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.