Today people are fully aware of the fact that credit expansion is the ultimate cause of the slump. All public declarations on the state of business, even those uttered by bureaucrats, are based upon a full acknowledgment of the monetary doctrine of the trade cycle. It is precisely the cognition of this theory's correctness that in the present boom period alarms businessmen and prompts them to inquire nervously about the date of the turning point. Not Quantitative Economics predicts the outcome of definite modes of conduct, in our case, of a policy of credit expansion. But this prediction is qualitative only.
Economic prediction can never disclose anything about the quantitative relations concerned.
There is not, and there cannot be such a thing as quantitative economics. In the field of the natural sciences there prevail constant relations between definite magnitudes. By means of laboratory experiments the scientists are in a position to determine these constants and to make practical use of them in predictions and in technological design. But in human action there are no such constant relations between magnitudes. There, all quantities are variables or, as a more appropriate term describes them, historical data.