I N T R O D U C T I O N T O V O L U M E 1 6 l x x i i i has nothing to do with physics (Reichenbach to Einstein, 24 March 1926 [Vol. 15, Doc. 235]). Einstein found more than one “fly in the soup” with the physics and mathematics of Reichenbach’s attempt to “come among theoretical physicists” (Einstein to Reichenbach, 31 March 1926 [Vol. 15, Doc. 239] see also sec. III of the Introduction to Vol. 15). However, during this exchange, Reichenbach also asked for Einstein’s assis- tance in securing a professorship, and Einstein wrote a glowing recommendation in June 1926 (Vol. 15, Doc. 297). Likewise, when asked to write about historical and philosophical issues related to relativity theory for newspapers and journals in the current volume, Einstein twice suggested that Reichenbach be asked instead (Abs. 98, 388). Whereas in their correspondence both Einstein and Reichenbach quickly agreed that casting physical fields into a geometrical form is not at the heart of general rel- ativity or unified field theory, most of their correspondence focused on the details of Reichenbach’s toy theory. In the present volume, Einstein was confronted with another philosopher’s work, one who did think that general relativity reduces grav- ity to geometry, and that unified field theories were the next logical step in an attempt at reducing all of physics to the structure of space or space-time, quite in line with Descartes’s identification of space and matter. The philosopher in ques- tion was Émile Meyerson, whom Einstein had met at the Société Française de Philosophie in 1922, where they had joined forces spoken out against Machian pos- itivism and Kantianism. In the draft (Doc. 6) of what was otherwise a very positive review of Meyerson’s recent book on the philosophical interpretation of relativity theory, Einstein pointed out that “my opinion differs strongly” from Meyerson’s on one particular point, echoing his earlier agreement with Reichenbach: “To be spe- cific, I cannot admit that the assertion that relativity theory reduces physics to ge- ometry has any sort of a clear-cut meaning.” He went on to elaborate (in much more detail than in his correspondence with Reichenbach the year before) why he did not believe that the use of a metric tensor in general relativity made the theory any more “geometrical” than electrodynamics, in which the (in Einstein’s view) equal- ly geometrical concept of a vector is used. Likewise, he noted “… the theories of Weyl and of Eddington for representing the electromagnetic field thus do not lie in the fact that these theories have incorporated geometry into the theory of that field, but rather that they have shown possible paths for representing gravitation and electromagnetism within a unified point of view…” (Doc. 6).[53] After Einstein sent Meyerson the draft of his book review on 15 June 1927, at- tached to Doc. 7, a correspondence between the two ensued. On 20 July, Meyerson sent Einstein five pages of comments to the review, answering, in unfailing humble politeness and blaming himself for having been unclear, the few points on which Einstein had criticized Meyerson’s book (Abs. 66).[54] The issue of geometrization