In Reason, Truth and History, Hilary Putnam offers an argument against the skeptical hypothesis that we are brains in a vat. He believes that whether or not such a scenario obtains, when I utter the words “I am a brain in a vat” I am saying something false.
Putnam’s argument, however, argument has little force against the skeptic because: (1) it does not address the skeptic’s real concern and (2) even if it succeeds, it only applies to a very limited number of skeptical scenarios.
Putnam presents a very specific skeptical scenario in which the universe consists solely of a vat full of brains (and nervous systems) and a computer that is programmed to feed these brains a “collective hallucination.” This situation is not the result of an evil demon (as suggested by Descartes) or a
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I can utter the same sentences as a human in the real world can, but not the same propositions—because I lack the appropriate causal contact with the real world. However, just because I cannot refer to my own situation, it does not follow that that situation does not obtain. In other words, just because a BIV cannot utter the proposition, “I am a brain in a vat,” it does not follow that it is not a BIV. Thus, Putnam’s argument fails to address the skeptic’s real concern.
A further objection to this argument is that it seems to trade one kind of skepticism for another. According to Thomas Nagel, if the skeptic accepts that he/she cannot actually express a skeptical proposition such as “Maybe I am a brain in a vat,” then he/she can recast the skeptical argument as follows: “Perhaps I can’t even think the truth about what I am, because I lack the necessary concepts and my circumstances make it impossible for me to acquire them!” Nagel concludes that if this is not skepticism, he does not know what