Could a skeptical hypothesis be true? Could we all be brains in a vat? It is conceivable that someone is a brain in a vat. It thus seems conceivable that I am a brain in a vat, and it may seem conceivable that we are all brains in a vat. But that is an illusion, argues Putnam – at least for certain ways of constraining the thought experiment (Putnam 1981b). In a nutshell, I can't conceive that I am a brain in a vat because in order to do so I have to be a non-envatted brain. If I am envatted, my concepts are also envatted, and so I am not able to think my hypothetical situation. Since I can come to know this by reading Putnam's paper (or seeming to read it), I come to realize that, like Cartesian skepticism about my own existence, brain-in-a-vat skepticism about myself is not really conceivable if true. Moreover, since we can conceive of brain-in-a-vat skepticism, it cannot possibly be true.
Now, I am no skeptic. I don't regard it as a worthy philosophical position. But I also don't think it is refutable. My non-skepticism is more like an axiom than the result of an argument. Similar broad principles include naturalism and induction. They all seem like good first principles, even though attempts to justify them appear circular. Moreover, I am convinced by Wittgenstein's arguments in On Certainty both that the skeptic's demands are unreasonable and (thus) that they cannot be met. The skeptic's does not engage in the ordinary language “game” of asking for and providing justification, though he seems to. So although one cannot meet the skeptic's demand we can reject those demands as unreasonable.
Having been convinced by Wittgenstein that skepticism is not refutable, any argument that presents itself as refuting skepticism awakens my – well – skepticism. Now Putnam's more general target is “metaphysical realism” – the approach to truth that seems to imply that global skepticism presents us with a real possibility. Since a view can be possible but false, the mere falsity of a skeptical hypothesis does not undermine realism. For that Putnam needs to show that skepticism cannot possibly be true.
Did Putnam do the impossible? Did he refute skepticism? I will argue that what Putnam shows is the unthinkability of BIV skepticism (if it is true).
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