The knowledge argument (also known as Mary's room or Mary the super-scientist) is a philosophical thought experiment proposed by Frank Jackson in his article "Epiphenomenal Qualia" (1982) and extended in "What Mary Didn't Know" (1986). The experiment is intended to argue against physicalism — the view that the universe, including all that is mental, is entirely physical. The debate that emerged following its publication became the subject of an edited volume — There's Something About Mary (2004) — which includes replies from such philosophers as Daniel Dennett, David Lewis, and Paul Churchland.
Mary's Room is a thought experiment that attempts to establish that there are non-physical properties and attainable knowledge that can be discovered only through conscious experience. It attempts to refute the theory that all knowledge is physical knowledge. C. D. Broad, Herbert Feigl, and Thomas Nagel, over a fifty-year span, presented insight to the subject, which led to Jackson's proposed thought experiment. Broad makes the following remarks, describing a thought experiment where an archangel has unlimited mathematical competences:
He would know exactly what the microscopic structure of ammonia must be; but he would be totally unable to predict that a substance with this structure must smell as ammonia does when it gets into the human nose. The utmost that he could predict on this subject would be that certain changes would take place in the mucous membrane, the olfactory nerves and so on. But he could not possibly know that these changes would be accompanied by the appearance of a smell in general or of the peculiar smell of ammonia in particular, unless someone told him so or he had smelled it for himself.
Roughly thirty years later, Feigl expresses a similar notion. He concerns himself with a Martian, studying human behavior, but lacking human sentiments. Feigl says:
...the Martian would be lacking completely in the sort of imagery and empathy which depends on familiarity (direct acquaintance) with the kinds of qualia to be imaged or empathized.
Nagel takes a slightly different approach. In an effort to make his argument more adaptable and relatable, he takes the stand of humans attempting to understand the sonar capabilities of bats. Even with the entire physical database at one's fingertips, humans would not be able to fully perceive or understand a bat's sonar system, namely what it is like to perceive something with a bat's sonar.
The thought experiment was originally proposed by Frank Jackson as follows:
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? 
In other words, Jackson's Mary is a scientist who knows everything there is to know about the science of color, but has never experienced color. The question that Jackson raises is: once she experiences color, does she learn anything new?
Ontologically, the following argument is contained in the thought experiment:
- Premise: Any and every piece of physical knowledge in regard to human color vision has been obtained (by the test subject, Mary) prior to her release from the black-and-white room. She has all the physical knowledge on the subject.
- Premise: Upon leaving the room and witnessing color first-hand, she obtains new knowledge.
- Conclusion: There was some knowledge about human color vision she did not have prior to her release. Therefore, not all knowledge is physical knowledge.
Most authors who discuss the knowledge argument cite the case of Mary, but Frank Jackson used a further example in his seminal article: the case of a person, Fred, who sees a color unknown to normal human perceivers. We might want to know what color Fred experiences when looking at things that appear to him in that particular way. It seems clear that no amount of knowledge about what happens in his brain and about how color information is processed in his visual system will help us to find an answer to that question. In both cases cited by Jackson, an epistemic subject A appears to have no access to particular items of knowledge about a subject B: A cannot know that B has an experience of a particular quality Q on certain occasions. This particular item of knowledge about B is inaccessible to A because A never had experiences of Q herself.
Whether Mary learns something new upon experiencing color has two major implications: the existence of qualia and the knowledge argument against physicalism.
First, if Mary does learn something new, it shows that qualia (the subjective, qualitative properties of experiences, conceived as wholly independent of behavior and disposition) exist. If Mary gains something after she leaves the room — if she acquires knowledge of a particular thing that she did not possess before — then that knowledge, Jackson argues, is knowledge of the qualia of seeing red. Therefore, it must be conceded that qualia are real properties, since there is a difference between a person who has access to a particular quale and one who does not.
Refutation of physicalism
The knowledge argument is that if Mary does learn something new upon experiencing color, then physicalism is false. Specifically, the knowledge argument is an attack on the physicalist claim about the completeness of physical explanations of mental states. Mary may know everything about the science of color perception, but can she know what the experience of red is like if she has never seen red? Jackson contends that, yes, she has learned something new, via experience, and hence, physicalism is false. Jackson states:
It seems just obvious that she will learn something about the world and our visual experience of it. But then it is inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo there is more to have than that, and Physicalism is false.
Objections have been raised that have required the argument to be refined. Doubters cite various holes in the thought experiment that have arisen through critical examination.
Nemirow and Lewis present the "ability hypothesis", and Conee argues for the "acquaintance hypothesis". Both approaches attempt to demonstrate that Mary gains no new knowledge, but instead gains something else. If she in fact gains no new propositional knowledge, they contend, then what she does gain may be accounted for within the physicalist framework. These are the two most notable objections to Jackson's thought experiment, and the claim it sets out to make.
Objection: Nemirow claims that "knowing what an experience is like is the same as knowing how to imagine having the experience". He argues that Mary only obtained the ability to do something, not the knowledge of something new. Lewis put forth a similar argument, claiming that Mary gained an ability to "remember, imagine and recognize." There have been arguments against the ability hypothesis as well, namely that being able to imagine having a particular experience is neither necessary nor sufficient for having the knowledge of exactly what it is like to have that kind of experience.
Effect: Earl Conee (1994) and Alter (1998) cite the example of someone without the capacity to imagine having color experiences in an effort to show that imaginative abilities are not necessary for knowing what it is like to have a certain kind of experience. Their claim is that despite having a defect of this nature, she would still know what it is like to have an experience of e.g. red while staring at something that looks red to her.
