Einstein's 1935 Letters to Schr\"odinger and Popper and the Boundaries of the PBR $\psi$-Epistemic Framework

Abstract

Einstein's 1935 critique of quantum mechanics is often associated with the Einstein-Podolsky-Rosen (EPR) argument, yet his private correspondence from that year reveals a more exact conceptual structure guiding his claim that the $\psi$-function is incomplete. This paper reconstructs Einstein's reasoning in his letters to Schr\"odinger and Popper and examines how it engages, and fails to engage with contemporary $\psi$-ontic/$\psi$-epistemic distinctions. Recent scholarship, most notably by Ben-Menahem, has interpreted Einstein as an early representative of the modern $\psi$-epistemic tradition within the Harrigan-Spekkens ontological models framework and the Pusey-Barrett-Rudolph (PBR) theorem. I argue, however, that this retrospective classification is undermined by Ben-Menahem's own distinction between realist and radical epistemic interpretations: Einstein's 1935 view lacks the structural assumptions - defined ontic state space, preparation distributions, and overlap structure - required for membership in the HS/PBR class of $\psi$-epistemic models. Any such identification, therefore, requires importing formal machinery foreign to Einstein's original argument.