His fifth postulate, called the Parallel Postulate, states that for any line and any point not on that line, there exists a unique line passing through the point and never intersecting the line. It was long assumed to follow from the other axioms, but in the 19-th century, [Janos Bolyai]? (and probably Carl Friedrich Gauss before him) realized that its negation leads to consistent non-euclidean geometries, which were later developed by Lobatchevsky? and Riemann.