The Philosopher Who Built Mary's Room — and Then Changed His Mind

Frank Jackson published "Epiphenomenal Qualia" in 1982 with one of the cleanest thought experiments in modern philosophy of mind. Mary is a vision scientist who has lived her whole life in a black-and-white room, watching the world through a black-and-white monitor. She knows everything physical there is to know about color vision: every wavelength, every retinal response, every neural pathway, every word printed on color. One day she walks out and sees a red tomato.

Does she learn something new? If yes — and Jackson's intuition was that obviously yes — then physical knowledge cannot be all the knowledge there is. There is something it is like to see red, and that something can't be captured by any list of physical facts. Physicalism, the view that everything reduces to the physical, is therefore false. The argument got known as the Knowledge Argument.

It detonated. Over a thousand academic papers responded. Daniel Dennett, in particular, attacked the framing: he thought Mary, knowing literally everything about vision, would not be surprised by red, because her physical knowledge would already include the disposition to recognize and respond to it. The intuition that she'd learn something, he argued, was just a failure of imagination about what "knowing everything physical" really means.

Jackson eventually agreed. By the late 1990s he was calling himself a physicalist. In a 2023 interview he said plainly, "I no longer accept the argument," while still recommending it as the strongest case a non-physicalist can make. The most influential anti-physicalist thought experiment of the late twentieth century is, by its author's own account, wrong — and the field is still arguing about why.