Knowing that one knows what one is talking about

Twin-earth thought experiments, standardly construed, support the externalist doctrine that the content of propositional attitudes involving natural-kind terms supervenes upon properties external to those who entertain them. But this doctrine in conjunction with a common view of self-knowledge might have the intolerable consequence that substantial propositions concerning the environment could be knowable a priori. Since both doctrines, externalism and privileged self-knowledge, appear independently plausible, there is then a paradox facing the attempt to hold them concurrently. I shall argue, however, that externalist claims about the dependence of content on environmental factors presuppose certain theses about the semantics of natural-kind terms that, if sound, would make those claims eligible for empirical justification instead. In fact, that is the only interpretation of their epistemic status that could square with the standard conclusion from twin-earth cases. Furthermore, it can be shown to solve the paradox of externalism and self-knowledge in a more doxastically conservative way--accommodating precisely each of the well-accepted intuitions about knowledge, closure and the individuation of content given up by available competitors.