A well-publicized pithy maxim about the stock market in the late 1990s. Alan Greenspan was Chairman of the Fed in those heady days and he couldn't figure out why stocks traded at such high multiples relative to their earnings. Or maybe he was just torqued because his buddies were making the sick money and he wasn't. Anyway, he testified before Congress and uttered his deathless opinion that valuations were "irrational" as the exuberance of the market had pushed the S&P 500 to trade at all time high multiples of about 27x earnings. For a few days, the markets collapsed, but then exuberance again took hold and the markets continued their rise.