A Brain in a Vat

Descartes: "an evil genius, as clever and deceitful as he is powerful, who has directed his entire effort to misleading me." ("An evil genuius not less powerful than deceitful, [who has] employed his energies in deceiving me ...") (IP3 45a)

Note: Descartes' evil genius hypothesis is not the same as the "brain in a vat" hypothesis, since the latter, although not the former, assumes the existence of material things.