The jargon of economics and finance contains numerous colorful terms for market-asset prices at odds with any reasonable economic explanation. Examples include "bubble," "tulipmania," "chain letter," "Ponzi scheme," "panic," "crash," "herding," and "irrational exuberance." Although such a term suggests that an event is inexplicably crowd-driven, what it really means, claims Peter Garber, is that we have grasped a near-empty explanation rather than...