How do you write a group biography of people who never actually formed a group? Such is the challenge Wolfram Eilenberger sets himself in a book about the philosophers Martin Heidegger, Ludwig Wittgenstein, Walter Benjamin and — the surprisingly unstarry fourth subject — Ernst Cassirer, an urbane and now nearly forgotten neo-Kantian who might have deserved the made-up title of ‘symbologist’, thus far reserved for the heroes of Dan Brown’s novels.