Kant, in the course of his critique of classical empiricism, in particular that of David Hume, developed the idea that all our propositions can be classified in a two-fold way:
- On the one hand they are either analytic or synthetic,
- and on the other they are either a priori or a posteriori.
The meaning of these distinctions is, in short, the following. Propositions are analytic when ever the means of formal logic are sufficient in order to find out whether they are true or not; otherwise propositions are synthetic ones. And propositions are a posteriori whenever observations are necessary in order to establish their truth or at least confirm them. If observations are not necessary then propositions are a priori.
The characteristic mark of Kantian philosophy is the claim that true a priori synthetic propositions exist—and it is because Mises subscribes to this claim that he can be called a Kantian. Synthetic a priori propositions are those whose truth-value can be definitely established, even though in order to do so the means of formal logic are not sufficient (while, of course, necessary) and observations are unnecessary.
According to Kant, mathematics and geometry provide examples of true a priori synthetic propositions. Yet he also thinks that a proposition such as the general principle of causality—i.e.,the statement that there are time-invariantly operating causes, and every event is embedded into a network of such causes—is a true synthetic a priori proposition. I cannot go in to great detail here to explain how Kant justifies this view. A few remarks will have to suffice. First, how is the truth of such propositions derived, if formal logic is not sufficient and observations are unnecessary? Kant’s answer is that the truth follows from self-evident material axioms.
What makes these axioms self-evident? Kant answers, it is not because they are evident in a psychological sense, in which case we would be immediately aware of them. On the contrary Kant insists, it is usually much more painstaking to discover such axioms than it is to discover some empirical truth such as that the leaves of trees are green. They are self-evident because one cannot deny their truth without self-contradiction; that is, in attempting to deny the money would actually implicitly admit their truth.
How do we find such axioms? Kant answers, by reflecting upon ourselves, by understanding ourselves as knowing subjects. And this fact—that the truth of a priori synthetic propositions derives ultimately from inner, reflectively produced experience—also explains why such propositions can possibly have the status of being understood as necessarily true. Observational experience can only reveal things as they happen to be; there is nothing in it that indicates why things must be the way they are. Contrary to this, however, writes Kant, our reason can understand such things as being necessarily the way they are, “which it has itself produced according to its own design.”
In all this Mises follows Kant. Yet, as I said earlier, Mises adds one more extremely important insight that Kant had only vaguely glimpsed. It has been a common quarrel with Kantianism that this philosophy seemed to imply some sort of idealism. For if, as Kant sees it, true synthetic a priori propositions are propositions about how our mind works and must of necessity work, how can it be explained that such mental categories fit reality? How can it be explained, for instance, that reality conforms to the principle of causality if this principle has to be understood as one to which the operation of our mind must conform? Don’t we have to make the absurd idealistic assumption that this is possible only because reality was actually created by the mind? So that I am not misunderstood, I do not think that such a charge against Kantianism is justified.
And yet, through parts of his formulations Kant has no doubt given this charge some plausibility.
Consider, for example, this programmatic statement of his: “So far it has been assumed that our knowledge had to conform to observational reality”; instead it should be assumed “that observational reality conform to our knowledge.”
Mises provides the solution to this challenge. It is true, as Kant says, that true synthetic a priori propositions are grounded in self-evident axioms and that these axioms have to be understood by reflection upon ourselves rather than being in any meaningful sense “observable.” Yet we have to go one step further. We must recognize that such necessary truths are not simply categories of our mind, but that our mind is one of acting persons. Our mental categories have to be understood as ultimately grounded in categories of action. And as soon as this is recognized, all idealistic suggestions immediately disappear. Instead, an epistemology claiming the existence of true synthetic a priori propositions becomes a realistic epistemology. Since it is understood as ultimately grounded in categories of action, the gulf between the mental and the real, outside, physical world is bridged. As categories of action, they must be mental things as much as they are characteristics of reality: For it is through actions that the mind and reality make contact.
Kant had hinted at this solution. He thought mathematics, for instance, had to be grounded in our knowledge of the meaning of repetition, of repetitive operations. And he also realized, if only some what vaguely that the principle of causality is implied in our understanding of what it is and means to act.
Yet it is Mises who brings this insight to the foreground: Causality; he realizes, is a category of action. To act means to interfere at some earlier point in time in order to produce some later result, and thus every act or must presuppose the existence of constantly operating causes. Causality is a prerequisite of acting, as Mises puts it.
But Mises is not, as is Kant, interested in epistemology as such. With his recognition of action as the bridge between the mind and the outside reality; he has found a solution to the Kantian problem of how true synthetic a priori propositions can be possible. And he has offered some extremely valuable insights regarding the ultimate foundation of other central epistemological propositions besides the principle of causality; such as the law of contradiction as the cornerstone of logic. And he has thereby opened a path for future philosophical research that, to my knowledge, has hardly been traveled. Yet Mises’s subject matter is economics, and so I will have to lay to rest the problem of explaining in more detail the causality principle as an a priori true proposition.
— Hans-Hermann Hoppe, Economic Science and the Austrian Method, pp. 17-21
 A brilliant interpretation and justification of Kant’s a prioristic epistemology is to be found in F. Kambartel, Erfahrung und Struktur. Bausteine zu einer Kritik des Empirismus und Formalismus (Frankfurt/M.: 1968), esp. chapter 3; see also Hans-Hermann Hoppe, Handeln und Erkennen (Bern: 1976).
 Immanuel Kant, Kritik der reinen Vernunft, in Kant, Werke, vol.2, W. Weis-chedel, ed. (Wiesbaden: 1956), p.23.
 See in particular F. Kambartel’s work cited in note 12; instructive is also the Kant interpretation given by the biologist-ethologist K. Lorenz, Vom Weltbild des Verhaltensforschers (Munich: 1964); idem, Die Ruckseite des Spiegels. Ursucheiner Naturgeschichte menschlichen Erkennens (Munich: 1973). Among some followers of Austrianism, the Kant interpretation of Ayn Rand (see, for instance, her Introduction to Objectivist Epistemology (New York: New American Library, 1979); or For the New Intellectual (New York: Random House, 1961) enjoys great popularity. Her interpretation, replete with sweeping denunciatory pronouncements, however, is characterized by a complete absence of any interpretive documentation whatsoever. See, on Rand’s arrogant ignorance regarding Kant, B. Goldberg, ‘Ayn Rand’s ‘For the New Intellectual’,” New Individualist Review 1, no. 3 (1961).
 For Kantian interpretations of mathematics see H. Dingler, Philosophic der Logik und Mathematik (Munich: 1931); Paul Lorenzen, Einfiihrung in die operative Logik und Mathematik (Frankfurt/M.: 1970); Ludwig Wittgenstein, Remarks on the Foundations of Mathematics (Cambridge, Mass.: M.L.T Press, 1978); also Kambartel, Erfahrung und Struktur, pp. 118-22; for an unusually careful and cautious interpretation of Kantianism from the point of view of modern physics, see P.Mittelstaedt, Philosophische Probleme der modernen Physik (Mannheim: 1967).
 For some farther reaching considerations on these matters, see Hoppe “In Defense of Extreme Rationalism.”