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Pondering at the arguments at wiki:

In other words, if a brain in a vat stated "I am a brain in a vat", it would always be stating a falsehood. If the brain making this statement lives in the "real" world, then it is not a brain in a vat. On the other hand, if the brain making this statement is really just a brain in the vat then by stating "I am a brain in a vat" what the brain is really stating is "I am what nerve stimuli have convinced me is a 'brain,' and I reside in an image that I have been convinced is called a 'vat'." That is, a brain in a vat would never be thinking about real brains or real vats, but rather about images sent into it that resemble real brains or real vats. This of course makes our definition of "real" even more muddled.

We sometimes are aware of the dream and say I'm dreaming. When we wake up from a dream, we realize that we were dreaming, that is, suddenly we are connected to a second reality, and also retain memory of previous, disconnected reality. Also, somehow, we are able to differentiate between the two.

Now consider a brain in the vat. Let's say, the real camera feed from lab (where the whole vat thing is setup), is rendered when the vat brain dreams, and is woken up to the virtual world after sleep. So even in this upside down rendered reality, there must be some thing to convince the vat brain, to make it capable of differentiate between the two. (may be one of the rendered reality deliberately be made fuzzy etc)

So, doesn't this leads to the fact that the brain in the vat can

  1. acknowledge two realities?
  2. be convinced to some extent (even wrongly) to believe one of them?
  3. finally, have doubts about reality?
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2 Answers 2

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That is, a brain in a vat would never be thinking about real brains or real vats, but rather about images sent into it that resemble real brains or real vats.

This is the problematic part of the argument. Because, of course, all our actual brain gets is neural firing patterns sent into it. So there is no particular distinction between a brain in a vat being sent pictures of brains and vats and understanding that this is what it is; and us looking at the output of an electron microscope and understanding that this is what we are built of.

The passage is problematic, but it is hard to show that based on analogies with dreaming. Rather, it seems to implicitly assume some sort of direct transfer from "real" objects to a representation in the brain that does not agree with how it actually happens. Whether we interpose merely photoreceptor cells in our eyes or add in an electron microscope or interplanetary distance and cameras and radio waves is philosophically immaterial: the access is indirect and subject to all the philosophical problems that arise when perception is indirect.

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True. Perception cannot even verify if there's a level of indirectness all or not. But it still might wonder about this, and all other possibilities. –  user2411 Dec 19 '12 at 16:17

When I dream in my sleep I'm most of the time convinced that what I experience is real. When I'm awake on the other hand, I often have doubts.

Suppose there is a particular center in the brain that makes one doubt - which in my case is apparently off during sleep. If the experimenter makes sure that this particular area isn't excited, then the brain will be convinced everything that they show it is real.

Now we:

  • show it the second reality. the brain's convinced that it's in it.
  • make it remember the first reality while showing the second one. It acknowledges, that there are two realities, but believes in the second one since it's apparently current and it's doubting circuit is off.

If the current (not remembered) reality is to some extent true, the brain believes in truth, if it's the fake one, the brain is mistaken.

To make it have doubts, we excite the doubts center.

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