since one so far does not always distinguish clearly the objective ground of a truth from its subjective means of knowledge, so it follows that also the grounding proofs cannot always be distinguished from the most certain proofs

bernard bolzano looking at his best

this is a crucial passage in bolzano's Anmerkung.

do you believe in reductio ad absurdum? which brings me to:

if 9 were divisible by 4 without a remainder, then it would be an even number.

(for the bolzano curious, this is a good intro)