The thought experiment
Imagine Mary, a brilliant scientist confined from birth to a black-and-white room and educated through black-and-white screens. She specializes in the science of color and learns everything physical there is to know: the physics of light, the wiring of the retina, the neuroscience of visual processing, every fact expressible in the language of a completed physical science. By stipulation, her physical knowledge of color vision is total.
Then one day Mary leaves the room and sees a ripe tomato, or a red rose, for the first time. The argument asks a simple question: does she learn anything new? The strong intuition is that she does. She learns what it is like to see red, a fact that all her prior physical knowledge did not give her. If she gains new knowledge, then there was a fact she did not previously possess. But she already had all the physical facts. So there must be facts about experience that are not physical facts, and physicalism, the view that everything is physical, would be false.
Who made the argument
The Australian philosopher Frank Jackson introduced the argument in a 1982 paper titled Epiphenomenal Qualia and developed it further in a 1986 paper, What Mary Didn't Know. Jackson intended it as a direct challenge to physicalism, defending the reality of qualia, the felt qualities of experience, as something the physical story leaves out.
In a striking turn, Jackson later changed his mind and rejected his own argument. By the late 1990s and in papers such as Mind and Illusion (2003), he came to hold that the intuition driving the argument is misleading and that phenomenal facts are, in principle, physical facts after all. He adopted a representational account of experience and argued that the pull of Mary's case is a kind of cognitive illusion. It is rare and instructive for the author of a famous argument to become one of its sharpest critics.
The live debate
Physicalists have offered several replies. The ability hypothesis holds that what Mary gains is not new factual knowledge but new abilities: to recognize, imagine, and remember red. The acquaintance response says she gains a new kind of direct acquaintance with a quality she already knew about descriptively. The old fact, new mode reply argues that she learns an old physical fact under a new, first-person way of thinking, so no new fact enters the world.
Defenders of the argument counter that these moves either understate what Mary plainly seems to acquire or quietly smuggle in the very phenomenal facts they mean to deny. The knowledge argument remains one of the most discussed pieces in the philosophy of mind precisely because the intuition is so vivid and the physicalist responses so ingenious. It continues to shape debates about consciousness, and by extension about whether a system that has processed every fact about a thing could still be missing something by never having experienced it.