Mary knows everything there is to know about the colour red. As a scientist, it has been her life's work. If you want to know why we can't see infrared, why tomatoes are red, or why red is the colour of passion, Mary is your woman.
All this would be unremarkable, if it weren't for the fact that Mary is an achromat: she has no colour vision at all. The world, for Mary, looks like a black and white movie.
Now, however, all that is to change. The cones on her retina are not themselves defective, it is simply that the signals are not processed by the brain. Advances in neurosurgery now mean that this can be fixed. Mary will soon see the world in colour for the first time.
So despite her wide knowledge, perhaps she doesn't know everything about the colour red after all. There is one thing left for her to find out: what red looks like.
Baggini, J., The Pig That Wants to Be Eaten, 2005, p. 37.
To put a finer point on this and understand just how subversive this line of argument is attempting to be, it's helpful to go back to Jackson's 1986 paper and quote the first paragraph from it:
Mary is confined to a black-and-white room, is educated through black-and-white-books, and through lectures relayed on black-and-white television. In this way she learns everything there is to know about the physical nature of the world. She knows all the physical facts about us and our environment, in a wide sense of 'physical' which includes everything in completed physics, chemistry, and neurophysiology, and all there is to know about the causal and relational facts consequent upon all this, including of course functional roles. If physicalism is true, she knows all there is to know. For to suppose otherwise is to suppose that there is more to know than every physical fact, and that is just what physicalism denies.
In logical form, the argument goes something like this:
(1) Mary has all the physical information concerning human color vision before her release.
(2) But there is some information about human color vision that she does not have before her release.
(3) Not all information is physical information.
Hogwash! The first premise is patently false because Mary does not have "all the physical information" and cannot know "all there is to know" about this subject without having experienced it firsthand. Why? Precisely because we live in a physical universe where mental imaginings are not enough to move the physical atoms that make up the nerves in our eyes and the synapses in our brains. In philosophical terms, there is a real epistemic barrier to what we can learn no matter how much we sit in our rooms and read and think. All the travel books in the world cannot tell you what it feels like to walk around in Thailand. Nor can all the cook books in the world tell you what a peanut butter and mustard sandwich would taste like, even if you've separately already eaten lots of peanut butter and lots of mustard. (I imagine it would be quite horrible, but honestly the salty crunch with the vinegary spice might be appealing if it was balanced just right. I don't yet know.) All over the world and all throughout history, people have experienced new things for the first time and learned what that feels like. At no time did that create a non-physical entity in the universe. Our mental world is composed from different brain states, which a cursory study of neuroscience will readily explain.
"Hang on," the idealist might still say. "You physicalists may have seen around this particular trick of ours to try to separate experience from knowledge and place it out there in the ether, but you still haven't explained what it feels like to see red. Isn't knowledge about that subjective bit of consciousness something else entirely?"
I'm sure I'll have more to say about this in about 60 weeks when we get to Thomas Nagel's Bat and what that means for the study of consciousness, but as far as this week's argument against physicalism goes, no, it's not something else entirely and the idealist is just using different words to try to make it seem so. If the experience of seeing red was truly separable from the knowledge of what it feels like to see red, then not only could Mary know what it feels like to see red without having the actual experience (which she couldn't), but someone else could have the experience of seeing red without then knowing what it feels like to see red, and that cannot be done either. The physical experience is directly tied to the mental state of feeling the experience, and vice versa. This is because they are both formed from physical stuff and tied together by physical stuff. And with that, I'm done with this and going out to experience more new physical sensations.