I N T R O D U C T I O N T O V O L U M E 1 6 l x v i i made a living with various teaching appointments or private tutoring. His collabo- ration with Einstein probably began during 1927, and it may have been supported by a grant from the Notgemeinschaft der deutschen Wissenschaft, although finan- cial compensation is not mentioned in the correspondence.[45] Indeed, Müntz made his appearance earlier in the volume with an undated letter in an unidentified con- text by Einstein sent to Müntz, and another one dealing with a cosmological question (Docs. 53, 54). The subsequent correspondence, however, consisting of seven letters by Müntz to Einstein and twenty letters by Einstein to Müntz, carries through to the end of this volume and deals almost exclusively with technical prob- lems of the teleparallel approach.[46] In his first letter to Einstein on the new teleparallel approach, Müntz showed how to integrate exactly the linear field equations of the new theory (Doc. 244). In his reply, Einstein expressed interest in Müntz’s result but confronted Müntz with the worries prompted by Grommer’s concerns (Doc. 245). After discussing the problem in person, Müntz continued to provide exact solutions for the field equations and also argued that Grommer’s concern was unfounded (Docs. 250, 255, 256). Einstein was not convinced (Doc. 257), but continued to en- gage Müntz in finding solutions to the field equations (Doc. 258). He was encour- aged by Müntz’s success in integrating the field equations not only because he hoped it would provide particle-like solutions, but also because it might help with the problem of motion. Indeed, Einstein wrote to Müntz that “I have looked again at the problem of motion in the original theory of gravity, in order to generalize the method of deriving the law of motion. For a serious assessment of field theories is possible only if one has mastered the laws of motion of the singularities” (Doc. 257). In a letter on the very next day, Einstein stated that deriving the equations of motion of electrons from the teleparallel field equations should make it possible to also settle the problem whether it is justified to identify the electromagnetic poten- tial with the contraction of the torsion tensor (Doc. 258). In a long letter a week later (Doc. 260), Einstein laid out another approximation scheme to obtain equations of motion for uncharged and charged mass points from relativistic field equations. The scheme was a generalization of the earlier treat- ment of the problem of motion contained in Einstein 1928b (Doc. 91). He claimed that the new scheme, at the core of which an approximation treatment of the dynamical electromagnetic field was introduced, would eventually be applicable to any field theory, not just the teleparallel one. Mathematically, he admitted, it was all still “rather ugly” and urged Müntz to take some time “to smooth it out and to work it out fully,” an effort that would be worthwhile because they would “thereby create an instrument that can be utilized for the interpretation of every relativistic