 A number of skeptical hypotheses or scenarios have been proposed which can be used as the basis for arguments to the effect that we lack knowledge of various propositions about objects in the external world, propositions that we normally take for granted and that we assume are obviously true. Descartes’ Evil Genius hypothesis is perhaps the best known such hypothesis. Another is the brain-in-a-vat hypothesis, according to which human beings are brains in vats whose mental experiences, although qualitatively the same as in their normal lives, are all caused by a supercomputer. The skeptic then argues that many of the propositions about the external world that we take for granted (such as “Trees are green”) are propositions that we don’t know, since our evidence for such propositions is often exactly the same as the evidence we would have in a vat-world in which these propositions are false.