Ludwig von Mises
The Task and Scope of
the Science of Human Action
II. The Scope and Meaning of the System of A Priori Theorems
3. Theory and the Facts of Experience
The science of action deals only with those problems whose solution directly or indirectly affects practical interests. It does not concern itself, for reasons already explained, with the complete development of a comprehensive system embracing all the conceivable categories of action in their broadest generality. The peculiar advantage of this procedure is that, by giving preference to the problems encountered under the actual conditions in which action takes place, our science is obliged to direct its attention to the facts of experience. It is thereby prevented from forgetting that one of its tasks consists in the determination of the boundary between the conditions of action accessible to and requiring categorial comprehension, on the one hand, and the concrete data of the individual case, on the other. The theory must constantly concern itself with the actual facts of the individual and nonrepeatable case because only this offers it the possibility of showing where (conceptually, though perhaps not spatially, temporally, or in some other respect that would be perceptible to the senses) the realm of theoretical comprehension ends and that of historical understanding begins. When the science that aims at universally valid knowledge has so perfected its methods as to reach the furthest limit to which the theory can be pursued?that is, the point at which no condition of action open to categorial comprehension remains outside its range if experience has demonstrated the advisability of its inclusion?that science will still be obliged to treat also a part of the problems of descriptive, statistical, and historical research. Otherwise it could in no way succeed in recognizing and marking off its own domain. This task of demarcation is proper to it, and not to the empirical, descriptive sciences, because it is logically prior to them.
To be sure, even this procedure conceals many dangers. Sometimes one neglects to distinguish the universally valid from the historical; the methods are confounded, and then unsatisfactory results are obtained. Thus Böhm-Bawerk's ingenious exposition of the theory of interest, for example, suffered especially from an insufficient separation between the two modes of procedure.
 Supra, pp. 14 ff.