 Hilary Putnam (1981) proposed an interesting and much discussed attempt to refute a skeptical argument that is based on one form of the brain-and-a-vat scenario. In turn, Putnam’s attempted refutation is based on content externalism (also known as semantic externalism). On this view, the referents and meanings of various types of singular and general terms, as well as the propositions expressed by sentences containing such terms, are determined by aspects of the speaker’s external environment. In this entry, we will consider the basic features of and problems with Putnam’s original argument, and we will also present and discuss several of the most important attempts to reconstruct or improve upon that argument.