I N T R O D U C T I O N T O V O L U M E 1 6 l x i i i V. Relativity and Unified Field Theory In early 1927, in the last paper cowritten with his long-time assistant Jakob Grom- mer, Einstein had embarked on an investigation of whether, and if so, to what extent, the equations of motion in a given field theory might be implied by the field equations. Inspired by correspondence with George Yuri Rainich, Einstein pon- dered the difference between a linear field theory, such as classical electrodynam- ics, and a nonlinear field theory, such as general relativity or each of the unified field theories that had been proposed thus far. Einstein and Grommer 1927 (Vol. 15, Doc. 443) eventually argued that if a certain equilibrium condition is ful- filled, then the equations of motion of an uncharged particle, whose presence was signified by a singularity in the metric field, followed from the vacuum Einstein equations.[41] Even though the published paper reads like a work containing a novel result in classical general relativity, Einstein’s correspondence shows that he had hopes that the derivation of equations of motion from the gravitational field equations would show the way toward a comprehensive account of quantum matter in the context of classical field theory, a research program he had already outlined in Einstein 1924d (Vol. 14, Doc. 170) and saw in ever stronger contrast with the newly emerging quantum mechanics (see sec. I). In a follow-up paper to Einstein and Grommer 1927 submitted in November 1927, Einstein makes this contrast abundantly clear when he writes in the introduc- tion that “The majority of physicists today are convinced that the empirical facts of quantum phenomena exclude a field theory in the usual sense of the word. But this belief is not founded on a sufficient knowledge of the consequences of field theory” (Einstein 1928b [Doc. 91]). With this new paper, Einstein hoped to achieve three aims. First, he wanted to generalize his and Grommer’s treatment of the problem of motion such that it starts not from the purely gravitational field equations but from the combined gravita- tional and electromagnetic equations, now often called the Einstein-Maxwell equa- tions. This should then allow the derivation of the equations of motion of a charged particle subject to both gravitational and electromagnetic fields, rather than just the equations of motion of an uncharged particle subject only to gravitational fields (the geodesic equation), as in the predecessor paper. Second, Einstein likely wanted