On the Classification between $ψ$-Ontic and $ψ$-Epistemic Ontological Models

Abstract: Harrigan and Spekkens (2010) provided a categorization of quantum ontological models classifying them as $\psi$-ontic or $\psi$-epistemic if the quantum state describes respectively either a physical reality or mere observers' knowledge. Moreover, they claimed that Einstein - who was a supporter of the statistical interpretation of quantum mechanics - endorsed an epistemic view of $\psi$. In this essay we critically assess such a classification and some of its consequences by proposing a two-fold argumentation. Firstly, we show that Harrigan and Spekkens' categorization implicitly assumes that a complete description of a quantum system (its ontic state, $\lambda$) only concerns single, individual systems instantiating absolute, intrinsic properties. Secondly, we argue that such assumptions conflict with some current interpretations of quantum mechanics, which employ different ontic states as a complete description of quantum systems. In particular, we will show that, since in the statistical interpretation ontic states describe ensembles rather than individuals, such a view cannot be considered psi-epistemic. As a consequence, the authors misinterpreted Einstein's view concerning the nature of the quantum state. Next, we will focus on Relationalist Quantum Mechanics and Perspectival Quantum Mechanics, which in virtue of their relational and perspectival metaphysics employ ontic states lambda dealing with relational properties. We conclude that Harrigan and Spekkens' categorization is too narrow and entails an inadequate classification of the mentioned interpretations of quantum theory. Hence, any satisfactory classification of quantum ontological models ought to take into account the variations of $\lambda$ across different interpretations of quantum mechanics.