In chapter 1 of Reason, Truth, and History, Hilary Putnam argues from some plausible assumptions about the nature of reference to the conclusion that it is not possible that all sentient creatures are brains in a vat. If this argument is successful, it seemingly refutes an updated form of Cartesian skepticism concerning knowledge of physical objects. In this paper, I will state what I take to be the most promising interpretation of Putnam's argument. My reconstructed argument differs from an argument strongly suggested by Putnam's text. I will show that the latter argument obviously does not work. The more promising argument which I reconstruct on behalf of Putnam raises some interesting questions about the relation between the contents of one's beliefs and one's environment and about how this relation affects the evaluation of anti-skeptical arguments. I conclude that my reconstructed argument ultimately fails as a response to Cartesian skepticism: the argument engenders a skepticism about knowledge of meaning, or propositional content, which undercuts its anti-skeptical force.