Social externalism must allow that subjects can misunderstand the content of their own thoughts. I argue that we can exploit this commitment to create a dilemma for the view’s account of communication. To arrive at the first horn of the dilemma, I argue that, on social externalism, it is understanding which is the measure of communicative success. This would be a highly revisionary account of communication. The only way that the social externalist can salvage the claim that mental content is (...) central to explaining communicative success is by adopting an account which gives unacceptable diagnoses as to the success of communicative exchanges. This is the second horn of the dilemma. Contrastingly, certain internalist views of content, which deny that subjects share thought content, do not face the dilemma. I argue that, as such, we should prefer accounts of communication which deny that subjects speak the same language. (shrink)