l x i v I N T R O D U C T I O N T O V O L U M E 1 6 to generalize the schema in such a way that it would not rely on a counterpart of the equilibrium condition that he and Grommer had employed. And third, he hoped that he would derive results beyond just a derivation of the classical Lorentz force law, results that would say something about the quantum nature of matter. Yet again, we mostly learn of these hopes from deleted passages in the manu- script that were not included in the published paper and from Einstein’s correspondence after the paper was completed. The paper’s conclusion emphasizes only the importance of the classical result, namely, that the equations of motion of a charged particle subject to both gravita- tional and electromagnetic fields had now been derived from the Einstein-Maxwell equations rather than being assumed independently, and that higher orders in the approximation schema might bring about constraints on the motion of the charged particle. The manuscript, however, was much more explicit about the possible link of these considerations to quantum theory. Here Einstein wrote that the constraints that might occur at higher orders could correspond to “quantum conditions.” One such quantum condition can be found in another passage of the introduction to the manuscript, where Einstein made, and later deleted, the announcement that the present paper would deliver a generalization of the equations of motion as derived from the field equations, a generalization that would make possible the attribution of a characteristic frequency and a characteristic magnetic field to the electron.[42] In the published paper, however, Einstein ends up with classical equations of mo- tion, without any hint of implied quantum conditions. He voiced satisfaction with the positive result but also disappointment with its purely classical nature in a letter to Paul Ehrenfest two months later, on 21 January 1928. He noted that in order to proceed any further, he was turning his attention back to the Kaluza-Klein theory, proclaiming: “Long live the fifth dimension” (Doc. 137). Einstein had thus returned to modifying the field equations themselves, hoping, as before, to find field equations that would describe the gravitational and the elec- tromagnetic fields as a truly unified field. He also hoped that they would allow a solution that could be interpreted as an electron. However, he did not stay focused on the five-dimensional approach to unified field theory for long. Neither did he return to it later, when engaged in correspondence with Heinrich Mandel and Raschko Zaycoff, both of whom had written to Einstein about results in five- dimensional theory. Whereas his work in this domain had thus far relied on gener- alizations of pseudo-Riemannian geometry introduced by Weyl, Kaluza, and Eddington, Einstein in 1929 began exploring a new approach to unified field theory that would keep him engaged for the next few years. At some point in May 1929, while convalescing at home in Berlin,[43] Einstein had an idea for what he thought was “an entirely new way of realizing the general theory of relativity and that may be groundbreaking” (Doc. 217). Key to this new