Black-and-White MaryJackson is a property dualist. He believes that physicalist and functionalist stories about the mind cannot capture the qualitative features of experience. As he puts it:
I am what is sometimes known as a "qualia freak." I think that there are certain features of the bodily sensations especially, but also of certain perceptual experiences, which no amount of purely physical information includes. Tell me everything there is to tell about what is going on in a living brain, the kinds of states, their functional role, their relation to what goes on at other times and in other brains, and so on and so forth, and be I as clever as can be in fitting it all together, you won't have told me about the hurtfulness of pains, the itchiness of itches, pangs of jealousy, or about the characteristic experience of tasting a lemon, smelling a rose, hearing a loud noise or seeing the sky. ("Epiphenomenal qualia," p. 127)Jackson tells us two stories to persuade us that his view about the qualitative features of experience is correct.
In the first of these stories, a man Fred sees two colors where we only see the one color, red. (Fred calls these two colors "red1" and "red2.") Jackson imagines that we obtain all the physical information we can about Fred's brain and optic system, about Fred's dispositions to behavior when confronted with various stimuli, and so on. Even if we possess all that information, there will still be something about Fred's color experiences that we won't know. We won't know what Fred's experiences are like, qualitatively. To learn that, we'd have to undergo an operation which made our visual system like Fred's; or we'd have to have his visual system transplanted into our brain. This shows there is more to know about Fred's experiences than is captured by the physical information we have about Fred. So the physical information must not be all the information. As Jackson puts it, "Physicalism leaves something out."
In the second story, a brilliant scientist Mary lives in a room where she has never seen any colors other than black, white, and shades of gray. We suppose that Mary studies the neurophysiology of vision. Eventually, she acquires all the physical information about what goes on in us when we see red objects. (Just as, in the first story, we had all the physical information about what goes on in Fred.) But Mary does not know all there is to know about our experiences. When she leaves the room, and sees a ripe tomato for the first time, there is something new she will learn about our experiences. She will learn what it is like to see red. This shows that there is more to know about our experiences than is captured by the physical information which Mary had when she was still in the room. So, once again, the physical information must not be all the information.
We can formalize Jackson's reasoning as follows:
Some ClarificationsIn thinking about Mary and Jackson's argument, it is helpful to split up Mary's progress into four stages. This is not part of the story as Jackson tells it, but it is a helpful exercise nonetheless:
Jackson's Knowledge Argument focuses primarily on Stage 4. In a nutshell, his argument is that since Mary knows all the physical information about us, but has not yet reached Stage 4, there must be something to know about us which the physical information leaves out.
Jackson's argument is not concerned so much with Stage 2. He does not concentrate any special attention on Mary's lack of experiential acquaintance with the qualitative features our experiences of red have. Nor does he discuss Mary's ability, or lack of ability, to imagine or visualize seeing red. Why not? Because Jackson's aim is to show that the physicalist is committed to saying something false about Mary. It is not obvious why the physicalist should be committed to saying that Mary would have experiential acquaintance with the qualitative features of our experiences of red, when she's still in the room. Nor is there any reason why the physicalist should be committed to saying that Mary's knowledge of all the physical information would give her the ability to have new visual experiences, or to have new visual imagery. Learning all the physical information wouldn't make a blind man see. Why should it make Mary have new sorts of visual experiences? The physicalist is under no pressure to say that it would. (Jackson stresses this point several times in "What Mary Didn't Know." See also his discussion of Nagel in "Epiphenomenal Qualia.")
So Jackson does not think there is any objection to physicalism to be found in considerations about what Mary can or can not imagine or visualize, when she's in the room. Rather, Jackson's complaint is that the physicalist is committed to saying that Mary would know all the facts there are to know about our experiences, when she's still in the room. For she has all the physical information about our experiences. And according to the physicalist, the physical facts are all the facts there are to know.
Notice that the knowledge Mary lacks when she's in the room, and then gains when she leaves the room, is knowledge about our experiences, and what they are like. This is the knowledge Mary gains at Stage 4. Jackson and the physicalist can agree that Mary comes to learn something new about her own experiences when she leaves the room--for new facts come to be true about Mary's experiences when she leaves the room. But no new facts have come to be true about our experiences. What Jackson and the physicalist dispute is whether Mary could have known all the facts about our experiences, while she was still in the room. Jackson writes:
The knowledge Mary lacked which is of particular point for the knowledge argument against physicalism is knowledge about the experiences of others, not about her own. When she is let out, she has new experiences, color experiences she has never had before. It is not, therefore, an objection to physicalism that she learns something on being let out. Before she was let out, she could not have known facts about her experience of red, for there were no such facts to know. That physicalist and non-physicalist alike can agree on. After she is let out, things change; and physicalism can happily admit that she learns this; after all, some physical things will change, for instance, her brain states and their functional roles. The trouble for physicalism is that, after Mary sees her first ripe tomato, she will realize how impoverished her conception of the mental life of others has been all along. She will realize that there was, all the time she was carrying out her laborious investigations into the neurophysiologies of others and into the functional roles of their internal states, something about these people she was quite unaware of. All along their experiences (or many of them, those got from tomatoes, the sky...) had a feature conspicuous to them but until now hidden from her... But she knew all the physical facts about them all along; hence, what she did not know until her release is not a physical fact about their experiences. But it is a fact about them. That is the trouble for functionalism. ("What Mary Didn't Know," p. 393)