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Saturday, May 1, 2010

What Zombie Mary Knows

[This post was revised and reposted on May 2, 2010.]

As I mentioned in my last post, Torin Alter alerted me to the fact that Victoria McGeer (2003) makes an argument about Zombie Mary which is similar to my own. I emailed McGeer and she was kind enough to send me a copy of her paper. It is not as similar to my argument as I was expecting, but we do cover some of the same ground. She makes strong use of Zombie Mary, though she does not construct an incompatability argument. Her aim is rather to show that (1) the knowledge argument relies on the very modal intuition it was supposed to establish, and (2) the knowledge argument is implausible. I will discuss her main arguments below, and I will also discuss some related issues.

First, McGeer makes a very important point about David Chalmers' position in this whole affair. Chalmers (1996) observes that the modal intuition--the intuition that zombies are conceivable--entails that zombies make judgments just like human beings do. This follows from the fact that judgments are behaviors--they are things we say and do. Yet, because zombies do not have phenomenal consciousness, their judgments about phenomenal consciousness are not true. Zombie Mary can say, "now I know what it is like to see red!" However, when she says it, she is wrong. This leads to a sort of paradox: What makes a phenomenal judgment true or false--what justifies it--cannot be a part of its causal history. As McGeer writes, "it seems that consciousness in the phenomenal sense is explanatorily and causally irrelevant to the making of phenomenal judgments, though not to their justification or truth-value" (McGeer, 2003, 386.) According to Chalmers, this paradox is "delightful and disturbing" (Chalmers, 1996, 181). Disturbing, indeed, for we must marvel at the fact that Zombie Mary makes judgments about phenomenal properties at all!

What I show in my defense of the incompatibility argument is that the paradox is wider than Chalmers may have realized. It is not simply a paradox of phenomenal judgment. It is a paradox of judgment, period. The knowledge and conceivability arguments cannot both work unless we completely detach knowledge from observable behavior. To save the knowledge argument, we must stipulate that knowledge cannot be observed, even when we observe clear cases of learning. Zombie Mary can learn how to identify colors, but learning is not sufficient for knowledge. Phenomenal experience is required for knowledge of any sort, but this experience has no causal or explanatory relation to the behaviors we regard as expressions of knowledge! It turns out Zombie Mary is not a super-scientist, though she looks and acts exactly like one in every observable detail.

The paradox of judgment does not just stem from the combination of the knowledge and conceivability arguments. It follows directly from the knowledge argument itself. McGeer notes that, if the knowledge argument is sound, then physicalism is false, and thus the modal intuition is correct. According to the incompatibility argument, if the modal intuition is correct, then the knowledge argument is not sound. Thus, the knowledge argument is self-defeating--unless we reject the incompatibility argument in favor of the paradox of judgment.

The paradox of judgment may also follow directly from the conceivability argument itself, if we accept Bertrand Russell's distinction between knowledge-by-acquaintance and knowledge-by-description. Russell (1912) maintained that all knowledge is composed of facts learned by direct acquaintance; even facts we learn by description are only known via facts we have learned by acquaintance. If this is true, then zombies cannot know anything at all, because they cannot be acquainted with anything. Thus, the modal intuition produces the paradox of judgment regardless of whether or not we combine it with the knowledge argument.

Chalmers might be willing to accept the paradox of judgment. He already accepts the paradox of phenomenal judgment, so why not also accept that all expressions of knowledge are unrelated to what makes them true or false? Indeed, it would be simpler to claim that whatever makes any judgment true or false is causally and explanatorily unrelated to the judgment itself than it would be to claim that phenomenal judgments are somehow distinct in this matter. Chalmers can say that this is just the way knowledge works, without having to make a special case for phenomenal knowledge.

The main reason to reject the paradox of judgment is that it makes it impossible for an anti-physicalist to explain zombie behavior. An explanation of zombie behavior in terms of evolutionary biology, for example, would be out of the question. Perhaps Chalmers is willing to purchase anti-physicalism at this price, but I cannot imagine how he could justify that move.

Furthermore, the paradox of judgment causes a problem for the knowledge argument, which stipulates that Mary can learn a completed science. If we stipulate that zombie behavior cannot be explained even in physical terms, then we are making a strong critique of the physical sciences. There would have to be physical facts which could not be deduced from the completed science Mary learns from inside her room, and the knowledge argument would no longer work against physicalism.

