows everything there is to know about the brain’s processing the color red. Everything. From the time the light photons enter the eye and activate nerves in the retina, to the time it is processed in the visual cortex and on until that information is processed, distributed, and used by the rest of the brain.

Mary knows everything about seeing red—except Mary is colorblind. She has never seen red. Jackson then asks what happens when her abilities are restored through some surgical manipulation. Does Mary learn something? Upon seeing red does she say, “Ah yes, just as I expected.” Or does she say, “Wow. I had no idea?” She knew everything science could possibly find out about seeing red, yet there appears to be something she did not know. The implication is that there are at least some things that science cannot reach—in particular the nature of the ‘experience’ of subjectivity.

This thought experiment has generated much spilled ink in the consciousness literature. It also seems to offer something about the nature of science and its ability to get at some questions.

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18 Responses to Thought-experiment August: (1) What Mary—a future neurobiologist—knows

  1. S.Faux says:


    Learning boils down to a change in behavior. If Mary changed her behavior as a consequence of “seeing,” then she “learned.”

    Some philosophers seem to want there to be something magical or mystical about the mind. From examples like Mary’s, much discussion about qualia has resulted. But to me, it is much ado about nuthin’.

    The issue is not whether computers could ever appreciate qualia, or whether Mary could, but whether normal humans do, period.

    Seeing “red” is really just an act of discrimination, and NOT a magical something that everyone privately and passively experiences the same way.

    Besides, Mary’s cortical area V4 probably did not develop properly. Proper brain development requires input.

    Science works best when it sticks with observations, although I will admit philosophy is an awful lot of fun.

    –signed, your friendly LDS Skinnerian and Darwinian — S.Faux

  2. SteveP says:


    “Seeing “red” is really just an act of discrimination, and NOT a magical something that everyone privately and passively experiences the same way.”

    I think I disagree here. From an objective, what can science get at, point of view, perhaps, but computers can make that kind of discrimination, but there is little to suggest that they are experiencing something. That there is ‘something that it is like’ to be a computer.’ When I see red, I experience something red. Seeing red is a meaningful subjective experience weather it effects my behavior or not. If you are studying behavior it might not have any effects that pre-color-mary changes upon sing red and that seeing “red” is really just an act of discrimination, and NOT a magical something that everyone privately and passively experiences the same way. with any differently.” But still from the inside it is a different experience.

    That we don’t ‘experience the same way seems’ an untestable point and drives home the problem this thought experiment gets at. We can never know how it is experienced by anyone but ourselves (but having evolved the same wet stuff it seems the burden of proof that we don’t would fall on those making the claim).

    Anyway, thanks for weighing in. Many behavioral scientists agree with you (as you point out Skinner) but I side on the phenomenologists on this one.

  3. ujlapana says:

    Why are you dubious that there is “something that it is like” to be a computer? If you built a brain from scratch and watched it’s neurons fire, you would have no sense that there’s something “conscious” in there.

    The excitement about qualia eludes me. It’s the representation of data to the brain. So it’s clearly different from rational knowledge about that data–it’s the actual input of the data. It’s like having a computer that keeps a database of heat sensors, even though it doesn’t have one installed itself. When you subsequently install one, the input from the sensor itself is completely different from the database. They’re unrelated domains of experience. But that doesn’t make having a heat sensor “magical” or proof of non-material processes.

  4. SteveP says:

    ujlapana, I wonder what it’s like to be a heat sensor? (There is a philosopher named David Chalmers that says there is something it’s like to be a thermostat.) The point here is that as a consciousness, we experience the change. Certainly, there is no difference between a heat sensor and a neural signal from the third person perspective. But are you saying the recipient of the heat sensor data experiences the input? The claim that there is something that it is like to be a computer is a bold claim. (I hope some of the data I input does not cause something like pain to it.) To argue that machines experience something from a first person perspective is an untestable claim (I can’t even prove scientifically that you experience something), but I believe it is not only untestable but unwarranted.

  5. ujlapana says:

    Is it so outlandish? A materialist would have to admit that a petaflop machine is arguably faster-brained than a mouse (or some lower life form). If a mouse is conscious, which I think it is, than why isn’t Roadrunner? Small portions of the human brain have been precisely modeled and “activated” in software. If we modeled the whole thing and started it up, why would it be less conscious than a newborn?

    A spiritualist–particularly a Mormon one–would have to admit that if the Earth has a spirit (as evidenced by a dialog with Enoch in the PoGP), then lots of “inanimate” entities might be conscious.

