How can we know if we are a brain in a vat? Can we be sure that we are not the playthings of evil demons? These questions have been discussed by many philosophers in the past and still we do not have a proof that we are not some demon’s plaything. Yet, at least two prominent philosophers, René Descartes and John Locke believed there are ways to prove that we are not brains in vats. At one point in his philosophy, Descartes reflected that perhaps God or some evil spirit was constantly tricking his mind, causing him to believe what was false. ....