Putnam's proof (1981b) that we are not brains in vats (BIVs) is often construed as a semantic response to epistemological skepticism. In particular, the proof has typically been assumed to rely on semantic externalism, that is, the view that the semantic contents of referring terms depend on features of the external environment in a constitutive sense. This chapter provides a critical assessment of that assumption. Crucially, all that the best formulation of the proof relies on is a causal constraint on reference, which should be distinguished from a causal theory of reference. Some semantic internalists accept such a constraint in virtue of combining their view with the claim that reference is determined by satisfaction of causal descriptions. It turns out the semantic content of such descriptions constitutively depends neither on internal features of speakers nor on the sorts of environmental features which semantic externalists typically point to. So, if semantic externalists can appeal to Putnam's proof as a semantic response to epistemological skepticism, then so can those semantic internalists who endorse causal descriptivism.