第1页 Are Stock prices predictable? It is doubtful

1900 Louis Bachelier 'The Theory of Speculation' 
Past, present, and even discounted future events are reflected in market  price, but often show no apparent relation to price changes. . . .  [A]rtificial causes also intervene: the Exchange reacts on itself, and  the current fluctuation is a function, not only of the previous  fluctuations, but also of the current state. The determination of these  fluctuations depends on an infinite number of factors; it is, therefore,  impossible to aspire to mathematical predictions of it. . . . [T]he  dynamics of the Exchange will never be an exact science. 
这是一种自我强化的过程（self-reinforcing momentum ）
The key to Bachelier’s insight is his observation, expressed in a notably modern manner, that “contradictory opinions concerning [market] changes diverge so much that at the same instant buyers believe in a price increase and sellers believe in a price decrease.” Convinced that there is no basis for believing that—on the average—either sellers or buyers consistently know any more about the future than the other, he arrived at an astonishing conjecture: “It seems that the market, the aggregate of speculators, at a given instant can believe in neither a market rise nor a market fall, since, for each quoted price, there are as many buyers as sellers.”
 Now comes the real punch, in Bachelier’s words and with his own emphasis: “The mathematical expectation of the speculator is zero.” 法预测的
 In a disarmingly simple but perceptive statement about the nature of  security markets, he sums up his case: The probability of a rise in  price at any moment is the same as the probability of a fall in price,  because “Clearly the price considered most likely by the market is the  true current price: if the market judged otherwise, it would quote not  this price, but another price higher or lower.” 
 This conclusion led Bachelier to another important insight. The size of a  market fluctuation tends to grow larger as the time horizon stretches  out. In the course of a minute, fluctuations will be small—less than a  point in most instances. During a full day’s trading, moves of a full  point are not unusual. As the time horizon moves from a day to a week to  a month to a year and then to a series of years, the range within which  prices swing back and forth will grow ever wider. 
Bachelier认为股票的价格是无法预测的，在任何时候，股价上涨和下跌的概率都相等，当市场不再认同原有价格时，价格会发生变化，但没有人知道市场何时会变，朝上面方向变化。
 ————————————————————————————————————————
The Dow Theory, developed by Charles Dow, co-founder of Dow, Jones & Co. in 1882 and the first editor of the company’s flagship publication. The Wall Street Journal, launched in 1889.
Underlying the so-called Dow Theory is the assumption that trends in stock prices, once under way, will tend to persist until the market itself sends out a signal that these trends are about to lose their momentum and go into reverse.
道氏理论的核心假设是趋势。
Dow theorists boast that they can identify the very moment when what appears to be only a slight fluctuation is actually the first sign of the reversal of a major trend. They do not always agree among themselves, however. Disputes often arise over whether a slight fluctuation away from a trend is just a temporary setback—a “correction,” in market patois—or the onset of a new trend. Sometimes the signal appears so late that the main trend has almost exhausted itself, and stock prices are about ready to turn around and start a new trend headed in the other direction.
然而，道氏理论用来判断趋势反转的信号并不时时可靠。
Hamilton repeatedly stressed a central idea of Dow Theory that prices on the New York Stock Exchange are “sufficient in themselves” to reveal everything worth knowing about business conditions. Here Hamilton was anticipating a radical concept that was to appear long after his death. In the 1960s, a group of college professors would develop the Efficient Market Hypothesis, based on the notion that stock prices reflect all available information about individual companies and about the economy as a whole. The Efficient Market Hypothesis, however, also looks back to Bachelier, for it assumes that information is so rapidly reflected in stock prices that no single investor can consistently know more than the market as a whole knows. Hamilton, on the contrary, believed that the market itself revealed what stock prices would do in the future.
William Peter Hamilton反复强调道氏理论中价格已经包含了一切信息，即“股市是经济晴雨表”的概念，这一点与有效市场假说EMH类似。