The actual independence of the parallel postulate from the other postulates of Euclidean geometry was not unquestionably established until consistency proofs of the hypothesis of the acute angle were furnished. These were now not long in coming and were supplied by Beltrami, Arthur Cayley, Felix Klein, Henri Poincare, and others. The method was to set up a model in Euclidean geometry so that the abstract development of the
hypothesis of the acute angle could be given a concrete interpretation in a part of Euclidean space. Then any inconsistency in the non-Euclidean geometry would imply a corresponding inconsistency in Euclidean geometry (see Problem Study 5.8)