Louis Bachelier

“The factors that determine activity on the Exchange are innumerable, with events, current or expected, often bearing no apparent relation to price variation. Beside the somewhat natural causes for variation come artificial causes: The Exchange reacts to itself, and the current trading is a function, not only of prior trading, but also of its relationship to the rest of the market. The determination of this activity depends on an infinite number of factors. It is thus impossible to hope for mathematical forecasting. Contradictory opinions about these variations are so evenly divided that at the same instant buyers expect a rise and sellers a fall.

The calculus of probability can doubtless never be applied to market activity, and the dynamics of the Exchange will never be an exact science. But it is possible to study mathematically the state of the market at a given instant- that is to say, to establish the laws of probability for price variation that the market at the instant dictates. If the market, in effect, does not predict its fluctuations, it does not assess them as being more or less likely, and this likelihood can be evaluated mathematically.”
― Louis Bachelier, Louis Bachelier's Theory of Speculation: The Origins of Modern Finance