Several authors have argued that, assuming we have apriori knowledge of our own thought-contents, semantic externalism implies that we can know apriori contingent facts about the empirical world. After presenting the argument, I shall respond by resisting the premise that an externalist can know apriori: If s/he has the concept water, then water exists. In particular, Boghossian's Dry Earth example suggests that such thought-experiments do not provide such apriori knowledge. Boghossian himself rejects the Dry Earth experiment, however, since it would (...) imply that externalism is true of empty concepts as well as non-empty concepts. Yet in this paper I respond by defending empty-concept externalism, from criticisms suggested by Boghossian and Brown, and recently developed further by Besson. My contention is that an externalist can give a non-ad hoc descriptivist account of empty concepts. Accordingly, apriori self-knowledge does not enable an externalist to know contingent features of the external world. (shrink)