 But even if we leave aside the question of whether we in fact know anything in the incredibly strong sense required for Stoic epistêmê, there are still some serious puzzles about the cognitive impression. The Stoics insist that the cognitive impression not only “arises from what is and is stamped and impressed in accordance” with the thing from which it arises, but also that it is “such as could not arise from that which is not.” But it seems that we can imagine all kinds of situations in which we might be in a position where the sense impressions that we have are indistinguishable from ones that misrepresent the world. Thus, consider Descartes’ evil demon hypothesis or its modern counterpart, the brain in a vat scenario. In the latter example it is stipulated that electrical stimulation of your brain by incredibly clever but unscrupulous scientists produces sense impressions that are indistinguishable from the ones that you are presently having. Surely here we have a demonstration that there could not be a true impression which is such that it could not arise from what is not. No sane person thinks that these skeptical hypotheses are actually true. The point is rather that if one of them were true, our sense experience would be indistinguishable from what (we take to be) our true and accurate sense impressions of real tables, chairs and fireplaces. Doesn’t this show that there is no such thing as a cognitive impression?