Marcus Aurelius ruled the Roman Empire from 161 to 180 CE, presiding over one of the most powerful political entities in human history at a time of almost continuous crisis: devastating plague, relentless military campaigns on multiple frontiers, economic strain, and the constant pressure of court politics. He is remembered by historians as one of the most capable and conscientious rulers of antiquity. He is remembered by philosophers as the clearest example in recorded history of a person who actually lived by the philosophical principles they professed. And he left behind, entirely by accident, one of the most intimate and...