In a sense, the entire novel is built around a favorite anecdote from classical times, the yarn about the philosopher who was killed when an eagle mistook his bald head for a rock and dropped a tortoise on it. All the rest falls rather neatly in place around the central joke. The philosopher turns into Ephebe, a whole city full of marvelous parodies of Greek thinkers. The tortoise naturally calls to mind the galactic turtle on which the Discworld muddles through the universe, which in turn suggests a new religion for the Omnians to persecute. Once the philosophical and religious conflicts are established as central concerns, Pratchett has ample scope for his whimsy, and the pages are full of allusions which are opposite to the story and at the same time a delight for the educated reader, references to Galileo, the Church Fathers, Greek...