Hm one answer mentions some of Euclid’s proofs, but I think you could add that there were also false/widely accepted proofs that his Fifth Postulate could be de

Hm one answer mentions some of Euclid’s proofs, but I think you could add that there were also false/widely accepted proofs that his Fifth Postulate could be deduced from the others and was therefore redundant. IIRC, such proofs made subtle use of the geometry being flat, which you (of course) need the Fifth to pin down. Edit: Wikipedia mentions some such false proofs but doesn’t say how widely accepted they were: https://en.wikipedia.org/wiki/Parallel_postulate#History https://en.wikipedia.org/wiki/Parallel_postulate#History