Brain in a vat (January 2004):
More on brains in vats .... The threat of actually being a BIV seems minimal. If we were BIV, wouldn't there be some way we could tell? Is there anything which is necessarily unique to the real world, any seal of authenticity that stamps out the world as real?
But this isn't how I would actually answer the question of why I believe the world is real. I don't point to any official seal. Instead, I point to the fact that the world seems indistinguishable from a real world.
Consider the Matrix. The Matrix is discernible from the real world. The Matrix is "buggy". So the Matrix isn't actually an example of the hypothetical BIV scenario, according to which being a BIV was supposed to be indiscernible. Obviously the real world and the BIV world are different, but the difference is supposed to be one that as BIV we would be shielded from discovering. That is, there might be a solution out there, but one that is not available to us within the system. In the Matrix, the difference is discoverable from within the system.
Maybe we're in something more like the Matrix. How could we go about looking for bugs, for clues that would let us know this world wasn't real? And if we don't find any such bugs, can we conclude that the world we perceive is real? I find this logic appealing. We have a method of discovering if we are BIV. Every day we don't discover we have BIV, we have a bit more evidence for believing we are not BIV. This sounds remarkably like fallibilism, also like how much scientific knowledge is obtained. Or maybe it's not even possible to have non-discoverable bugs; perhaps given enough time, all bugs are obvious.
Is it even possible that the experience of being a BIV could be qualitatively indiscernible from the real thing? The makers of the Matrix tried it and failed. Do you think you could do any better? How would you go about creating a Matrix without any artifacts? Is every implementation of the BIV doomed to be buggy or inconsistent in some way or another? Is it even conceivable that the experience of being a BIV could be qualitatively indiscernible from the real thing? I have trouble conceiving of any actual implementation of the hypothetical BIV scenario.
Maybe our belief that we aren't BIV isn't something we can -- or need to -- explain. If we cannot conceive of really being BIV, it doesn't seem like something we have to seriously consider as a real possibility. Could it be that certain truths, such as that we aren't BIV, are just obvious to us without any need of further examination?
This doesn't seem like a very satisfying solution to the BIV problem to me (it doesn't explain how we know, just claims that it's obvious), but if it's true, it does answer the threat of the BIV problem. We know we are not BIVs precisely because we know it in some direct and unquestionable why. Why couldn't there be certain truths which are directly known to us? And if there are such truths, it seems likely that the fact that we are not BIV would be one of these truths.