" The fundamentals of the economy remain strong.' That cliche is repeated by authorities as they try to restore public confidence after every major stock market decline. They have the opportunity to say this because just about every major stock market decline appears inexplicable if one looks only at the factors that logically ought to influence stock markets. It is practically always the stock market that has changed; indeed the fundamentals haven't.

How do we know that these changes could not be generated by fundamentals? If prices reflect fundamentals, they do so because those fundamentals are useful in forecasting future stock payoffs. In theory the stock prices are the predictors of the discounted value of those future income streams, in the form of future dividends or future earnings. But stock prices are much too variable. They are even much more variable than those discounted streams of dividends (or earnings) that they are trying to predict. "

― George A. Akerlof , Animal Spirits: How Human Psychology Drives the Economy, and Why It Matters for Global Capitalism