Rene Descartes' Meditations on First Philosophy, first published in 1641, is one of the most famous and influential thought experiments in the history of ideas. In six meditations the author sets out a long and subtle argument describing his quest for certainty. Casting aside all knowledge that he can in any way doubt, he explores what he can know with absolute certainty. This exercise brings him to the famous conclusion: I am thinking; therefore, I am. Having established this basic truth, Descartes then proceeds to build an argument to demonstrate other certainties. These include the existence of God and of external material objects.The argument in the Mediations has frequently been criticized, even when it first appeared, but it remains a fascinating and influential work and an important introduction to modern philosophy. Not the least of its contributions to modern thought is the famous Cartesian dualism: the radical split between a natural world (including the human body) governed by mechanical laws and the human mind. This dualism is still a major problem (perhaps the major problem) in modern biology.