The above derivation is an illustrative example of Bolzano’s importation of proof methods from mathematics — such as the methods of indirect proof and of mathematical induction — into philosophy. (For certain reservations concerning these methods cf. WL IV, 269–285.) Nevertheless, this alleged proof for the existence of a true proposition does not reach its goal for a simple reason: The proof is in no way peculiar for truths in the sense of true propositions as Bolzano needs to have it. If succe