Mary the Color Scientist

Does this famous dualist thought experiment imply a contradiction?

Mary the Color Scientist is a famous thought experiment that was originally described like this:

Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘red’, ‘blue’, and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal cords and expulsion of air from the lungs that results in the uttering of the sentence ‘The sky is blue’. [...] What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not? (1)


The idea is that even though Mary knows everything physical about color and vision, when she experiences colors for the first time she’ll learn something new. It was argued that we should, therefore, conclude that the experience of color must be non-physical, but does that make sense? It really depends on what definition of “physical” we’re using. Often the definition is something like “relating to the body instead of to the mind” or even “non- mental”. Obviously, those definitions aren’t useful because they presuppose the conclusion. So, what does it mean for something to be physical? I’ll propose that we use a definition like “physical things are the set of all things that can interact with each other as described by physical laws.” Anything that’s physical can affect any other physical thing through gravity or chemistry or electromagnetic waves, etc. When we use this definition, which doesn’t presuppose that anything mental is non-physical, Mary’s seeing color implies a contradiction. We know that Mary has learned something because her behavior has changed - she says different things, or writes about her experience, or acts differently. Those are all physical behaviors: They’re changes in the way Mary’s muscles act, and her muscles are controlled by her nerves.. If her physical behavior changes it must be caused by a physical change in her nervous system, which means that experiencing color must have had a physical effect on her nerves. The experience made some sort of physical change in Mary, e.g. a chemical change or an electrical change. Therefore, the experience of color must be physical, unless we want to accept that something non-physical can cause physical changes.

We now have an apparent contradiction: The conscious experience of color was a physical change in Mary, and she now knows something new, yet she already knew everything physical about color and vision. What new thing could she have learned?

The problem is that the original thought experiment assumes that the human brain is a monolithic storage/processing unit for a single kind of information. It assumes that any information available to any part of the brain, in any format, can be transferred or used by any other part of the brain. That assumption is clearly not true, and some of the responses to the original thought experiment are based on it. They suggest that there are different kinds of knowledge, like an Ability (2) or an Acquaintance(3), and there are many examples of this kind of distinction. If I know every physical fact about riding a bike, that doesn’t mean I can ride a bike. Those are two different kinds of information, which are stored in different places in my brain and processed in different ways. The same could be true of the qualia of conscious experiences and the knowledge that we acquire through language and studying. Qualia and language might be as different as analog and digital versions of a record. If we have a digital music player and a vinyl record, we can’t process the information on the record. Even if we can physically read the record, if the player doesn’t have an analog to digital converter, it’s useless. We could even assume that the information on how to convert an analog to digital signal is stored on the record, say a description of an Analog to Digital converter, and it still wouldn’t be useful.

There are examples of a similar thing actually happening with patients who were blind and were able to see later in life (often by having cataracts fixed). They experienced the world through their other physical senses, but then when they could see they had problems reconciling their previous experience of the world with their new experience of it through sight (4). Previously blind patients could have the ability to distinguish different objects by touch, but when they saw objects, they couldn’t pick out a round from a flat object by sight. The physical information stored in their brains from touch isn’t accessible by the part of their brain that can see the objects. Even if the patient knew every physical fact about geometry, when she could see objects, she would gain new physical information. Or the same information, only in a different format than what she had before.

Qualia seems like it must be physical since it can change our behavior, but it’s in a “format” that we can’t transfer or can’t be processed by other parts of our brain, and so we can’t communicate it to other people. It’s not that the information isn’t there, it’s just that the pathways to transfer it don’t exist. And yet we accept that Mary could know all physical information. Why is this? One possibility is that Mary knows all physical facts about vision that can be stored as language, and those are the same facts that could be stored by qualia. In many ways qualia doesn’t add any useful information about the world. If I know that an apple reflects light in a wavelength of 650 nanometers, and I experience a red apple, that’s the same information, just in two different formats. Maybe qualia is one way we can processes information from our senses? It probably has a lot of advantages like speed and efficiency and the fact that we can form memories of it. But it’s not the only way, and so, even without access to it, Mary can store information about color and vision in a different format in a different part of her brain. We all have duplicates of the same information about color stored in different ways in our brains, but Mary only has one set of facts. Mary can have all the information about what the world looks like, and if she’s able to process it in a non-qualia way, she won’t gain any new information about the world when she can see it in color; she’ll just be able to process the same information in a different way. But when she first sees color she’ll gain the same information in a format that she didn’t have access to previously, and it will be something that’s a physical change in her brain.

When we think about this thought experiment, it’s not just about color or vision. The assumption is that it’s about all of consciousness, and that the same lesson can be applied to all conscious experiences. We can imagine different scientists that are experts in different types of physical facts -sound, heat, smell, etc. even though they can’t directly experience those things. And in every case they could learn all the facts about their subject and then experience that same information in a new format when they finally gained the ability to experience the relevant senses. The thought experiment plays out the same way for every kind of qualia, whether it’s sound or color or just about anything else. Except for pain and pleasure--these feelings aren’t just the same information about the world processed in a different way; in other words, they’re not duplicates of physical facts; they’re entirely new information about the world. They add new information to our experience, roughly that some things are good and others are bad.

We can see this difference by imagining what would happen if Mary had red and blue switched. Let’s say she was testing her new color vision and picked up two color chips that happened to be mislabeled. If the red chip was labeled “blue” there’d be no way for her to tell. She’s taking some physical facts she knows through learning -- the name and wavelength of a color -- and trying to pair them up with their duplicate fact that she’s experiencing for the first time. Her brain (or anyone else’s) isn’t wired to be able to transfer or compare these two kinds of information, so she has no way to check to make sure the label is correct. Compare that to the experience of someone who couldn’t feel pain or pleasure, and then had those feeling restored. If my first experience was having my finger pricked, I’d know immediately if that was pain or pleasure. There’s no question because there’s no duplicate facts to get confused by. Pain and pleasure only exist in the brain, and while they exist in a format that can’t be directly transferred to someone else, they have a physical effect on our behavior that’s observable and comparable.

We can see then the range of possibilities of how qualia overlap with other kinds of information. Most senses create duplicates of information that exists in the world, while pain and pleasure aren’t duplicates of anything. They’re entirely new information that have obvious survival and evolutionary benefits. Without these two specific kinds of qualia the physical history of the human species would be very different. There are genes in our DNA that control which nerves create which experiences, those experiences change our behavior, and that behavior plays a role in determining which genes survive in the population and which die out. In the case of pain and pleasure this chain of causation is clear because there’s no duplicate information to get confused by, but the same could be true for other senses. And, in fact, I’d argue that the color scientist thought experiment should lead us to conclude that consciousness is a physical property.


  1. "Epiphenomenal Qualia", Philosophical Quarterly, 32, 127, pp. 127–136
  2. Lycan, William G., ed. (1990-01-01). Mind and cognition: a reader
  3. Conee, Earl (1994-06-01). "Phenomenal knowledge". Australasian Journal of Philosophy 72
  4. Held, R.; Ostrovsky, Y.; De Gelder, B.; Gandhi, T.; Ganesh, S.; Mathur, U.; Sinha, P. (2011). "The newly sighted fail to match seen with felt". Nature Neuroscience 14 (5): 551–553.