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Friday, April 23, 2010

Are The Knowledge and Conceivability Arguments Compatible?

Two of the most popular and widely discussed arguments against physicalism are the knowledge argument and the conceivability argument. The knowledge argument is often described in terms of Mary, a super-scientist who learns every physical fact there is to know about color vision before she ever sees a color. She does this by reading black-and-white books and watching black-and-white science lectures from inside a black-and-white room. When she leaves the room and enters the polychromatic world, she experiences the color red for the first time. Only then does she learn what it is like to see red. Thus, the argument goes, Mary learns new facts about color vision. These new facts cannot be physical facts, because she had learned all of those already. So there must be facts about color vision which are not physical facts. Thus, physicalism is false--not all of the facts are physical facts.

One response to the knowledge argument is to reject the premise that anyone could ever learn all of the physical facts, if there even is such a thing as "all of the physical facts." I thus have offered a revised knowledge argument (see my last post and also this older one). In the revised version, Mary does not learn every physical fact. She only learns an arbitrarily large number of physical facts. Yet, no matter what she learns from inside her room, she will only learn what it is like to see red after she leaves the room and experiences red for the first time. Therefore, what she learns from seeing red cannot be a physical fact.

In my last post, I argued for the ability hypothesis, which claims that what Mary learns from seeing red is not factual knowledge. It is a more primitive sort of knowledge which is not propositional, and which cannot be learned by didactic discourse. I am not going to argue for the ability hypothesis again here. I am not going to critique the knowledge argument, either. I only want to demonstrate that it is incompatible with the conceivability argument.

The conceivability argument is commonly discussed in terms of zombies. Zombies, if they are possible at all, are beings which are physically and functionally identical to human beings, but which lack phenomenal consciousness. There is nothing it is like to be a zombie. Obviously, zombies cannot know what it is like to see red, because there is nothing it is like for a zombie to see red. This is not to say that zombies cannot see. Zombies are physically and functionally identical to human beings, so there is a sense in which they can see and even recall visual images. It's just that their vision does not have any phenomenal properties (whatever that means).

The zombie version of the conceivability argument goes like this (See David Chalmers, The Conscious Mind, 1996):

  1. In our world, human beings have phenomenal consciousness.
  2. We can conceive of a zombie world: a world identical to ours in every physical way, but in which there are no conscious beings.
  3. Therefore, the positive facts about consciousness in our world are not physical facts.
  4. Therefore, physicalism is false.
It is common to take issue with the second premise in Chalmers' argument. On the one hand, some claim that we cannot conceive of zombies at all. On the other hand, it is claimed that the second premise assumes that physicalism is false, and therefore cannot be used to argue for the conclusion that physicalism is false. I am sympathetic with both of these complaints. It does seem that any argument for premise 2 would be a direct argument against physicalism. And it does seem that zombies are inconceivable: I cannot imagine what phenomenal consciousness might be, if it could be absent from a being which was physically and functionally identical to a human being.

Obviously I am not sympathetic to the knowledge and conceivability arguments. Both should be rejected. However, instead of arguing directly against these two arguments, I want to pit them against each other. I want to suggest that these two arguments, though often supported together, are in fact incompatible. If I am right, then those who wish to reject physicalism cannot rely on both the knowledge and conceivability arguments. They have to choose.

Before I give my argument, I must present another thought experiment. Imagine the knowledge argument as before, but this time, Mary is a zombie. Zombie Mary learns all of the physical facts--or she learns any arbitrarily large number of physical facts--while inside her black-and-white room. Will Zombie Mary learn something new about color vision when she leaves her room and sees the color red for the first time?

According to Hugh Chandler (University of Illinois, Urbana-Champaign), who first brought up Zombie Mary at PhilPapers, she will. He writes: "She can now invoke an inner picture of red roses, can’t she? She can induce the necessary brain states. This is something she couldn’t do before" (Chandler, PhilPapers post 3539). The suggestion is that Mary will gain a new physico-functional knowledge of color vision. According to Hugh Chandler, this suggests that the difference between Mary and Zombie Mary is impossible to conceive.

While I give Hugh Chandler credit for introducing Zombie Mary to the discussion, he has not made the argument I am about to make here. Rather, his aim is only to show that Zombie Mary is not clearly conceivable. Now, I agree with Hugh that Zombie Mary is inconceivable. However, I don't think Zombie Mary gives us any new reasons to doubt the conceivability of zombies. A defender of Zombie Mary might respond to Hugh by saying that, while Zombie Mary does learn something--let's say she learns how to imagine, remember and recognize visual states of color vision--what she learns does not involve phenomenal consciousness. Zombie Mary can imagine red, for example, but when she does, there are no phenomenal properties of redness involved. So she does not gain the phenomenal knowledge which ordinary Mary gains. There is still nothing it is like to be Zombie Mary. While Mary gains phenomenal knowledge, Zombie Mary only gains a set of non-propositional abilities. So Hugh is wrong to think that a difference between Mary and Zombie Mary is inconceivable.

According to the ability hypothesis, possessing the abilities to remember, recognize, and imagine colors is identical to knowing what it is like to see red. If the ability hypotheses is correct, Mary and Zombie Mary both learn the same thing, and we have no basis for distinguishing between them. I think the burden is on Zombie Mary's supporters to argue for a distinction between phenomenal knowledge and these abilities, though others might say the burden is on supporters of the ability hypothesis to demonstrate that zombies are inconceivable. Either way, Zombie Mary has not helped us decide whether or not zombies are conceivable.

The knowledge argument is sometimes used to motivate the claim that zombies are conceivable. Chalmers (1996), for example, utilizes the knowledge argument to help motivate the second premise of his zombie argument. As I will now show, the knowledge argument cannot be used to support the conceivability argument. If Zombie Mary is conceivable, then the knowledge argument is invalid.

Recall how the knowledge argument goes. Mary, upon seeing colors for the first time, learns something new about color vision. She learns what it is like to see colors. Therefore, she could not have known these facts from inside her black-and-white room. But Zombie Mary also learns something when she sees colors for the first time, and what she learns cannot be a fact about phenomenal consciousness, because she never experiences that. Zombie Mary only learns physical facts, or gains physical capacities. Therefore, the fact that Mary learns something new upon seeing red for the first time is not sufficient to draw the conclusion that she learns non-physical facts about color vision. As I wrote in response to Hugh (where "Mary Z" refers to Zombie Mary, and "Mary O" refers to the original Mary):

"Our intuition tells us that Mary Z and Mary O both learn something new when they leave their respective black-and-white rooms. Presumably, whatever Mary Z learns, Mary O will learn as well. But then we cannot say that whatever Mary O learns is [non-physical] phenomenal knowledge, since Mary Z cannot gain [non-physical] phenomenal knowledge. Whatever Mary Z learns, it can supervene on the physical. We have thus lost the motivation to claim that Mary O learns something which does not supervene on the physical.

Thus, Mary Z, if conceivable, defeats the knowledge argument" (Streitfeld, Philpapers post 3565).

If you accept the knowledge argument, you cannot support the second premise of the conceivability argument, and vice versa. Therefore, the knowledge and conceivability arguments cannot both be sound. At least one has to go.