Indeed, we can all vividly imagine her, seeing a red rose for the first time and exclaiming, "So that's what red looks like!" And it may also occur to us that if the first colored things she is shown are, say, unlabeled wooden blocks, and she is told only that one of them is red and the other blue, she won't have the faintest idea which is which until she somehow learns which color words go with her newfound experiences.
That is how almost everyone imagines this thought experiment- not just the uninitiated, but the shrewdest, most battle-hardened philosophers (Tye, 1986; Lewis, 1988; Loar, 1990; Lycan, 1990; Nemirov, 1990; Harman, 1990; Block, 1990, van Gulick, 1990). Only Paul Churchland (1985, 1990) has offered any serious resistance to the image, so vividly conjured up by the thought experiment, of Mary's dramatic discovery. The image is wrong; if that is the way you imagine the case, you are simply not following directions! The reason no one follows directions is because what they ask you to imagine is so preposterously immense, you can't even try. The crucial premise is that "She has all the physical information." That is not readily imaginable, so no one bothers. They just imagine that she knows lots and lots- perhaps they imagine she knows everything that anyone knows today about the neurophysiology of color vision. But that's just a drop in the bucket, and it's not surprising that Mary would learn something if that were all she knew.
To bring out the illusion of imagination here, let me continue the story in a surprising- but legitimate - way:And so, one day, Mary's captors decided it was time for her to see colors. As a trick, they prepared a bright blue banana to present as her first color experience ever. Mary took one look at it and said "Hey! You tried to trick me! Bananas are yellow, but this one is blue!" Her captors were dumfounded. How did she do it? "Simple," she replied. "You have to remember that I know everything -absolutely everything- that could ever be known about the physical causes and effects of color vision. So of course before you brought the banana in, I had already written down, in exquisite detail, exactly what physical impression a yellow object or a blue object (or a green object, etc.) would make on my nervous system. So I already know exactly what thoughts I would have (because, after all, the "mere disposition" to think about this or that is not one of your famous qualia, is it?). I was not in the slightest surprised by my experience of blue (what surprised me was that you would try such a second-rate trick on me). I realize it is hard for you to imagine that I could know so much about my reactive dispositions that the way blue affected me came as no surprise. Of course it's hard for you to imagine. It's hard for anyone to imagine the consequences of someone knowing absolutely everything physical about anything!"
Surely I've cheated, you think. I must be hiding some impossibility behind the veil of Mary's remarks. Can you prove it? My point is not that my way of telling the rest of the story proves that Mary doesn't learn anything, but that the usual way of imagining the story doesn't prove that she does. It doesn't prove anything; it simply pumps the intuition that she does ("it seems just obvious") by lulling you into imagining something other than what the premises require.
It is of course true that in any realistic, readily imaginable version of the story, Mary would come to learn something, but in any realistic, readily imaginable version she might know a lot, but she would not know everything physical. Simply imagining that Mary knows a lot, and leaving it at that, is not a good way to figure out the implications of her having "all the physical information"-any more than imagining she is filthy rich would be a good way to figure out the implications of the hypothesis that she owned everything. It may help us imagine the extent of the powers her knowledge gives her if we begin by enumerating a few of the things she obviously knows in advance. She knows black and white and shades of gray, and she knows the difference between the color of any object and such surface properties as glossiness versus matte, and she knows all about the difference between luminance boundaries and color boundaries (luminance boundaries are those that show up on black-and-white television, to put it roughly). And she knows precisely which effects -described in neurophysiological terms- each particular color will have on her nervous system. So the only task that remains is for her to figure out a way of identifying those neurophysiological effects "from the inside." You may find you can readily imagine her making a little progress on this- for instance, figuring out tricky ways in which she would be able to tell that some color, whatever it is, is not yellow, or not red. How? By noting some salient and specific reaction that her brain would have only for yellow or only for red. But if you allow her even a little entry into her color space in this way, you should conclude that she can leverage her way to complete advance knowledge, because she doesn't just know the salient reactions, she knows them all.
It's what Dennet calls 'Philosopher's Syndrome': mistaking a failure of imagination for an insight into necessity, a term that I love because I think it describes something that happens all the time. The real driver of the thought experiment is exactly that: "it seems just obvious" she would learn something new.
I suppose the real problem is why the third person isn't the first person. We want more than an equivalence, we want an identity. And so even if you tell me that this brain state, when instantiated in my brain (but not until then!), will indeed give me red, it doesn't solve the problem of why it wasn't already, as a third person description already red, prior to any requirement that it be instantiated. Why should there even be descriptions at all? If our senses stretched down into the world to match the depth of our knowledge and realized every true statement about physics as a corresponding qualitative "raw feel," we wouldn't need representations and there wouldn't be such a thing as a description that wasn't identical to the first person experience.
Representation involves the "overhead" cost of getting to something that isn't qualitative content by means of qualitative content. Black and white give themselves immediately but also, when we have enough power over them, give lines, a cube, or a map. I could instead have a cube-feeling or a map-feeling, in which case it might not be necessary to "represent" either.
If physicalism is true (as think it is) the real problem posed by red is that we call it intrinsic and thereby postulate an irreducible character and then take up the definitionally impossible task of trying to reduce it. What we mean by "intrinsic" here has to be mistaken. Red can be broken apart to some prior components, physical components which really are combined to make it, and this can be done without transgression of an ontological divide. Physicalist stuff really can combine in such a way as make green, and tone, and the snake's directional olfaction and any of innumerable senses we could yet evolve. To use a dangerous word that I generally don't use, the being of physical ingredients is imparted onto red.
That red and blue combine to 'make' purple is a metaphor to be extended in the other direction to describe the ingredients coming together to make red. Red can not be 'intrinsically' so in the philosophically significant sense, but rather produced by the coming together of prior ingredients, of which there is nothing but.