The 1990s have witnessed one of the great bull stock markets in American history. Whether that means an unstable bubble has developed in its wake is difficult to assess. A large number of analysts have judged the level of equity prices to be excessive, even taking into account the rise in "fair value" resulting from the acceleration of productivity and the associated long-term corporate earnings outlook.
But bubbles generally are perceptible only after the fact. To spot a bubble in advance requires a judgment that hundreds of thousands of informed investors have it all wrong. -- Alan Greenspan, testimony before the Joint Economic Committee of Congress, June 17, 1999
"The consensus of judgment of the millions whose valuations function on that admirable market, the Stock Exchange, is that stocks are not at present over-valued.... Where is that group of men with all-embracing wisdom which will entitle them to veto the judgment of the intelligent multitude?" -- Prof. Joseph Lawrence of Princeton in 1929, before the Great Crash (quoted in Robert Shiller, Irrational Exuberance)