In fact, is is not a mere empirical accident that not all the contingent markets needed for efficiency exist but a necessary fact with deep implicati… - Kenneth Arrow

There is little warrant for the belief that we know the laws of history well enough to make projections of any great reliability. Most of the turning points of history, great and small, were surprises to both their participants and the analysts of the day, whatever their doctrine.

The businessman may be compared with two other types of individuals who are essentially concerned with behavior under uncertainty ― the scientist and the statistician. The scientist must choose, on the basis of limited information, among the innumerable logically conceivable laws of nature, a limited number. He cannot know whether his decisions are right or wrong, and, indeed, it is none too clear what is meant by those terms. There is a long history of attempts to reduce scientific method to system, including many which introduce probability theory, but it cannot be said that any great formal success has attended these efforts. If we were to compare the businessman to the scientist, we would be forced to the melancholy conclusion that little of a systematic nature can be said about the former’s decision-making processes.
The statistician typically finds himself in situations more similar to that of the businessman. The problem of statistics can be formulated roughly as follows. It is known that one out of a number of hypotheses about a given situation is true. The statistician has the choice of one of a number of different experiments (a series of experiments can be regarded as a single experiment, so that drawing a sample of any size can be included in this schema), the outcome of any one of which is a random variable with a probability distribution depending on which of the unknown hypotheses is correct. On the basis of that outcome, the statistician must take some action (accept or reject a hypothesis, estimate the mean of a distribution to be some particular value, accept or reject a lot of goods, recommend a change in production methods, and so on), the consequences of which depend on the action taken and on the hypothesis that is actually true.

L. Walras first formulated the state of the economic system at any point of time as the solution of a system of simultaneous equations representing the demand for goods by consumers, the supply of goods by producers and the equilibrium condition that supply equal demand on every market.