  Russell (1988, 148; 1996) notes that CORNEA seems to open the door to something like Cartesian skepticism. Recall the condition CORNEA puts in place, the thing it says one needs a reason to believe: if p were not the case, s would likely be different than it is in some way discernible by her. Now consider the following possibility: you are a brain being kept alive in a vat of nutrients and stimulated in a way that causes your current experiences and memory impressions. Let p be I am not a brain in a vat, indexed to you, and consider whether you have reason to think things would look different for you if that were false, that is, if you were a brain in a vat. Ex hypothesi, they would not. So CORNEA seems to lead quickly to the skeptical conclusion that we do not know that we are not mere brains in vats. McBrayer (2009) expands at length on this, making the argument in more detail, also incorporating analogies to the epistemology of Nozick and Dretske. 