The Author of the Most Famous Anti-Physicalist Argument Stopped Believing It

Frank Jackson published "Epiphenomenal Qualia" in 1982 and "What Mary Didn't Know" in 1986. The thought experiment those papers introduced became one of the most-taught arguments in philosophy of mind. Mary is a neuroscientist who has spent her life inside a black-and-white room. She knows every physical fact about color vision — wavelengths, retinal cones, the neural pathways, every functional state involved in seeing red. Then the door opens and she walks outside.

Jackson's claim was that Mary learns something. She finally knows what red looks like. If physicalism were true, the argument runs, she would already have known: the physical facts would have been all the facts. Since she didn't, they aren't. There is something — phenomenal experience, qualia — that escapes the physical story.

The argument took. It got named the knowledge argument and became the standard challenge a physicalist had to answer. Jackson, meanwhile, kept thinking about it. By the late 1990s he was telling colleagues he no longer believed his own conclusion, and he eventually said so in print.

The move that broke it for him was the causal one. If Mary really has a non-physical experience when she sees red, that experience would have to be doing nothing — physics, by hypothesis, already explains every body movement, including her saying "wow." Epiphenomenalism saves the qualia by making them inert, and inert qualia struck Jackson, on reflection, as a worse position than just being a physicalist. He decided that Mary was learning a new way her brain represented colors she had been describing all along, not encountering a new metaphysical category.

Jackson tells the story with some amusement. Other philosophers, he has said, come up to him at conferences to express disappointment. He had given them an argument; they had built careers refining it; and the inventor had gone over to the other side.