That it's impossible to create an accurate economic model because the economy is far too complex and has too many variables to model with any degree of certainty.
(This is apparently an outrageous proposition to some folks, even though it's essentially chaos theory).
Oh. That's just because they don't really understand modern economics.
I haven't studied Austrian economics that much (I have better things to do) but I think what they claim is very different from what I claim.
My claim is simply that future movements of things like stock prices are based on everyone's predictions. Predicting stock prices is predicting what others will predict that others will predict...that can by definition not really be done accurately.
Economic theory is hardly tautological, since a lot of people don't accept even basic concepts of it. Still, a lot of it has considerable empirical support.