Even a proposition that is self-evident, in the sense that the action axiom is, must be shown to be so. One way to do so involves an argument: the action axiom, persons act, must be assumed true in any attempt to demonstrate it false, therefore, it must be true. Alternatively, one could just start with the common sense claim that everyone knows action exists by their own experience.
Hans Hoppe advances the former:
David Gordon advances the latter: