Frank Jackson set the trap gently, in a 1982 paper, and it has never quite been sprung. Imagine Mary, a scientist who has learned everything physical science could ever teach about colour vision — the wavelengths, the retina, the cortex, the lot — but who has spent her whole life inside a black-and-white room, seeing the world only on a monochrome monitor. By hypothesis, she knows every physical fact there is. Then one day she walks out, and for the first time sees a red rose.
The question is whether she learns anything. And the pull of the intuition is strong: it certainly seems she does — she finally learns what seeing red is like. But here is the sting. She already knew every physical fact. So if she learns something new, that new something cannot be a physical fact. And if there are facts that are not physical facts, then physicalism — the view that reality is exhaustively physical — is false. That is the knowledge argument, and it fits into three tidy lines that are far harder to escape than they look.
The physicalist repliesWhere the argument might break.
Physicalists do not concede. They argue the experiment trades on a slide between very different senses of "knowing," and that once you pull them apart, the mystery thins. Here are the three most influential responses.
The ability hypothesis
Mary gains no new fact — she gains an ability: to imagine, remember, and recognise red. Knowing-how is not knowing-that, and only facts threaten physicalism.
The acquaintance reply
She becomes acquainted with red for the first time. Acquaintance is a third kind of knowing — neither a new fact nor merely a skill — and it leaves physicalism untouched.
Old fact, new guise
She learns an old physical fact under a new mode of presentation — the same fact grasped a new way, as "water" and "H₂O" pick out one thing. No new fact, so no threat.
Each reply has force, and each has critics. Does "knowing what red is like" really reduce to a knack for imagining it? Can a fact you genuinely already possessed still ambush you when you finally live it? The debate turns, in the end, on whether the felt quality of experience — its qualia — is the kind of thing a complete physical description could ever contain.
The twistThe man who changed his mind.
Here is the detail that keeps the story honest. Frank Jackson, who built the argument and made it famous, later abandoned it. He came to think the knowledge argument, however seductive, must be going wrong somewhere, and he returned to a physicalist view — arguing that our conviction that Mary learns a new fact is an illusion produced by how differently experience and description present the same reality. The inventor of philosophy's sharpest anti-physicalist weapon decided, on reflection, to lay it down. Whether he was right to is still, very much, an open question — and now that you have opened the door yourself, it is your question too.