The brain-in-a-vat skeptical argument (and its terrible image(!?) really is needed in making clear what Putnam is known for in epistemology. I am a professional epistemologist and when Putnam comes to mind, we first think of his externalism-about-reference argument against the brain-in-the-vat skeptical argument. The reason BIVs must be explicitly referred to here is that the BIV argument for skepticism is actually supposed to take on, in its structure, the kind of argument Descartes had in mind in books 1 and 2 of the Meditations. however, Putnam's external reference strategy exploits features of the BIV scenarioa scenario that could come about even if one rejects (a priori) supernaturalism of the sort Descartes envisions in his evil demon argument. Put another way, Putnam's external reference argument not only tries to challenge a skeptical argument with the power of Descartes' evil demon argument, but moreover, one for which the relevant thought experiment is consistent with philosophical naturalism.