Noe points out that the question, "How do you know that you aren't a brain in a vat?" has been used in order to undercut our tendency to take for granted that we have an immediate experience of and interaction with our environment. The expected answer, "I don't know," is supposed to lead us to conclude that our brain is directly aware only of internally produced representations while our body is a kind of vat that houses, nourishes, stimulates and responds to the brain. For if it could be a brain in a vat, then one two different stimuli may cause the same cognitive act; in which case it would follow that this cognitive act is not intrinsically ordered toward either of the possible causes.
The most appropriate reply would be to show how representationalism runs into problems, regardless of any concern about brains, vats, and the like. But apart from those types of replies I might also argue that merely not knowing with 2+2=4 certainty that one's everyday beliefs are true does not compel one to conclude to that the world that serves as the object of belief is a mere representation. In order to make that point I would argue that the brain-in-vat (assuming this scenario is possible) would in fact be experiencing the vat rather than inner representations. Likewise with the fully embodied person is experiencing the word in which he seems to be moving. So the question in play is whether nature is deceptive, not whether I live in a world of representations. And the question about deception is not 100% resolvable through speculation: it is a pragmatic one.
(edit) Crucial to my argument would be to establish that the rest of my body is part of who I am and how I experience the world, even if I grant the unnatural state of a brain being in a vat is possible. One resource would be Noe's point about the non-fixity of where the "I" ends and environment begins. If I become blind, I will sense the ground through my cane when I walk. Strong claim: no representation involved even there. Of course, the non-brain part of my body is not a cane. In order to cash out that point I would need to talk about teleology, habit, stuff like that. More later. Also helpful would be Noe's observation that the brain has "no mission control." Just as no part of the brain is mission control, so too with the body. Finally, I now see why Noe addresses the phantom limb issue, as this phenomenon would seem to argue against the veracity of my perception of the non-brainy-part-of-my-body as part of me.
Back to the pragmatic question: I would address that by asking, "Do you believe that you have a brain? Why/why not?" I expect that we would end up discussing component questions, such as, have you ever seen a brain, have you seen yours, have you seen the correlation between your brain activity and your environment, have you seen the lack of correlation between cognition and the rest of your body, do you rely on the testimony of others, how is it that you know how to use words meaningfully, what do you assumed when you use words to communicate, to think, etc.)
These questions would show, I hope, that the posing the brain-in-vat question presupposes beliefs about the way the world is, beliefs that are had regardless of whether the questioner can demonstrate that they are true. He might object that since there is no demonstration in this matter, he can suspend judgment about the truth of his common sense convictions. I would argue in reply that a pure suspension of judgment is impossible, and I would do so by analyzing the use of language by one who attempts this suspension of judgment. If the person is a fan of science, then I would also point out that common sense is more consistent with Ockham's razor: non-representationalism is simpler and more elegant than its alternative: why clutter the world with deceivers, let alone unwarranted claims about representations?
Another, even more fanciful counterargument: point out that if I am a brain in a vat, then there is someone who placed me there, who is not a brain in a vat. We can then ask questions about the brain of the deceiver: Can this deceiver PROVE to himself that he is not a vatted brain? I suspect the answer would be negative. Does that answer imply that they are dealing only with representations and not directly with reality? I propose that the answer would be that it doesn't. If the answers are as I hope, then ask why can't we answer these questions in the same manner when they concern ourselves?
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