Ten years ago yesterday, Alan Greenspan made what was to become the most famous speech in his 18-year tenure as chairman of the Federal Reserve. Against a backdrop of a strong economy and soaring stock market, Mr. Greenspan said: "How do we know when irrational exuberance has unduly escalated asset values? . . . We should not underestimate . . . the interactions of asset markets and the economy. Asset prices, particularly, must be an integral part of the development of monetary policy." Stocks indeed were surging when Mr. Greenspan spoke at the annual dinner of the American Enterprise Institute...