 The metrical account led to a progressive elucidation of a significant obscurity in Euclid’s Elements: the parallel postulate. For much of the 19th century, non-Euclidean geometries (like Lobachevskii's) were the only alternative to Euclid’s that was proposed as an intelligible geometry, even though it was generally agreed that only the most delicate experiments could hope to decide the matter. Poincaré’s contested view was that no experiment could so decide, and this raised important issues about the way abstract terms are to be interpreted and the role of hypotheses in scientific theory.