x l i v I N T R O D U C T I O N T O V O L U M E 1 6 eventually requested that his contribution be incorporated into Wander J. de Haas’s presentation instead (see Vol. 7, Appendix B). Moreover, after Chaim Weizmann invited Einstein to participate in a fund-raising tour to bolster the Zionist cause, Einstein decided not to attend the conference at all.[5] By the time the fourth Solvay meeting was being planned, Arnold Sommer- feld had convinced Einstein that it would be wrong for him to attend while other German researchers were being excluded. Einstein thus asked Lorentz to en- sure that no formal invitation be sent to him so as to avoid inadvertently harm- ing a return to “enthusiastic collaboration among physicists of different countries” by having to decline (Einstein to Lorentz, 16 August 1923 [Vol. 14, Doc. 102]). Hence it was only at the Fifth Solvay Congress that Einstein returned to the scene. Lorentz invited him to deliver a report and also to succeed Heike Kamerlingh Onnes on the Solvay Institute’s Scientific Committee (see Lorentz to Einstein, 6 April 1926 [Vol. 15, Doc. 246]). Einstein was happy to be involved and, after some coaxing, agreed to present a report (see Einstein to Lorentz, 1 May 1926 [Vol. 15, Doc. 272]). However, more than a year later he wrote to Lorentz to say that “after much reflection one way and the other, I have come to the conviction that I am incapable of producing a report that really corresponds to the state of affairs” (Doc. 8). Einstein explained that he had not followed recent developments in quantum mechanics closely enough. He did not approve “of the purely statistical way of thinking on which the new theories are based” and was “still looking for a theory that is fully deterministic.” His work on this front, however, was still at an early stage: I have not advanced enough in my own efforts to be able to say whether they promise any success. You will be justifiably annoyed at me for telling you this only now. But up to this point I still nourished the hope that I would be able to contribute something valuable in Brussels now I have given it up. I beg you from the bottom of my heart not to be angry with me because of this I didn’t undertake it lightly, but rather tried as hard as I could. (Doc. 8) The fully deterministic theory that Einstein had hoped to contribute was probably a developed version of his fledgling hidden variable theory, which had important affinities with the theory that De Broglie did, in fact, present at the Solvay Congress.[6] According to De Broglie’s theory, the trajectory of the corpuscle is determined by the evolution of the wave function. Einstein presented his version of this kind of theory to the Prussian Academy of Sciences on 5 May 1927 (“Does Schrödinger’s Wave Mechanics Completely Determine the Motion of a System, or Only Statistically?,” [Vol. 15, Doc. 516]) but shortly thereafter with- drew it from publication.[7] Whatever his reasons for abandoning the paper, by the time the Solvay meeting convened in October 1927, he was evidently inclined to fa- vor De Broglie’s approach. During the general discussion period at the end of the