Descartes is one of the greatest of all thinkers. Modern philosophy is generally taken to begin with him. His unique contribution to Western thought covers not only philosophy but also science and mathematics; his studies in mechanics and optics have provided modern science with tools still used and work still built on today. This is the first intellectual biography of Descartes in English. Stephen Gaukroger traces his intellectual development from childhood, establishes the connections between his intellectual and personal life, and placing these in the context of the cultural environment of the time, offers a fundamental reassessment of all aspects of his life and work. It is usually assumed that there is a little development in Descartes' thought, but this biography shows evidence of very significant changes of view and a general shift in his concern away from natural philosophy following the condemnation of Galileo by the Church in 1633. Starting with a full account of Descartes' early scientific work, Dr Gaukroger shows how it informed and influenced his later philosophical studies. On this new view, Descartes' philosophical work was meant not a self-contained exercise in epistemology and scepticism, but rather as a defence of his physical doctrines against a hostile Church. This book allows for the first time a full understanding of Descartes' ideas in the context of his life and times. It will be welcomed by all readers interested in the origins of modern thought.