“How may praxeology be applied to forecasting, to the prediction of future historical events? The process is essentially that of a the historian, except that the difficulties are greater. Thus, using the above example the forecaster may see a considerable increase in the money supply take place. He asserts [(2)]; [(3)] he knows as a praxeological truth. In order to forecast the probable future course of purchasing power, he must make an estimate of the probable course of the demand for money in the period under consideration.
If, on the basis of his judgement, he decides that the relative change in demand will be negligible, he is in a position to predict that the purchasing power of the money unit will decline in that period. With the help of praxeology, his judgement is he best he can offer, but it is still inexact, dependent on the correctness of his estimate—in this case, of the movement in demand for money. If he wishes to make a quantitative estimate of the change in purchasing power, his estimate is still more inexact, for praxeology can be of no help in this attempt. If his prediction proves erroneous, it is not praxeology that has failed, but his judgment of the future behavior of the elements in the praxeological theorem.
Praxeology is indispensable, but it does not provide omniscience. It furnishes laws in the form of: If X, and if Y remains unchanged, then Z. It is up to the historian, and his counterpart, the forecaster, to determine the specific cases in which the law is applicable. It should now be clear that there are no praxeological laws of historical development, and that neither Mises nor myself need “reconcile” any “dilemmas” in setting forth such a law. If there were, then the task of the historian would be far easier than it is. Historical events are complex results of numerous causal factors, praxeological, psychologic, physical, chemical, biological, etc. The historian must determine which science and its laws apply, and, more difficult, the extent to which each causal factor operated in the events he is attempting to explain or predicts.”
— Murray N. Rothbard, Praxeology: Reply to Mr Schuller