The so-called ‘possibility proof’ in Kant's pre-CriticalBeweisgrundhas been widely discussed in the literature, and it is a common view that he never really abandoned it. As I shall argue, this reading is mistaken. I aim to show that the natural illusion in theCritique of Pure Reason, which is usually taken to be the possibility proof turned into a transcendental illusion, has both a different conclusion and a different argument than the possibility proof. Rather, what remains fromBeweisgrundis what I will call (...) the ‘proofa posteriori’, which theCritiqueturns into a transcendental illusion that is of regulative use for reason. (shrink)