I N T R O D U C T I O N T O V O L U M E 1 4 x l v i i In his reexamination of the relationship between general relativity and quantum theory, Einstein now returned to an old pet idea, namely, of creating quantum dis- creteness through the overdetermination of a system of partial differential equa- tions. The suggestion was to obtain a discrete set of solutions to a system of partial differential equations by postulating more equations than independent variables. He had earlier considered this idea, notably in the first half of 1920 (see Doc. 170, note 6). Its revival may have been initiated by a discussion in October 1923 with his student Hans Joachim, who had sent Einstein a manuscript entitled “Geometry and the Atomic Hypothesis,” in which he advanced suggestions about giving up the assumption of the continuity of space in the atomic realm. During their conversa- tion, Einstein had apparently defended the assumption of a continuity of space in view of quantum challenges, but the question of accounting for quantum discrete- ness in a field-theoretic framework was clearly on the agenda here (see Doc. 170, note 5). We do not know what the specific new idea, or Ansatz, was that Einstein pursued in the fall of 1923 when he reapplied himself to the overdetermination approach, but we do know that he again enlisted Jakob Grommer’s help in exploring the fea- tures and solutions of some trial overdetermined systems of differential equations (Docs. 153, 155, 173). When Einstein hurriedly left Berlin in November 1923, he “continued to calcu- late quietly on my problems” (Doc. 150). He was working “without restraints on a theory—a little escape from life is always present while doing so” (Doc. 161). Sure enough, he again ran into problems of mathematical complexity regarding the over- determination approach and could not find a satisfactory solution. But this time he decided he would at least publish the idea and invite mathematicians to devote their attention to this issue as well. He could thereby also somewhat justify his long absence from Berlin (Doc. 165). On 6 December 1923, while still in Leyden, Einstein sent Planck the manuscript of a paper entitled “Does Field Theory Offer Possibilities for the Solution of the Quantum Problem?” (Einstein 1924d [Doc. 170]). He asked Planck to present the paper for publication at the next Academy meeting, and added a comment ex- plaining the question mark at the end of the title: “What is set forth there is surely more a wish than its fulfillment. But it involves a matter whose formal mastery sur- passes my mathematical abilities. In my opinion, it nevertheless cannot be denied that this constitutes a logical possibility for real insight into the nature of quantum processes” (Doc. 171). There are hints that Einstein continued to think along the lines of the over- determination problem for some time. By the end of 1923 he revealed his frustra- tion to Lorentz: “Unfortunately, the mathematical difficulties are too great for my