SA constitutes a puzzle because (a) each of the premises enjoys a fair amount of plausibility. As to P1, how could I know that I’m not a bodiless brain in a vat? By waving my arms around? Wouldn’t that just give me the evidence of some arm-waving experiences that a mere brain in a vat could have? As to P2, it is just an instance of the closure principle for knowledge,[6] which many regard as axiomatic. But (b) given our intuitive anti-skepticism, C seems immensely implausible, even though (c) SA appears to be valid.