Jason Brennan talks a priori theory again. What follows is something I originally intended as a comment at that link, but ultimately decided to turn into a blog post of my own.
Jason and the (possibly entirely imaginary?) psychologist are debating an aspect of epistemology; it has nothing to do with either psychology or economics. Consider the following.
Every child must become aware of the concept of duality. At a certain point, the child discovers that any object - let's say her security blanket, for example - is not the same entity as her self.
Now, in order to discover that there is a "me" and a "not-me," the child presumably has to do some kind of experimentation: it hurts when I bite my own hand, it doesn't hurt when i bite the security blanket, and so on.
The entire debate amounts to this: When the child conducts this kind of investigation, is it purely by accident, or does the child have some theory involved that is confirmed when she experiences pain or not? Perhaps even the first experience of pain versus not pain is an empirical observation through pure accident; but then it is at that point that the child develops the theory (a priori theorizing), and subsequently tests to confirm (empiricism).
I'm not sure we can answer this question definitively, but I think that e.g. Mises makes a strong case for the fact that it's impossible to empirically observe something without getting a priori theory involved first. Consider the following excerpt from Epistemological Problems Of Economics (emphasis added):
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.
And, much later, in Human Action:
In asserting the a priori character of praxeology we are not drafting a plan for a future new science different from the traditional sciences of human action. We do not maintain that the theoretical science of human action should be aprioristic, but that it is and always has been so. Every attempt to reflect upon the problems raised by human action is necessarily bound to aprioristic reasoning. It does not make any difference in this regard whether the men discussing a problem are theorists aiming at pure knowledge only or statesmen, politicians, and regular citizens eager to comprehend occurring changes and to discover what kind of public policy or private conduct would best suit their own interests. People may begin arguing about the significance [p. 41] of any concrete experience, but the debate inevitably turns away from the accidental and environmental features of the event concerned to an analysis of fundamental principles, and imperceptibly abandons any reference to the factual happenings which evoked the argument. The history of the natural sciences is a record of theories and hypotheses discarded because they were disproved by experience. Remember for instance the fallacies of older mechanics disproved by Galileo or the fate of the phlogiston theory. No such case is recorded by the history of economics. The champions of logically incompatible theories claim the same events as the proof that their point of view has been tested by experience. The truth is that the experience of a complex phenomenon--and there is no other experience in the realm of human action--can always be interpreted on the ground of various antithetic theories. Whether the interpretation is considered satisfactory or unsatisfactory depends on the appreciation of the theories in question established beforehand on the ground of aprioristic reasoning .