3 Scholars believe that Euclid’s fifth postulate was not considered to be as self-evident as the others. It is less terse, and more like a proposition (a theorem) than a postulate (it is the converse of proposition I 17). Euclid does not use it until proposition I 29. For this reason, over the ages, many attempts to prove the populate wade including Sacchieri’s attempt to prove it by reduction ad absurdam based on its denial (Eves, 1953).