Johann H Lambert (1728 - 1777)

Proofs 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.

Quoted in C B Boyer, A History of Mathematics (New York 1968)