What is the Mary's Room thought experiment?

Mary's Room, also called the knowledge argument, is a thought experiment proposed by the philosopher Frank Jackson in his 1982 article "Epiphenomenal Qualia" and extended in "What Mary Didn't Know" in 1986. It imagines a scientist who knows every physical fact about color vision but has never seen color, and asks whether she learns anything when she finally does. Jackson used it to argue against physicalism, the view that everything is physical. [1] [2]

In Jackson's telling, Mary is a brilliant scientist who must investigate the world from a black and white room, using a black and white television monitor. She specializes in the neurophysiology of vision and acquires all the physical information there is about what happens when people see ripe tomatoes or the sky. This includes which wavelength combinations stimulate the retina and how the nervous system produces the sentence "The sky is blue." Popular retellings, such as the one in Philosophy Now, add that she then leaves the room and sees a red apple for the first time. [2] [3]

The central question is whether Mary gains new knowledge on leaving the room. Jackson says it seems obvious that she will learn something about the world and about our visual experience of it. [2] [1]

The reasoning runs as follows. Before her release Mary has all the physical information about human color vision. Yet she learns something when she sees color, what it is like to see red, so her earlier knowledge was incomplete. If she had all the physical information and still lacked something, then not all information is physical, and physicalism is false. A course summary from Jim Pryor's teaching notes puts the conclusion in terms of facts: there is a fact about other people's experiences that the physical information did not capture, and so it is a non-physical fact. [2] [4]

Jackson also used a second case, Fred, who sees a color unknown to normal perceivers. No amount of knowledge about Fred's brain seems to tell us what color he experiences. [2]

The Stanford Encyclopedia of Philosophy distinguishes two versions. The weaker version concludes only that there is some kind of knowledge about color vision that is non-physical, which is an epistemological claim. The stronger version concludes that there are non-physical facts, an ontological claim that a physicalist must reject. The entry says Jackson clearly had the stronger version in mind, and that the weaker conclusion does not by itself imply the stronger one, since incomplete knowledge does not automatically point to a specific fact that is unknown. [2]

Responses to the argument divide accordingly. Many authors accept the weaker version but reject the stronger, holding that Mary gains new propositional knowledge but not knowledge of a new fact. This is called the New Knowledge/Old Fact view. Others deny even the weaker version and claim Mary gains no new propositional knowledge at all, a position called the No Propositional Knowledge View. The results provided do not say which view most specialists hold, so that is left open here. [2]