I N T R O D U C T I O N T O V O L U M E 1 5 l x x x v i i Einstein’s theory would belong to the latter category.[55] On the same day in May 1927 that Einstein presented this paper to the Prussian Academy, he wrote to Ehrenfest that he had shown that “solutions [to the Schrödinger equation] can be uniquely assigned to particular motions, which makes any statistical interpretation unnecessary” (Doc. 517). A few days later he also reported the same to Born (Doc. 520) and thereby ignited Heisenberg’s “burn- ing interest” (Doc. 524). Upon receiving the proofs, Einstein composed an adden- dum that suggests that his outlook on wave mechanics, and his own modification to it, had changed fundamentally within the two weeks since presenting the paper to the Academy. The addendum is concerned with an issue that Einstein had al- ready puzzled about in his initial evaluation of both wave mechanics and matrix mechanics, an issue that resonates with questions raised almost a decade later in Einstein, Podolsky, and Rosen 1935. In his first letter of 16 April 1926 to Schrödinger (Doc. 256), Einstein considered the question whether Schrödinger’s equation allowed the description of what he would ten days later call the “requirement of system additivity” (Doc. 267). Einstein demanded that, if is a possible energy eigenvalue of a first system and a possible energy-eigenvalue of a second system, and if the two systems are not coupled to one another, then should be a possible energy eigenvalue of the total system. Believing that Schrödinger’s equation lacked this property, Einstein had suggested an allegedly better one and had argued that this equation satisfied the requirement of system additivity. It turned out, however, that Schrödinger’s paper had in fact contained precisely the equation suggested by Einstein as an alternative. Six days later, Einstein admitted his error in a postcard (Doc. 261). But Schrödinger did not receive the card before replying to the previ- ous letter and pointing out that his equation was exactly the one that Einstein had suggested (Doc. 264). Nevertheless, Schrödinger was rather happy about Einstein’s letter, for he saw it as reconstructing his equation from the requirement of system additivity and from a second requirement that absolute energy values are impermis- sible. In reply, Einstein praised Schrödinger’s theory for adhering to the require- ment of system-additivity, considering this a major advantage of wave mechanics over matrix mechanics (Doc. 267). In his addendum to Doc. 516, written almost a year after the first comments on the issue, Einstein defines the requirement of system additivity in exactly the same way. He does not take back his claim that wave mechanics (and his own modification of it presented in the paper) obeys the requirement. However, he adds E1 E2 E1 E2 + E =