The University of Tübingen has a history of innovative thought, particularly in theology, in which the university and the Tübinger Stift are famous to this day. Philipp Melanchthon (1497–1560), the prime mover in building the German school system and a chief figure in the Protestant Reformation, helped establish its direction. Among Tübingen's eminent students (and/or professors) have been the astronomer Johannes Kepler; the economist Horst Köhler (President of Germany); Joseph Ratzinger (Pope Benedict XVI), poet Friedrich Hölderlin, and the philosophers Friedrich Schelling and Georg Wilhelm Friedrich Hegel. "The Tübingen Three" refers to Hölderlin, Hegel and Schelling, who were roommates at the Tübinger Stift. Theologian Helmut Thielicke revived postwar Tübingen when he took over a professorship at the reopened theological faculty in 1947, being made administrative head of the university and President of the Chancellor's Conference in 1951.