    If consciousness isn’t tied to material activity, why wouldn’t any random object be conscious?

    Where along the continuum from ameoba to human do you believe consciousness occurs? I don’t know, and I’m not sure I even know what it means to have my own consciousness. After all, there’s evidence (e.g. Alien Hand syndrome, missing corpus callosum studies) that I may in fact be two consciousnesses in constant communication. Perhaps my stomach is it’s own consciousness–there are billions of independent neurons down there, after all. Perplexing stuff, but it doesn’t seem “literally” magic to me. Quite material, rather.

  6. SteveP says:

    ujlapana, all very interesting questions, and actually, I’m more sympathetic to those kind of questions than your claim that Qualia arn’t interesting. I’m pointing out that whatever is having experiences (and where that line is cut I don’t have any idea), is having something real that science can’t touch. And to me that is the very essence of mystery. Maybe not magic, but certainly something worthy of wonder and speculation.

    There is something about the universe we cannot detect directly, but suspect is there only because we each individually have the experience. There is something that it is like to be us. Descartes is still relevant here.

  7. DB says:

    I would disagree with your statement that “Mary knows everything about seeing red . . .” Mary knows nothing about seeing red. Maybe she knows everything about how the brain processes colors but that’s not the same as knowing about seeing red. When she finally does see red, does she learn anything? Maybe nothing about the neurological process, but she certainly understands red now, whereas before she had no way of understanding red until she experienced it. Knowledge can be gained by learning facts or by understanding an experience. I know nothing about how the brain processes colors but I’m sure Mary could teach me and I could learn it. Mary does not understand red but I cannot teach it to her even though I experience it everyday; she has to experience it herself in order to understand it.

  8. ujlapana says:

    Experiencing red teaches you almost nothing about red. It teaches you how your brain communicates from the visual cortex when stimulated by red light. Would you learn about infrared light by seeing it vs seeing simulated infrared images? Not in any way I can think of. Is your lack of experiencing how a bee sees red curtailing your understanding of the color? Again, I would say no.

  9. DB says:

    Really? You understand how your brain communicates from your visual cortex when stimulated by light just by looking at things? I use my vision everyday and I couldn’t tell you how my brain communicates from my visual cortex. You must be some kind of genius! Did you understand that the very first time you opened your eyes as an infant or was it some time in pre-school?

    Would I learn about infrared light by seeing it vs seeing simulated infrared images? I don’t know; I’ve never seen infrared light. I’d have to experience it to know for sure. I guess you have seen infrared light since you know that I wouldn’t learn anything. What’s it like?

    What would you say if we carried SteveP’s though experiment out further and included a neurobiologist who is completely blind. Let’s suppose that person has studied and learned everything there is to know about how the brain processes light. Does she understand light? I suppose you might say that she does. Let’s suppose that person has a surgery that completely restores her vision. When she sees the world for the first time after the surgery does she say, “Ah yes, just as I expected.” or does she say, “Wow, I had no idea?” I suppose you might say that she has learned nothing and I would disagree.

    Perhaps you and I have completely different learning styles. You learn strictly through academic study and learn nothing through experience, whereas I learn equally through both if not more through experience.

  10. ujlapana says:

    You understand how your brain communicates from your visual cortex when stimulated by light just by looking at things?

    I understand only that–certainly nothing about wavelengths or the particle-like behavior of light. My only understanding is within my brain.

    When she sees the world for the first time after the surgery does she say, “Ah yes, just as I expected.” or does she say, “Wow, I had no idea?”

    If she only studied light, the latter. If she studied how her brain would interact with light, and understood that relative to analogous personal experiences, quite possibly the former. Light and her qualia are completely different things. Do you not understand light because you haven’t been shot by a laser yet? That’s an experience you’re probably missing.

    This is best illustrated by a simple at-home experiment. Put each hand in a bowl of water–one steaming hot and one ice cold. Then put them both into a bowl of room-temperature water. What have you learned about the absolute temperature of the room-temperature water? Is it hot or cold? You’ve learned nothing about the temperature of the water, but you’ve learned a lot about how your brain and skin communicate temperature changes to each other.

    You learn strictly through academic study and learn nothing through experience…

    Academic study IS experience. If you aren’t introspecting, you’re experiencing (and the case could be made that even introspection is experience).

