PatSzar, another particular tactic you can use when confronted by your “defiant bloggers” is to ask whether they also deny the validity of the Ricardian Law of Comparative Advantage. After all, this is a pure thought experiment, devoid of any empirical verification – so it should surely be treated with similar suspicion.
If they attempt to bypass this by pretending that they are indeed skeptical of the Law of Comparative Advantage, you can remind them that none other than Paul Samuelson – certainly no friend of the Austrian approach – once identified this as an example of a true and non-trivial proposition that had been discovered by economic science.
Of course, none of this is intended to deny the philosophical contribution of Mises’s writings, which Professor Casey elucidated in his response. The point I am making is that much of the mainstream fuss against a priori reasoning in economics is no more than empty posturing.