The so-called ‘possibility proof’ in Kant's pre-Critical Beweisgrund has 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 the Critique 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 from Beweisgrund is what I will call the ‘proof a posteriori’, which the Critique turns into a transcendental illusion that is of regulative use for reason.
