We tend to think of Marcus Aurelius as a philosopher-king, and in a sense he was, though the ancient Romans were allergic to the word “king” and preferred Imperator, which technically just meant supreme commander of the troops, the equivalent of a modern American President being hailed as “commander-in-chief.” Emperors ever since the first one, Octavian Augustus, the adopted son of Julius Caesar, also liked to present themselves as “princeps,” which literally meant, somewhat oxymoronically,...