    Many comments

  11. DB says:

    You’re right, I’ve never been shot by a laser. Unless you count those laser pointers that teachers like to use in classrooms. But if I ever am shot by one, you bet I’ll learn something and I’ll understand lasers and light in a way that I didn’t before I was shot!

  12. ujlapana says:


    Many comments, and even the OP, seem to suggest that qualia (i.e. the firing of neurons) is part of the nature of the stimulus. It is not. The light stops when it hits my retina. It’s absorbed into the cells (partially reflected) and gone. My conscious experience of it is entirely indirect.

    Let’s take our surgery patient again, and give her a flashlight. She sits in the darkened recovery room and turns it on–my, so bright! It’s almost painful. Later that afternoon, while sitting on a park bench enjoying the sun, she turns it on again. Strange, it’s barely visible. Has she now learned something about the light from this flashlight? Quite the opposite, her qualia are corrupting her understanding of the flashlight’s intensity, which hasn’t changed at all.

    Good thing she studied light so much, so she’ll now understand that her experience is in her brain, and not intrinsic to light.

  13. ujlapana says:

    In response to #11, this leads me to another thought. (Sorry to be repeat commenting–just saw it after posting #12.)

    Suppose you live in a place utterly devoid of light, but not heat. You have experienced pain from burning–say touching a hot stove. You have studied light, and know all about it’s nature, but have never experienced it. You have deduced (from your vast knowledge) that concentrated light will elevate the temperature of a surface.

    One day, you get shot by a high-powered laser. (You don’t see it–those slimeballs got you in the back.) This is your first experience with light.

    And you would say, “Ah yes, just as I expected.”

  14. DB says:

    Just one last question. Do you honestly believe that you will learn nothing from an experience that you can’t learn from studying the experience?

  15. ujlapana says:

    It depends on the experience. Would I learn more about China by visiting than reading? Maybe. More about human qualia? Yes. More about light? No.

  16. Kari says:


    In response to your question in #14, let me ask you a question. What do you learn about the color red by actually seeing it, that you can’t learn by studying it?

    Ujlapana’s point, if I understand him correctly, with which I agree, is that there is nothing about the color red, i.e. the wavelength of light that we see as red, that cannot be learned purely through study. The act of seeing red gives us an experience, a qualia if you will, but doesn’t impart actual knowledge about something.

    Have you ever seen a picture taken with a camera that records infrared? Does seeing such an image actually impart any additional information about the infrared spectrum that you wouldn’t have had before? And just because we’ve seen such an image, can we now truly understand what it would be like to see the infrared spectrum with our eyes, like snake or a bee?

  17. Kari says:

    I also have a few thoughts about this particular thought experiment.

    If Mary were to suddenly see red after a lifetime of being red colorblind, what would we learn? Can we assume that her experience with seeing red would be the same as for those of us who have seen red all our lives? Is it possible that she would have the experience of hating it? That she would see red constantly? Red splotches everywhere that we’ve learned to ignore and filter out she would find distracting to the point of insanity?

    A similar situation, and not just a thought experiment, is what happens to people who experience hearing, through a cochlear implant, after a lifetime of deafness? When they turn on that implant for the first time do they say she say, “Ah yes, just as I expected.” Or do they say, “Wow. I had no idea?”

    Some people with cochlear implants can’t handle it, the constant noise is distracting and even painful and they turn the implants off forever. Some on the other hand, learn to adapt and live in a hearing world.

    But in the end, what does this teach us about the objective action of soundwaves striking the eardrum and being converted by the cochlea into electrical impulses in the cochlear nerve and how those impulses are then received and processed by the central nervous system? Nothing. It only teaches us about the qualia of hearing. And to my mind, really does mean that we can’t know how something is experienced by others, only by ourselves (the experience of the deaf being fulfilling the burden of proof).

  18. DB says:

    Let’s remember that SteveP’s question was “Does Mary learn something?” and not “Does Mary learn something about light or wavelengths or photons?” It seem like everyone commenting here has tunnel vision or something.
    Any new experience will always teach us something. You’re right Kari, maybe Mary will learn that she hates red and finds it distracting. Maybe she will learn that red is beautiful. Maybe she will learn that her favorite outfit that she thought matched, really doesn’t. Maybe she will finally understand why red is a symbol for danger. Maybe when she goes to an art gallery she will finally understand that one painting that she never understood before. How much of this could she have learned from a textbook? None.
    I see red everyday but that doesn’t teach me anything about light waves. Mary studies light waves everyday but that doesn’t teach her anything about red.

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