What Is Mary's Room?

Direct Answer

Mary's room is a thought experiment invented by the philosopher Frank Jackson in his 1982 paper "Epiphenomenal Qualia." Mary is a brilliant scientist who, since birth, has been confined to a black-and-white room and has never seen color. She has access to a complete scientific library and has come to know every physical fact there is to know about color vision: the wavelengths of light, the physiology of the retina, the neural pathways, the mechanisms of color discrimination, the physics and chemistry of the world — everything. She knows all the physical facts about color and color perception.

The question is: what happens when Mary first leaves the room and sees a red rose for the first time? Jackson's answer: Mary learns something she did not know before. She learns what it is like to see red — the qualitative character of the experience, its phenomenal feel, its quale. Before leaving the room, she knew all the physical facts, but she did not know what red looks like. Therefore, there are facts about experience that are not physical facts. If physicalism is the thesis that all facts are physical facts, then physicalism is false.

The argument has a simple logical structure. Premise 1: Mary knows all the physical facts about color vision. Premise 2: Mary does not know all the facts — specifically, she does not know what it is like to see red (she learns this on release). Conclusion: there are facts about experience that are not physical facts. The argument is an attack on physicalism from the inside: it grants the physicalist everything about physical knowledge and shows that something is still left out — the phenomenal character of experience.

The argument targets not only physicalism but also functionalism: Mary could know every functional fact about color perception — every causal role, every behavioral disposition, every computational state — and still not know what red looks like. The knowledge argument thus supports the claim that qualia are real and irreducible, and it is one of the central pillars of the case for property dualism and against reductive accounts of consciousness.

Historical Context

The knowledge argument is the modern descendant of a long tradition of arguments that experience involves more than objective knowledge. In "What Is It Like to Be a Bat?" (1974), Thomas Nagel had argued that subjective experience escapes the objective, third-person standpoint. Jackson's thought experiment gave the idea its sharpest form: instead of comparing species, it imagines a single person who knows all the objective facts and shows that she still lacks knowledge of experience.

The argument appeared at a time when physicalism was the dominant orthodoxy in analytic philosophy of mind, and it became, with Nagel's bat and Kripke's modal argument, one of the three classic anti-physicalist arguments of the 1970s–1980s. Jackson's own position at the time was epiphenomenalism: he argued that qualia are real but causally inert — they exist but do not cause behavior, which is why a purely physical science of behavior could not reveal them.

The argument generated an enormous literature of responses, and in a 1998 paper ("Postscript" to "Mind and Illusion") Jackson himself renounced the argument, converting to physicalism, while acknowledging that the problem it posed remained deeply puzzling. The argument's power is shown by the fact that even its critics concede it captures something important about experience.

Key Arguments & Debates

The responses to the knowledge argument fall into several families.

The ability hypothesis (Lawrence Nemirow, David Lewis): Mary does not learn a new fact; she gains new abilities — the ability to imagine, recognize, and remember color experiences. Knowledge is not always propositional knowledge; knowing what red looks like is a know-how, not a know-that. The objection: the argument's force seems to survive the distinction — Mary seems to gain new propositional knowledge, not merely abilities.

The acquaintance hypothesis (Earl Conee): knowing what red looks like is a third kind of knowledge — acquaintance with a phenomenal property — that is neither propositional nor practical. This concedes the argument's conclusion (there are nonphysical facts) or at least a non-physical mode of knowledge.

The phenomenal concept strategy (Brian Loar, David Papineau, and others): Mary's new knowledge is knowledge of the same physical facts under different concepts — "phenomenal concepts" that present experiences from the inside. There are not two kinds of facts, but two ways of conceiving the same facts. The objection: this requires an account of phenomenal concepts that does not smuggle in what it explains, and it is not clear that the concepts are genuinely physical.

The "Mary learns what it is like" reply (Dennett): Dennett famously argues that the thought experiment is underdescribed — if Mary really knew everything physical, she would know what red looks like, because she could deduce it; the "awe" of first sight is a psychological fact, not a new fact. Jackson and others reply that Dennett's move is a bare assertion of physicalism, not an argument.

The debate has no consensus. The knowledge argument remains the most widely discussed argument in the philosophy of mind, because it captures, in a single vivid image, the intuition that experience exceeds objective description.

Contemporary Relevance

The knowledge argument is directly relevant to the science of consciousness and to AI. If the argument succeeds, then no matter how complete our neuroscience becomes — no matter how fully we map the neural correlates of consciousness — there will remain facts about experience that the physical story leaves out. This bears on the hard problem: the knowledge argument is one of the clearest statements of why the hard problem is hard.

For AI, the argument has a pointed implication: a machine that knows and computes every functional fact about color — a robot Mary, so to speak — might still not know what it is like to see red. Whether the machine has the experience is precisely what the knowledge argument puts in doubt. And for ethics, the argument matters for the question of which systems have experiences that count: if knowing all the facts about a system never settles whether it has qualia, then the question of machine or animal consciousness is not answerable by functional or behavioral tests alone.

Related Concepts

• Qualia: the phenomenal characters Mary learns about.
• What is it like to be a bat: Nagel's companion argument from the subjective standpoint.
• The explanatory gap: the gap the argument exposes between physical facts and experience.
• The hard problem vs easy problems: why physical knowledge leaves experience unexplained.
• Phenomenal consciousness: the subject matter of the argument.

Further Learning

• Stanford Encyclopedia of Philosophy: The Knowledge Argument
• Internet Encyclopedia of Philosophy: The Knowledge Argument
• Frank Jackson, "Epiphenomenal Qualia," Philosophical Quarterly 32 (1982).
• Frank Jackson, "What Mary Didn't Know," Journal of Philosophy 83 (1986).