  Klein's result led Bertrand Russell (b. 1873, d. 1970) to assert, in his neo-Kantian book on the foundations of geometry (1897), that the general “form of externality” is disclosed to us a priori in projective geometry, but its metric structure—which can only be Lobachevskian, Euclidean or elliptic—must be determined a posteriori by experiment. Henri Poincaré (b. 1854, d. 1912) took a more radical stance: If geometry is nothing but the study of a group,