ABSTRACT: AThe topic of this paper is the familiar problem of skepticism about the external world. How can you know that you are not a brain in a vat being fooled by alien scientists? And if you can’t know that, how can you know anything about the external world? The paper assumes Evidentialism as a theory about justification, and then argues that you are justified in believing that you are not a brain in a vat, in virtue of the fact that your evidence supports that belief. The paper also considers a number of different objections to this proposal. The upshot is that you do know that you are a not a brain in a vat, and that you also know lots of things about the external world.