The theory of valuation depends on the projection of a long-term stream of dividends whose growth rate is extraordinarily difficult to estimate. Thus, fundamental value is never a definite number. It is a fuzzy band of possible values, and prices can move sharply within this band whenever there is increased uncertainty or confusion. Moreover, the appropriate risk premiums for common equities are changeable and far from obvious either to investors or to financial economists. Thus, there is room for the hopes, fears, and favorite fashions of market participants to play a role in the valuation process.
History provides extraordinary examples of markets in which psychology seemed to dominate the pricing process, as in the tulip-bulb mania in seventeenth century Holland and the Internet bubble at the turn of the twenty-first century. It is doubtful that the current array of market prices ALWAYS represents the best estimate available of appropriate discounted value.
Nevertheless, the evidence suggest that stock prices display a remarkable degree of efficiency. Prices adjust so well to important information. Information contained in past prices or any publicly available fundamental information is rapidly assimilated into market prices. If some degree of mis-pricing exists, it does not persist for long.
"True value will always out" in the stock market. To paraphrase Benjamin Graham, ultimately the market is a weighing mechanism, not a voting mechanism.