The problem with writing a biography of Socrates, as Bettany Hughes merrily admits, is that he’s a “doughnut subject”: a rich and tasty topic with a big hole right in the middle where the main character should be. Despite his fame and his insistence on an examined life, Socrates never wrote anything, and our knowledge of him comes mainly from three contemporaries — his devoted pupils Plato and Xenophon, and the parodist Aristophanes — each of whom had his own agenda. He produced no great answers, only great questions, and the most enduring image we have of his life is his leaving of it, as the title of this book suggests.