 He wondered whether it was possible, at least in principle, to ascribe certain properties to a quantum system in the absence of measurement. Can we suppose, for instance, that the decay of an atom occurs at a definite moment in time even though such a definite decay time is not implied by the quantum state function? That is, Einstein began to ask whether the formalism provides a description of quantum systems that is complete. Can all physically relevant truths about systems be derived from quantum states? One can raise a similar question about a logical formalism: are all logical truths (or semantically valid formulas) derivable from the axioms. Completeness, in this sense, was a central focus for the Göttingen school of mathematical logic associated with David Hilbert. (See entry on   Hilbert’s Program.)  Werner Heisenberg, who had attended Hilbert’s lectures, picked up those concerns with questions about the completeness of his own, matrix approach to quantum mechanics. In response, Bohr (and others sympathetic to complementarity) made bold claims not just for the descriptive adequacy of the quantum theory but also for its “finality”, claims that enshrined the features of irrealism and indeterminism that worried Einstein. (See Beller 1999, Chapters 4 and 9, on the rhetoric of finality and Ryckman 2017, Chapter 4, for the connection to Hilbert.) Thus complementarity became Einstein’s target for investigation. In particular, Einstein had reservations about the uncontrollable physical effects invoked by Bohr in the context of measurement interactions, and about their role in fixing the interpretation of the wave function. EPR’s focus on completeness was intended to support those reservations in a particularly dramatic way. 