The study of the history of investment theory shows the merging of finance, statistics, predictability, and economics into the current investment theory. An analysis of the history of a field clarifies its current paradigms and contrasts them with the previous and the competing paradigms. The notion that investment theory is a young science since the 1960s is flawed. Finance as relevant for investment theory has existed since the thirteenth century. The legitimization in the 1860s of investing in stocks, which until the nineteenth century were associated with gambling, results in efficient market theories. Investment theory in France in the nineteenth century and in the United States in the first half of the twentieth century is full of treasures, illustrated by the work of Regnault, Bachelier, Fisher, Cowles, Macaulay, Williamson, and many others.