The French thinker René Descartes is one of the best-known philosophers in the world. His brief dictum "I think, therefore I am" has become part of all humanity's cultural heritage. Just as Columbus discovered an unknown continent, the so-called "New World", Descartes succeeded in opening up a new dimension of knowledge and altering our view of reality. Prior to Descartes, people in the Christian West believed, for over a thousand years, in the Bible as testimony to God's revealed truth. Then came Descartes with the radical demand that knowledge had from then on to rely on a basis of absolute certainty: "It was always my most earnest desire to learn to distinguish the true from the false". He posed the fundamental question: how does one arrive at sure and certain knowledge? On what can I really rely? On what I see and hear? On thought and logic? Or perhaps on what I have learned from my earliest childhood on? His radical answer runs: on nothing at all! We must doubt absolutely everything! In his famous Meditations on First Philosophy Descartes describes his search for this absolutely certain knowledge. The result he arrives at: I can doubt anything and everything but in the moment of my doubting I cannot doubt that there is an "I" that thinks and doubts: "I think, therefore I am". Is thinking really the human trait that decides everything about us? Is there really nothing in the world except "thinking" within us and thoughtless, soulless bodies outside of us? Is it science's task to subjugate matter, plants and animals and even the human body itself? Descartes does more than just lay the ground for modern science. In a sense, his thought has become our destiny, both for good and for bad. The book appears as part of the well-loved series Great Thinkers in 60 Minutes which has now been translated worldwide into six languages.