McGeer does not follow this line of thought exactly, but her approach is quite similar. She suggests a way for anti-physicalists to explain Zombie Mary's judgments: Zombie Mary experiences phenomenal illusions. Zombie Mary only thinks she is experiencing colors, as a result of physical transformations she undergoes when she sees a red tomato for the first time. However, as McGeer notes, this creates another problem for the knowledge argument. For, if Zombie Mary's physical transformation induces a phenomenal illusion in her, why wouldn't it induce such an illusion in the original Mary? Indeed, how do we know we are not experiencing phenomenal illusions when we think we are experiencing colors? The possibility of phenomenal illusions seems to undermine the knowledge argument, too.

I suspect supporters of the knowledge and conceivability arguments will reject the idea that Zombie Mary experiences a phenomenal illusion. How could a zombie experience an illusion, when they cannot experience anything at all?

Either way, the knowledge argument is in trouble. If anti-physicalists embrace the paradox of judgment and reject the claim that Zombie Mary experiences phenomenal illusions, they undermine the knowledge argument by claiming that not all physical facts can be accounted for with the physical sciences.

McGeer makes another argument which deserves mention. She notes that Zombie Mary seems to learn what it is like to see red, and even says, "Ah, so this is what it's like to see red!" Yet, the point of Zombie Mary is that she cannot know what Mary knows. So the fact that she seems to know what Mary knows is not enough to conclude that she knows what Mary knows. In the case of Zombie Mary, anti-physicalists ignore her behavior, because Zombie Mary does not really know what she claims she knows. Yet, in the case of the original Mary, anti-physicalists do not ignore her behavior, because Mary really knows what she says she knows. But then, anti-physicalists are licensing themselves to ignore evidence in one possible world, but not another. To justify this move, the anti-physicalist must first establish that the same evidence can be treated differently in different worlds, and that requires an argument for the modal intuition that zombies are conceivable. So anti-physicalists must first establish that zombies are conceivable before they can successfully argue for the knowledge argument. Thus, the knowledge argument relies on the very modal intuition it was supposed to establish.

Even if all of my previous arguments are flawed--even if the knowledge and conceivability arguments are compatible, and even if the knowledge argument is plausible--McGeer shows that the knowledge argument cannot be used to support the conceivability argument. The conceivability argument is thus significantly weaker, for the knowledge argument was supposed to establish the modal intuition which it requires. The knowledge argument is also significantly weaker, because it relies on the same modal intuition.

Before concluding, I will make one more observation about the poverty of the knowledge argument. Consider just what Mary is supposed to learn which is said to prove that there are non-physical facts. Frank Jackson (1986) puts it thus:

The trouble for physicalism is that, after Mary sees her first ripe tomato, . . . 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 that she was quite unaware of. All along their experiences . . . had a feature conspicuous to them but until now hidden from her.

Anti-physicalists claim that Zombie Mary could not learn this fact. But how could the original Mary learn it? Her own experience of color vision cannot tell her anything about what other people experience. Anti-physicalists claim that phenomenal experience is absolutely subjective--nobody has access to anybody else's. So nobody's color-perceptual experiences can tell them anything about what other people experience, if other people experience anything at all. Indeed, how does Mary know that the rest of the world isn't populated by zombies? Mary cannot learn what Jackson says she can learn, so the knowledge argument's threat against physicalism is idle.

I have here presented several arguments which together suggest that (1) the knowledge argument is self-defeating and poses no threat to physicalism, (2) both the knowledge and conceivability arguments rely on the same modal intuition, and (3) the modal intuition entails a paradoxical view of knowledge and judgment. If anti-physicalists want to make a case against physicalism, they need a new argument for the modal intuition--preferably one which either justifies or overcomes the paradox of judgment.


Chalmers, David (1996) The Conscious Mind: In Search of a Fundamental Theory. New York: Oxford University Press.

Jackson, Frank (1986) “What Mary Didn’t Know,” Journal of Philosophy, 83, pp. 291–5.

McGeer, Victoria (2003) "The Trouble With Mary" Pacific Philosophical Quarterly, 84, 4, pp. 384-393.

Russell, Bertrand. (1912). The Problems of Philosophy. Oxford: Oxford University Press.