Moore's 'Proof of an External World' has evoked a variety of responses from philosophers, including bafflement, indignation and sympathetic reconstruction. I argue that Moore should be understood as following Thomas Reid on a variety of points, both epistemological and methodological. Moreover, Moore and Reid are exactly right on all of these points. Hence what I present is a defence of Moore's 'Proof', as well as an interpretation. Finally, I argue that the Reid-Moore position is useful for resolving an issue that has recently received attention in epistemology, namely, how is it that one knows that one is not a brain in a vat?