In order to show precisely that imaginative abilities are not sufficient for knowing what it is like, Conee introduces the following example: Martha, "who is highly skilled at visualizing an intermediate shade that she has not experienced between pairs of shades that she has experienced...happens not to have any familiarity with the shade known as cherry red". Martha has been told that cherry red is exactly midway between burgundy red and fire red (she has experienced these two shades of red, but not cherry). With this, Martha has the ability to imagine cherry red if she so chooses, but as long as she does not exercise this ability, to imagine cherry red, she does not know what it is like to see cherry red.
Objection: Earl Conee (1994) presents another variant. Conee's acquaintance hypothesis identifies a third category of knowledge: knowledge that is not reducible to factual knowledge nor to knowing-how. He argues that the knowledge Mary actually acquires post-release is only acquaintance knowledge. Knowing something by acquaintance “requires the person to be familiar with the known entity in the most direct way that it is possible for a person to be aware of that thing”. Since “experiencing a quality is the most direct way to apprehend a quality”, Mary gains acquaintance with color qualia after release.
The physicalist can defend himself against the knowledge argument like this:
- Qualia are physical properties of experiences (and experiences are physical processes). Let Q be such a property.
- Mary can know all about Q and she can know that a given experience has Q before release, although — before release — she is not acquainted with Q.
- After release Mary gets acquainted with Q, but she does not acquire any new item of propositional knowledge by getting acquainted with Q (in particular she already knew under what conditions normal perceivers have experiences with the property Q).
Ramachandran and Hubbard
- Mary says she sees nothing but gray.
- She has the "Wow!" response from subjectively experiencing the color for the first time.
- She experiences a form of blindsight for color, in which she reports seeing no difference between a red apple and an apple painted gray, but when asked to point to the red apple, she correctly does.
They explain further: "Which of these three possible outcomes will actually occur? We believe we've learned the answer from a colorblind synesthete subject. Much like the theoretical Mary, our colorblind synesthete volunteer can not see certain hues, because of deficient color receptors. However, when he looks at numbers, his synesthesia enables him to experience colors in his mind that he has never seen in the real world. He calls these "Martian colors." The fact that color cells (and corresponding colors) can activate in his brain helps us answer the philosophical question: we suggest that the same thing will happen to Mary."
Effect: Ramachandran and Hubbard's contribution is in terms of exploring "the neural basis of qualia" by "using pre-existing, stable differences in the conscious experiences of people who experience synaesthesia compared with those who do not" but, they note that "this still doesn’t explain why these particular events are qualia laden and others are not (Chalmers’ ‘hard problem’) but at least it narrows the scope of the problem" (p. 25).
Objection: Evan Thompson  questioned the premise that Mary, simply by being confined to a monochromatic environment, would not have any color experiences. Furthermore, would it not be a possible case that Mary, upon release, still would not be able to see colors? Jackson would have to refine his thought experiment to account for these doubts.
Effect: One possible adjustment to the experiment would be to have Mary monochromatic from birth. She has a condition, and can only see in black-and-white from birth. At some point, a medical discovery is made and Mary's monochromatic condition is alleviated. Still, are we sure that monochromatics cannot have mental color experiences? The objection raises strong doubt to the thoroughness of Jackson's experiment.
Objection: Daniel Dennett argues that Mary would not, in fact, learn something new if she stepped out of her black and white room to see the color red. Dennett asserts that if she already truly knew "everything about color", that knowledge would necessarily include a deep understanding of why and how human neurology causes us to sense the "qualia" of color. Moreover, that knowledge would include the ability to functionally differentiate between red and other colors. Mary would therefore already know exactly what to expect of seeing red, before ever leaving the room. Dennett argues that functional knowledge is identical to the experience, with no ineffable 'qualia' left over. As a consequence, Dennett concludes that this is not a sound argument for the existence of qualia.
Support: Frank Jackson initially supported the anti-physicalist implications of the Mary's room thought experiment. Jackson believed in the explanatory completeness of physiology, that all behaviour is caused by physical forces of some kind. And the thought experiment seems to prove the existence of qualia, a non-physical part of the mind. Jackson argued that if both of these theses are true, then epiphenomenalism is true — the view that mental states are caused by physical states, but have no causal effects on the physical world.
Thus, at the conception of the thought experiment, Jackson was an epiphenomenalist.
Objection: However, he rejected epiphenomenalism later. This, he argues, is because when Mary first sees red, she says "wow", so it must be Mary's qualia that causes her to say "wow". This contradicts epiphenomenalism because it involves a conscious state causing an overt speech behavior. Since the Mary's room thought experiment seems to create this contradiction, there must be something wrong with it. This is often referred to as the "'there must be a reply' reply".
Jackson now believes that the physicalist approach (from a perspective of indirect realism) provides the better explanation. In contrast to epiphenominalism, Jackson says that the experience of red is entirely contained in the brain, and the experience immediately causes further changes in the brain (e.g. creating memories). This is more consilient with neuroscience's understanding of color vision. Jackson suggests that Mary is simply discovering a new way for her brain to represent qualities that exist in the world. He likens her to patients suffering from akinetopsia, the inability to perceive the motion of objects. If someone were cured of akinetopsia, they would not be surprised to discover any new facts about the world (they do, in fact, know that objects move). Instead, their surprise would come from their brain now allowing them to see this motion.
- Ludlow, Peter; Nagasawa, Yujin; Stoljar, Daniel, eds. (2004). There's Something about Mary: essays on phenomenal consciousness and Frank Jackson's knowledge argument. Cambridge: MIT Press. ISBN 0-262-12272-3.