The brain in a vat hypothesis is an example of radical scepticism, and is a contemporary reworking of Descartes’ evil demon argument. It may have no practical significance for me (even if true), but it does challenge us to consider and justify what we think we know about the external world, and so is a useful exercise.
The sceptic’s challenge is that, for all I know, I might exist only as a disembodied brain floating in a vat of nutrients. The brain is connected to a supercomputer that is producing electrical impulses that are stimulating it in an identical way to those that would arise from normal perception. So, when I currently think that I am sitting in my office typing on my computer, I actually exist only as a brain that is sitting in a vat and being fed the inputs for all of my perceptions of the world around me. Is there any way that I can answer the sceptic’s challenge?
Firstly, we should note that arguing that I cannot be a brain in a vat, as I can experience the external world in all of its rich texture (and I have detailed memories of it) gets me nowhere, as the hypothesis states that the inputs to my brain are identical to those that it would receive from my senses if I was really experiencing the external world, and therefore the two scenarios are indistinguishable to me.
One possible solution is to say that the hypothesis is an empty one. If I am actually a brain in a vat, but have no way to ever verify this, then it makes no difference to me, so it is as if I really do exist in the world. In this sense, it is a bit like having an invisible and intangible elephant in my garage – it is no different to having no elephant at all. However, there is a crucial difference here that makes the analogy false. At the moment the brain in a vat hypothesis makes no difference to my existence, but if it is actually true then there is the possibility that it might make a difference in the future (the inputs might be changed radically, or stopped completely, for example). So, in that sense, the brain in a vat hypothesis is not an empty one.
Another solution might be attempted by appeal to Occam’s razor. I might venture that the hypothesis that I am a brain in a vat is less parsimonious than the hypothesis that I really do experience the external world as I believe it to be. However, my idea of which hypothesis is the more parsimonious is based upon my experience of the world. If this experience is illusory, and the external world doesn’t exist as I seem to experience it, then my real world hypothesis may be the less parsimonious one. Having said that, there is an intuitive sense in which the brain in a vat hypothesis contains more ad-hoc elements. In this scenario, my experiences need to be based upon something – either upon some reality within the world in which my brain is en-vatted, or else pure fabrications (or some combination of the two). If the former, then some version of the world that I experience does exist somewhere but, for some unknown reason, I exist in it purely as a brain in a vat. If the latter, then it would seem to require a great deal of effort in order to fabricate an internally consistent world of the complexity that I experience, with no obvious reason as to why this should be done at all (note though that the simulation needn't be of an entire universe, as I don't actually experience the entire universe - only a very small subset of it). In both cases, intuition suggests that these options are less parsimonious than the hypothesis that the real world just exists with me in it.
A number of semantic responses have been attempted (by Putnam and others) along the following lines:
P1: If I am a brain in a vat, then my word ‘tree’ does not refer to trees.
P2: My word ‘tree’ refers to trees. So,
C: I am not a brain in a vat
However, all such responses appear to beg the question. The whole point of the sceptic’s argument is that I don’t know that my perception refers to anything in the real world, so P1 and P2 above are not both true (and might be both false). My word ‘tree’ may or may not refer to actual trees, if they exist, but I cannot deduce anything significant from that. For example, imagine that the brain in a vat world is identical to our world, with the exception that the expertise and technology exists to put brains into vats and feed them inputs that are indistinguishable from external perception in the BIV world, and that ‘I’ am one of those brains. In this case, there is a sense in which my word ‘tree’ does refer to trees (the stimulation of my brain is identical to the stimulation that would come from perceiving a real tree, as real trees do exist in the BIV world). In that case, P1 is false, and P2 is true, so the conclusion does not follow. There is also a sense in which my word tree does not refer to trees, since I have never actually seen a ‘real’ tree. In that case, P1 is true, but P2 is false, so again the conclusion does not follow.
In the end, I think that I am warranted in believing that I am not a brain in a vat - even though I cannot prove it. The most parsimonious explanation for all of the evidence that I observe is that I actually do exist in the real world. Any other 'fake world' hypothesis, including the brain in a vat hypothesis, requires me to posit some inscrutible other world in which some agency or agencies exist that have the means and motivation to create a fake world for me. This has to be less parsimonious than the real world hypothesis.
Furthermore, based upon my experience of my world (whether real or fake), the most rational course of action is to attempt to achieve my goals (happiness, for example). Whether my world is real or fake, this course of action is the same. There may be no way, even in principle, for me to determine whether I am a brain in a vat or not, but since the course of action is the same in both cases, it makes no difference to me now (although that might change in the future).