D O C . 6 R E V I E W O F M E Y E R S O N ’ S B O O K 5 5 Meyerson to add his own corrective remarks to Einstein’s text. In Abs. 221, Meyerson then expressed shock at the thought of appending critical remarks by himself to Einstein’s review, and asked, in light of what Einstein had written him in Doc. 7, whether Einstein might allow him instead to suggest changes to the text itself. Einstein authorized Meyerson to change the text “in light of our correspon- dence” accordingly and asked him to send him the modified draft once it was ready. Meyerson sent the modified draft attached to Abs. 326, which Einstein returned with a few modifications of his own attached to Abs. 334. [6] Einstein had often emphasized that special and general relativity were a natural continuation of previous developments. Most recently, he had made this point in his writings on the occasion of com- memorating the 200th anniversary of Isaac Newton’s death (see Vol. 15, Introduction, sec. XII). [7] Einstein here interprets the equivalence principle as suggesting a general principle of the rela- tivity of all motion, and the latter as motivating the formal principle of general covariance. He had long regarded the equivalence principle as a special case of the relativity principle, allowing to extend the latter from inertial to uniformly accelerated motion. For detailed reflections on how the relativity principle, the equivalence principle, and general covariance are related to one another, see Einstein 1918e (Vol. 7, Doc. 4), especially notes 3–5. [8] At this point in Doc. 152, the criticism is not mentioned. Instead, the French version claims that Meyerson and Einstein are in agreement on this point. [9] In Einstein 1924d (Vol. 14, Doc. 170), Einstein had formulated a research program that, if suc- cessful, would deliver the quantum features of matter as a result of unifying the metric/gravitational and the electromagnetic field. However, in between working on different concrete approaches for such a unified field theory, he had often doubted the viability of the program. His most recent previous period of doubts had brought about Einstein 1925w (Vol. 15, Doc. 92), in which Einstein argued against the viability of a whole class of such theories, only to revisit five-dimensional unified field theories in Einstein 1927i (Vol. 15, Doc. 459). [10] This sentence is missing in Doc. 152. [11] When Einstein first read Meyerson’s book while on a trip to South America, he noted in his travel diary that Meyerson unjustly made a comparison between relativistic physics and Hegel because he took the attempts at a unified field theory by Hermann Weyl and Arthur Eddington to be an essential part of the theory (Vol. 14, Doc. 455, entry of 12 March 1925). During Einstein’s and Meyerson’s second encounter at a dinner at Meyerson’s Paris home on 15 January 1926, they may have debated the connection further. In André Metz’s reconstruction of the dinner as described in Metz to Einstein, 20 January 1927 (Vol. 15, Doc. 460), Einstein declared himself convinced that he was, after all, possessed by the same ‘‘demon’’ (“démon”) as Descartes. Einstein confirmed Metz’s account in a letter of 23 January 1927 (Vol. 15, Doc. 463) and reaffirmed that “the real theoretical physicists” (“die echten theoretischen Physiker”) wanted the same kind of logical construction corre- sponding to physical reality that Descartes and Hegel imagined. However, he added that the differ- ence lay in the fact that he did not believe that this construction could be attained by pure thought alone, but only via “subtle empiricism” (“subtile Empirie”), with the aim of finding a foundation for what was to be deduced. [12] The first two sentences of this paragraph differ significantly in Doc. 152. [13] Here a sentence is added in Doc. 152. [14] This is Einstein’s most detailed opposition to the idea that general relativity reduces physics, and gravity in particular, to space-time geometry. In his correspondence with Hans Reichenbach the previous year, Einstein had already expressed the same opinion (see especially Einstein to Reichen- bach, 8 April 1926 [Vol. 15, Doc. 249] and Vol. 15, Introduction, sec. III, as well as Giovanelli 2016 for context). A detailed analysis of Einstein’s reasons for the position opposed here and elsewhere can be found in Lehmkuhl 2014. Einstein’s related opposition against seeing geometry as something that can be determined a priori is most clearly expressed in Einstein 1921c (Vol. 7, Doc. 52) and Einstein 1924n (Vol. 14, Doc. 321). [15] The last sentence of this paragraph is missing in Doc. 152. [16] Lehmkuhl 2014 interprets this statement as Einstein’s conceptual distinction between a field theory unifying two fields versus the theory geometrizing them, arguing that Einstein saw general rel- ativity as unifying gravity and inertia in a way analogous to how special relativistic electrodynamics