How both you, and the BIV can know whether or not you are envatted
Epistemic Externalism offers one of the most prominent responses to the sceptical challenge. Externalism has commonly been interpreted (not least by externalists themselves) as postulating a crucial asymmetry between the actual world agent and their brain-in-a-vat (BIV) counterpart: while the actual agent is in a position to know she is not envatted, her BIV counterpart is not in a position to know that she is envatted, or in other words only the former is in a position to know whether or not she is envatted.
In this paper, I argue that there is in fact no such asymmetry: assuming epistemic externalism, both the actual world agent and their BIV counterpart are in a position to know whether or not they are envatted. After presenting the main argument, I examine to what extent the argument survives when one accepts additional externalist-friendly commitments (such as semantic externalism), and I conclude by discussing some of the implications of my conclusion to a variety of debates in epistemology.