In his Meditations, Marcus Aurelius described a mental exercise he returned to repeatedly throughout his reign as emperor. He would imagine lifting himself out of his immediate situation, the palace intrigue, the military campaigns, the petitioner waiting outside his door, the pain in his joints, the grief over a dead child, and rising above it all, surveying the breadth of the empire, then the earth itself, then the cosmos. From that imagined height, the conflicts consuming his attention would appear in their actual proportions: small, temporary, contingent, part of an unimaginably vast pattern of cause and effect in which any...