l x x i i I N T R O D U C T I O N T O V O L U M E 1 6 VI. Reichenbach, Meyerson, and the Geometrization of Gravity The prominence of the concepts of Riemannian geometry for the general theory of relativity, as well as generalizations of Riemannian geometry and their potential role in unified field theories, raised questions about the overall role for geometric concepts in relativistic field theories. On several occasions, Einstein felt called upon to clarify his own understanding of the new conceptualization of the gravita- tional and electrodynamical forces in non-Euclidean manifolds. He was particu- larly explicit about this in debates with the philosophers Hans Reichenbach and Émile Meyerson. Drawing on undergraduate studies in engineering, physics, philosophy, and math- ematics, Hans Reichenbach completed a Ph.D. on the concept of probability in Er- langen in 1915. After being conscripted into the army while finishing up his dissertation, he returned to civilian life two years later and started work as an electri- cal engineer in Berlin. During this time, Reichenbach attended Einstein’s 1918 lec- tures on relativity and statistical mechanics in Berlin, and Einstein came to appreciate Reichenbach sufficiently to write to Georg Count von Arco to try to get a better position in engineering. He also encouraged Reichenbach to use him as a ref- erence “whenever this may help” (see Einstein to Reichenbach, 16 August 1919 [Vol. 9, Doc. 89]). In 1920, drawing on Einstein’s patronage (see Erich Regener to Ein- stein, 21 May 1920 [Vol. 10, Doc. 24]), Reichenbach obtained his Habilitation in physics at the Technical University of Stuttgart with work that develops a neo-Kan- tian epistemology based on an investigation of relativity theory. He became a Privat- dozent in Stuttgart and dedicated his first book to Einstein (Reichenbach 1920).[51] In the following years, Reichenbach published extensively on the philosophical analysis and consequences of both special and general relativity, and Einstein reg- ularly named him to others as one of those philosophers who best understood the theory, rivaled only by Moritz Schlick.[52] In the spring of 1926, Reichenbach wrote to Einstein that all recent attempts at a unified field theory, including Einstein’s, seemed “artificial” to him, very much in contrast to the general theory of relativity (Reichenbach to Einstein, 16 March 1926 [Vol. 15, Doc. 224]). In the ensuing correspondence, Einstein and Reichen- bach agreed that the alleged ‘geometrization’ of electromagnetism in these theories based on generalizations of pseudo-Riemannian geometry was not something physical but at best “a numerical crutch for discovering numerical laws” and in the end “a private matter” (Einstein to Reichenbach, 8 April 1926 [Vol. 15, Doc. 249]). Emboldened by Einstein’s assent, Reichenbach sent him a manuscript that created a toy unified field theory intended to show that even Einstein’s 1915 theory of gen- eral relativity can be rewritten such that the electromagnetic field can be made to look just as geometrical as the gravitational field, and that geometrizing a field thus