Several years before the financial crisis descended on us, I put forward the concept of "black swans": large events that are both unexpected and highly consequential. We never see black swans coming, but when they do arrive, they profoundly shape our world: Think of World War I, 9/11, the Internet, the rise of Google.

In economic life and history more generally, just about everything of consequence comes from black swans; ordinary events have paltry effects in the long term. Still, through some mental bias, people think in...