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 Having looked at how Einstein reacted to, and indeed partly inspired, the discov- ery of both matrix and wave mechanics, we shall now turn to his reaction to the emerging dominant probabilistic interpretation of the new quantum mechanics. In his letter of 18 February 1926, Heisenberg had already advocated a statistical interpretation (Doc. 198). But Einstein’s first explicit reaction is found in reply to Gustav Mie’s argument in April that causality had to be abandoned altogether and replaced with statistics (Doc. 268). For Einstein, this was too big a step: “Doing away with strict causality need not be final, for Heisenberg’s theory does not claim to be a complete theory at all, but is rather just a mathematical version of the correspondence principle” (Doc. 292). This is the first time in Einstein’s correspon- dence that the problem of jettisoning causality was raised in direct connection with the new quantum mechanics, although it had arisen previously in connection with the Bohr-Kramers-Slater theory and the Compton effect (see Vol. 14, Docs. 240, 256, 259). A few months later, Born supplied his own interpretation of Schrödinger’s wave function as giving the probability amplitude of finding quantum particles in a partic- ular state (Born 1926a, 1926b). For Born, this interpretation built on an idea expressed by Einstein five years earlier (Einstein 1922a [Vol. 7, Doc. 68]), as he emphasized both in the papers and in a letter to Einstein: “About myself I can report that in physics I am quite satisfied, because my thought of conceiving Schrödinger’s wave field as a ‘ghost field’ in your sense is increasingly proving its worth.” He elab- orated on this probability field: “Pauli and Jordan have made fine advances in this direction. Naturally, the probability field does not move in normal space, but in phase (or configuration) space.” He then concluded: “Schrödinger’s accomplishment reduces to something purely mathematical his physics is quite meager” (Doc. 422). Einstein responded with what has by now become one of his best known and most quoted letters, albeit in varying translations: “Quantum mechanics is very worthy of respect. But an inner voice tells me that it is not the genuine article after all. The theory delivers much, but does not really bring us any closer to the secret of the Old One. I, at any rate, am convinced that He does not play dice” (Doc. 426). Despite his opposition to Born’s interpretation of Schrödinger’s wave mechanics, the theory itself continued to preoccupy Einstein. In February 1927, writing to Lorentz, he noted: “Quantum theory has been completely Schrödinger- ized and has much practical success as a result. But surely it cannot be a description of the real process. It is a mystery” (Doc. 479). He wrote to Zangger in a similar vein a month later (Doc. 507). In the present volume, the only letter by Niels Bohr dates from 13 April 1927 (Doc. 513). At Heisenberg’s request, Bohr sent Einstein a copy of the proofs of Heisenberg 1927, in which Heisenberg introduced the uncertainty relations. Bohr’s accompanying letter contains a detailed discussion of the uncertainty relations as