One weekend last June, in an auditorium in the German city of Karlsruhe, the philosopher Peter Sloterdijk celebrated his seventieth birthday by listening to twenty lectures about himself. A cluster of Europe’s leading intellectuals, academics, and artists, along with a smattering of billionaires, were paying tribute to Germany’s most controversial thinker, in the town where he was born and where he recently concluded a two-decade tenure as the rector of the State Academy for Design. There were lectures on Sloterdijk’s thoughts on Europe, democracy, religion, love, war, anger, the family, and space. There were lectures on his commentaries on Shakespeare and Clausewitz, and on his witty diaries, and slides of buildings inspired by his insights. Between sessions, Sloterdijk, who has long, straw-colored hair and a straggly mustache, prowled among luminaries of the various disciplines he has strayed into, like a Frankish king greeting lords of recently subdued fiefdoms. The academy bookstore was selling most of his books—sixty-odd titles produced over the past forty years. The latest, “After God,” was displayed on a pedestal in a glass cube.