To a critic who doesn’t understand it:
Mises intends to be talking, not about some contrived concept of “action,” constructed to make his assertions about action true by stipulation, but rather about actions in the world, as we ordinarily understand them. Precisely his thesis is that we can gain knowledge of actions, understood this way, by thinking. We have a priori knowledge that applies to the world. What has Brennan to say against this? So far as I can see, he advances no argument. He merely states his own contrasting position.
Mises says that we have a priori knowledge of action; and Brennan responds, in effect, “No, we don’t.”