There’s a famous story in the philosophy of mind: the story of Mary. It goes something like this. Mary is the world’s greatest neuroscientist. She knows everything there is to know about the physical world, and about how our brains work to perceive and interpret the world. In particular, Mary knows everything there is to know about color and color vision: from the physics of light, to the structure of the eye and the nervous system in human beings and other organisms, to the ways that our brains recognize and distinguish particular colors, to the evolutionary origins of color vision, to the functions served in our minds by the apprehension of color, and the ways that our moods are affected by seeing one color or another. In short, Mary knows every physical and scientific fact that there is to know about color.

But there’s a catch: Mary herself has never perceived any color at all. She has lived for her entire life in a room that’s entirely black and white. She has read black-and-white textbooks, and watched black-and-white videos. And so she knows that the sky is blue, that grass is green, and that roses are red. But she has never actually seen the sky, the grass, or a rose. She has only read about them, or viewed black-and-white photos and videos of them.

The question is: what happens when Mary finally leaves her black-and-white room, goes outside, and sees a red rose for the very first time? What does it mean for her to feel, for herself, what she has previously only known about? What is it like for her to perceive the color red? Does the phenomenal experience of redness add anything to her store of knowledge about the color, and about how people respond to it? Does Mary learn something that she didn’t know before?

Our intuitions would seem to suggest that Mary does, at the very least, encounter something new when she leaves her room. Redness and blueness are what philosophers call qualia: phenomenal sensations, or “raw feels”, that seem to make up the very fabric of our mental experience. And the qualia of color vision, in particular, are precisely what Mary is missing inside her black-and-white room. Until she actually sees a red object, Mary cannot know what it is like to experience redness. But how does this square with the supposition that, while...