In philosophy of mind, physicalism is the view that mental states, like beliefs, feelings and desires, are nothing over and above the physical states of the brain: we don’t have souls or any non-physical features, and so all facts about our minds are, at bottom, physical facts. Physicalism is the theory of mind that many argue is most consistent with a scientific understanding of the universe.
But imagine this:
Mary is a brilliant super-scientist who has never left her black-and-white room, in which she has never seen colors. She has complete physical knowledge, that is, scientific knowledge expressible in physical and functional terms, about human color vision from books and black-and-white television. So, she knows every physical fact about human color vision.
One day, however, she leaves the room and sees a red tomato (she knows that tomatoes are red). She exclaims “This is what it is like to see red! This is what people experience when they see red!”
Now, we have an argument against physicalism, known as the knowledge argument: Mary apparently learns a new fact about human color experience. But she already knew all the physical facts before she left her room. So, what she learns must be a non-physical fact. Since there are non-physical facts, physicalism is false.
Let’s consider four of the most prominent responses to this argument.
1. The Ability Hypothesis
According to the ability hypothesis, Mary does learn something when she sees red, however, what she learns is not a new fact, but it is know-how; she just acquires new cognitive abilities. Knowing what it is like to have a certain experience, on this view, consists in the possession of the abilities to recognize, imagine, and remember the relevant experience. If what Mary learns is just know-how, which doesn’t involve a new truth, then her pre-release knowledge can still be complete.
By analogy, consider that Mary has complete scientific knowledge about swimming. But if she leaves the room and learns how to swim, that doesn’t mean that her scientific knowledge about swimming was incomplete: by learning how to swim she didn’t learn a new fact, she just acquired an ability.
A problem with the ability hypothesis, however, is that Mary does seem to learn a new truth when she sees red, beyond mere know-how. When she says “This is what people experience when they see red!” she states a true belief, which she didn’t have before seeing a red tomato.
2. The Acquaintance Hypothesis
A similar response is that Mary doesn’t learn a new truth but she gains nonfactual acquaintance knowledge.
Suppose I ask you whether you know Oprah, and, as an answer, you give me some facts about her. But, then, I say “No, what I am asking is do you know her?” I am asking whether you know her personally. According to the acquaintance hypothesis, Mary comes to know what it is like to see red in this sense of “know.” To know what it is like to experience a mental state is just to experience that mental state. Then, of course, Mary cannot know what it is like to see red before she leaves the room, but this doesn’t mean that there were some facts that she didn’t know. It is possible to know all the facts about Oprah without knowing her personally, and it is possible to know all the facts about the red seeing experience without actually having that experience.
The acquaintance hypothesis, however, like the ability hypothesis, denies that Mary learns a new truth, so the above objection to the ability hypothesis can be raised against it also.
3. The Phenomenal Concept Strategy
According to this view, Mary’s new knowledge is about the physical facts that she already knows about. First-personal knowledge of experiences involves phenomenal concepts, concepts that directly denote subjective experiences, and phenomenal concepts can only be acquired by subjective experience.
When Mary sees red, she says, “This is what it is like to see red.” Let’s say physicalism is true and the experiential redness Mary is talking about is actually a physical property of the brain. Let’s call this physical property “R.” So, what Mary says is equivalent to this: “R is what it is like to see red.” “This” in her remark expresses a phenomenal concept. So, what Mary learns upon seeing red is not a new truth, but a new way of apprehending a physical truth that she already knew. By analogy, this is just like “Superman can fly” expressing the same fact as “Clark Kent can fly.”
The main difficulty with the phenomenal concept strategy is to explain the nature of phenomenal concepts in completely physical terms, given that phenomenal concepts seem to involve subjective experiences from the first-person perspective.
4. The No Learning Objection
Some physicalists argue that the intuition that Mary learns something new upon seeing red is prima facie powerful only because we lack adequate understanding of what complete physical knowledge would be. According to them, even if we cannot see how, Mary would be able to deduce whatever fact is expressed by “this is what it is like to see red” from physical knowledge. By analogy, it seems that before Einstein we would think that complete information about matter would not give us complete knowledge of energy, but that’s because we did not yet have Einstein’s theory explaining how matter and energy are not distinct entities.
However, many physicalists disagree. They maintain that Mary learns something when she sees red for the first time and that, even though this is not a new truth, it is something new (either know-how or acquaintance knowledge or a phenomenal concept) that is not deducible from her physical knowledge.
