[Irving] Fisher was the country's first great economist, a pioneer of the mathematical approach that came to dominate the discipline after his death. Fisher saw the behavior of the market in rational, mathematical terms. He wasn't completely doctrinaire about this--earlier in his career, he had allowed that investors sometimes behaved like sheep. But in the 1920s, convinced that skilled monetary management at the Federal Reserve and the rise of new, professionally run investment trusts had reduced the riskiness of markets, he lulled himself into believing that the prices prevailing on Wall Street were a reflection of economic reality and not of investor mania or a credit bubble.
~ Justin Fox, Time, "The Myth of the Rational Market," June 22, 2009