As Voltaire once remarked, 'It is the privilege of the real genius, especially one who opens up a new path, to make great mistakes with impunity.' The Copernican revolution brought about by Kant was, I think, the most important single turning point in the history of philosophy. For that reason there has been, ever since, a watershed in understanding between those who have taken his work on board and those who have not. For a good many of the problems he uncovered, the solutions he put forward have not stood the test of time, but his uncovering of the problems remains the most illuminating thing a philosopher has ever done. Because of the fundamental character of these problems, and because Kant did not solve them, confronting them has been the most important challenge to philosophy ever since.