The knowledge argument is one of the most influential arguments against physicalism. Explaining the nature of our subjective knowledge of our experiences in physical terms seems to be very difficult, and, simply put, the knowledge argument says this: if our experiences were physical, then explaining our knowledge of them wouldn’t be this difficult.
 One version of physicalism holds that mental states are brain states or neural states: mental states are brains or the neurons of brains in different configurations.
Another version of physicalism holds that mental states are functional states of the brain. One way of understanding a functional state is this: If A is a functional state of B, then A is something B does and nothing more. For example, a particular computation is a functional state (or, process, to be precise) of the computer hardware.
 The main alternative to physicalism is dualism, of which there are different types. According to substance dualism, in addition to the physical body you have, there is also a separate non-physical thing, a soul. According to property dualism, which is the most common type of dualism today, you don’t have a soul but your brain has some non-physical properties. These non-physical properties would be subjectively-accessible properties of experiences, such as the feeling of pain, that are over and above the neural and functional properties of the brain.
 The qualitative features of experiences that are detectable only through introspection are called “qualia” (singular: quale). So, for example, red quale is the “raw feel” of seeing red; it is what distinguishes the experience of seeing red from all other experiences (such as seeing green) from the first person perspective. This is what Mary means by “this.”
 The “Knowledge argument” and the case of Mary is developed by Frank Jackson (1982, 1986).
 The ability hypothesis is developed by Lewis (2004) and Nemirow (1980, 1990, 2007).
 Developed by Earl Conee (1994).
 Note that acquaintance knowledge is different from know-how. I may have the ability to recognize Oprah (I have seen her pictures), but I have never met her, so I don’t have the acquaintance knowledge of her, in the relevant sense.
 See, for example, Horgan (1984), Loar (2004), Levin (2007), Papineau (2002, 2007).
 This view might seem similar to the acquaintance hypothesis, but, the key difference is, according to the phenomenal concept strategy, as opposed to the acquaintance hypothesis, Mary’s new knowledge is propositional.
 David Chalmers, for example, argues that any account of phenomenal concepts that is expressible in completely physical terms cannot explain our epistemic situation concerning our subjective experiences (since, subjectivity is an essential part of phenomenal concepts and we cannot capture subjectivity in objective/scientific terms), and any account that cannot be expressed in purely physical terms is not an adequately physicalistic account. This is a simplified version of Chalmers’ “Master Argument” against the phenomenal concept strategy. For his original argument, see Chalmers (2007). For a defense of the phenomenal concept strategy against his argument, see Balog (2007).
 Such as Churchland (1985) and Dennett (2004, 2007).
 Frank Jackson changed his mind about his own argument in the late 1990’s. He is a physicalist now.
Dennett, Daniel C. 2004. “Epiphenomenal Qualia?” In Peter Ludlow, Daniel Stoljar & Yujin Nagasawa (eds.), There’s Something About Mary: Essays on Phenomenal Consciousness and Frank Jackson’s Knowledge Argument. MIT Press. 59-68.
Lewis, David. 2004. “What Experience Teaches.” In Peter Ludlow, Daniel Stoljar & Yujin Nagasawa (eds.), There’s Something About Mary: Essays on Phenomenal Consciousness and Frank Jackson’s Knowledge Argument. MIT Press. 77-104.
Loar, Brian. 2004. “Phenomenal States (Revised Version).” In Peter Ludlow, Daniel Stoljar & Yujin Nagasawa (eds.), There’s Something About Mary: Essays on Phenomenal Consciousness and Frank Jackson’s Knowledge Argument. MIT Press. 219-240.
Van Gulick, Robert. 2004. “So Many Ways of Saying No to Mary”. In Peter Ludlow, Daniel Stoljar and Yujin Nagasawa (eds.), There’s Something About Mary: Essays on Phenomenal Consciousness and Frank Jackson’s Knowledge Argument. MIT Press. 365-406.
For Further Reading
The Gettier Problem & the Definition of Knowledge by Andrew Chapman
Philosophy and Its Contrast with Science by Thomas Metcalf
About the Author
Tufan Kıymaz is an Assistant Professor at Bilkent University, Ankara, Turkey. He received his PhD in Philosophy, with a minor in History and Philosophy of Science, from Indiana University, Bloomington. His main area of research is philosophy of mind. He is also interested in philosophy of religion and philosophy of well-being. http://www.phil.bilkent.edu.tr/index.php/tufan-kiymaz/