Descarte's Meditation one

…

…mislead a person into thinking for instance, that a triangle has four sides or some other false belief, there is no realm of perception that is not touched by the argument. The Evil Genius argument allows Descartes to logically distrust and doubt a priori and a posterior knowledge. This argument completes Descartes' mission for a logical argument that will allow him to doubt everything, and start from the bottom of the hierarchy of his beliefs.