Mises's Fundamental Axiom, the Nub of Praxeology, Is the Existence of Human Action; Men Have Some Ends and They Use Some Means To Try To Attain Them

We turn now to the Fundamental Axiom (the nub of praxeology): the existence of human action. From this absolutely true axiom can be spun almost the whole fabric of economic theory. Some of the immediate logical implications that flow from this premise are: the means-ends relationship, the time-structure of production, time-preference, the law of diminishing marginal utility, the law of optimum returns, etc. It is this crucial axiom that separates praxeology from the other methodological viewpoints-and it is this axiom that supplies the critical “apriori” element in economics.

First, it must be emphasized that whatever role “rationality” may play in  Professor Machlup's theory, it plays no role whatever for Professor Mises. Hutchison charges that Mises claims “all economic action was (or must be) ‘rational.’ ” This is flatly incorrect. Mises assumes nothing whatever about the rationality of human action (in fact, Mises does not use the concept at all). He assumes nothing about the wisdom of man's ends or about the correctness of his means. He “assumes” only that men act, i.e., that they have some ends, and use some means to try to attain them. This is Mises’ Fundamental Axiom, and it is this axiom that gives the whole praxeological structure of economic theory built upon it its absolute and apodictic certainty.

—Murray N. Rothbard, “In Defense of ‘Extreme Apriorism,’” Southern Economic Journal 23, no. 3 (January 1957): 317.