I should almost therefore put forward the proposal that the third hypothsis (angle sum of a triangle less than two right angles) holds on the surface of an imaginary sphere.
Johann Heinrich LambertProofs of the Euclidean [parallel] postulate can be developed to such an extent that apparently a mere trifle remains. But a careful analysis shows that in this seeming trifle lies the crux of the matter; usually it contains either the proposition that is being proved or a postulate equivalent to it.
Johann Heinrich Lambert