I N T R O D U C T I O N T O V O L U M E 1 5 x l i x deeply than the aforementioned equation, which is not unified and only phenome- nologically justified” (Doc. 348). It is exactly this project that Einstein and Grom- mer believed they had made significant advances on only a few months later: de- riving the equations of motion from the vacuum Einstein equations, without any appeal to an energy-momentum tensor.[11] Despite the relevance of this result for general relativity proper, Einstein’s cor- respondence teaches us why something he had deemed a pseudosolution to the pro- cess of giving an account of matter (considering particles as singularities in the field) was a worthwhile approach to describing the motion of matter in general rel- ativity. He expected that the pseudosolution would point the way to a proper solu- tion: a comprehensive account of (quantum) matter within the realm of classical field theory. He emphasized that hope in the final sentence of the paper, as he had done earlier when writing to Besso and to Ehrenfest (Doc. 450) five days after pre- senting it to the Prussian Academy on 6 January 1927. Shortly thereafter, Herglotz expressed enthusiasm for Einstein and Grommer’s result (Doc. 468). He handed the proofs of the paper to Weyl, who commented in detail, and was less enthusiastic, for he “did not see what in it goes beyond my own development” of this topic (Doc. 473). In his reply, he did not address Weyl’s crit- icism that Einstein had failed to acknowledge, or significantly improve upon, Weyl’s earlier work on the problem of motion in general relativity (Doc. 514).[12] Instead, he focused on his own motivation for taking up the problem of motion in the first place: the question of whether the “field equations as such are to be con- sidered as falsified because of the facts of quanta—or not.” On this occasion, Weyl also took the opportunity of returning to an earlier disagreement, namely, Ein- stein’s “measuring-rod objection” to Weyl’s unified field theory of 1918 (Einstein 1918g [Vol. 7, Doc. 8]). He observed that the new quantum mechanics justified his introduction of the scale factor in that earlier theory (Weyl 1918) when reinterpreted as a phase factor, by making imaginary the exponent quantity that depends on the electromag-netic potential. Thus, the scale invariance of Weyl’s original theory (from which the term “gauge invariance” is derived) was converted from a state- ment about scale (connected to measuring rods) to one about phase (connected to Schrödinger’s wave function).[13] As Weyl observed to Einstein, the theory thus has less to do with field unification than it does with field quantization. It may be argued that this repurposing of the mathematics from Weyl’s 1918 theory consti- tutes the inception of modern gauge theory. In the previous volume we found Einstein already firmly convinced that a satis- factory theory of geomagnetism would have to be connected with a fundamental