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
2. A Priori Theory and Empirical Confirmation
New experience can force us to discard or modify inferences we have drawn from previous experience. But no kind of experience can ever force us to discard or modify a priori theorems. They are not derived from experience; they are logically prior to it and cannot be either proved by corroborative experience or disproved by experience to the contrary. We can comprehend action only by means of a priori theorems. Nothing is more clearly an inversion of the truth than the thesis of empiricism that theoretical propositions are arrived at through induction on the basis of a presuppositionless observation of "facts." It is only with the aid of a theory that we can determine what the facts are. Even a complete stranger to scientific thinking, who naively believes in being nothing if not "practical," has a definite theoretical conception of what he is doing. Without a "theory" he could not speak about his action at all, he could not think about it, he could not even act. Scientific reasoning is distinguished from the daily thinking of everyone only in seeking to go further and in not stopping until it reaches a point beyond which it cannot go. Scientific theories are different from those of the average man only in that they attempt to build on a foundation that further reasoning cannot shake. Whereas in everyday living one is usually content to accept uncritically ideas that have been handed down, to carry a burden of prejudices and misunderstandings of all kinds, and to allow fallacies and errors to pass as true in cases where it is not easy to avoid them; scientific theories aim at unity and compactness, clarity, precision, apodictic evidence, and freedom from contradiction.
Theories about action are implicit in the very words we use in acting, and still more in those we use in speaking about action. The frequently lamented semantic ambiguities that plague our efforts to achieve precision in science have their roots precisely in the fact that the terms employed are themselves the outcome of definite theories held in common-sense thinking. The supporters of historicism were able to believe that facts can be understood without any theory only because they failed to recognize that a theory is already contained in the very linguistic terms involved in every act of thought. To apply language, with its words and concepts, to anything is at the same time to approach it with a theory. Even the empiricist, who allegedly works without presuppositions, makes use of theoretical tools. They are distinguished from those produced by a scientific theory only in being less perfect and therefore also less useful.
Consequently, a proposition of an aprioristic theory can never be refuted by experience. Human action always confronts experience as a complex phenomenon that first must be analyzed and interpreted by a theory before it can even be set in the context of an hypothesis that could be proved or disproved; hence the vexatious impasse created when supporters of conflicting doctrines point to the same historical data as evidence of their correctness. The statement that statistics can prove anything is a popular recognition of this truth. No political or economic program, no matter how absurd, can, in the eyes of its supporters, be contradicted by experience. Whoever is convinced a priori of the correctness of his doctrine can always point out that some condition essential for success according to his theory has not been met. Each of the German political parties seeks in the experience of the second Reich confirmation of the soundness of its program. Supporters and opponents of socialism draw opposite conclusions from the experience of Russian bolshevism. Disagreements concerning the probative power of concrete historical experience can be resolved only by reverting to the doctrines of the universally valid theory, which are independent of all experience. Every theoretical argument that is supposedly drawn from history necessarily becomes a logical argument about pure theory apart from all history. When arguments based on principle concern questions of action, one should always be ready to admit that nothing can "be found more dangerous and more unworthy of a philosopher than the vulgar pretension to appeal to an experience to the contrary," and not, like Kant and the socialists of all schools who follow him, only when such an appeal shows socialism in an unfavorable light.
Precisely because the phenomena of historical experience are complex, the inadequacies of an erroneous theory are less effectively revealed when experience contradicts it than when it is assessed in the light of the correct theory. The iron law of wages was not rejected because experience contradicted it, but because its fundamental absurdities were exposed. The conflict between its most clearly controvertible thesis?that wages tend toward the minimum needed for subsistence?and the facts of experience should have been easily recognized. Yet it is even today just as firmly entrenched in lay discussion and public opinion as in the Marxian theory of surplus value, which, incidentally, professes to reject the iron law of wages. No past experience prevented Knapp from presenting his state theory of money,* and no later experience has forced his supporters to give up the theory.
The obstinacy of such unwillingness to learn from experience should stand as a warning to science. If a contradiction appears between a theory and experience, we always have to assume that a condition presupposed by the theory was not present, or else that there is some error in our observation. Since the essential prerequisite of action?dissatisfaction and the possibility of removing it partly or entirely?is always present, only the second possibility?an error in observation?remains open. However, in science one cannot be too cautious. If the facts do not confirm the theory, the cause perhaps may lie in the imperfection of the theory. The disagreement between the theory and the facts of experience consequently forces us to think through the problems of the theory again. But so long as a re-examination of the theory uncovers no errors in our thinking, we are not entitled to doubt its truth.
On the other hand, a theory that does not appear to be contradicted by experience is by no means to be regarded as conclusively established. The great logician of empiricism, John Stuart Mill, was unable to find any contradiction whatever between the objective theory of value and the facts of experience. Otherwise he would certainly not have made the statement, precisely on the eve of a radical change in the theory of value and price, that as far as the laws of value were concerned, there remained nothing more to be explained either in the present or in the future; the theory was quite perfect." An error of this kind on the part of such a man must ever stand as a warning to all theorists.
* Cf. the English translation of his book with this title by H. M. Lucas and J. Bonar (London, 1924).
 Cf. Wieser, ?ber den Ursprung und die Haupigesetze des wirtschaftlichen Wertes (Wien, 1884), pp. I ff.
 Kant, Critique of Pure Reason, "Transcendental Doctrine of Elements," Part II, Second Division, Book I, Section I.
 J. S. Mill, Principles of Political Economy (London, 1867), III, 265.