Intersubjective Agreement about Quantum States Is Unnecessary in QBism

Abstract: The thought experiment called Wigner's Friend has experienced a renewal of interest for interrogating the meaning of intersubjectivity and objectivity in quantum mechanics. These new inquiries extend to investigations at the intersection of phenomenology and QBism. Philosopher of physics Steven French has criticized QBism on the grounds that it does not give assurances that Wigner and friend must agree on the same quantum state or the same set of facts in the experiment. In this paper, we draw on Wigner's Friend to argue that intersubjective agreement on quantum states is unnecessary. We defend the QBist notion of reciprocity, treating Wigner and friend as "peer observers" or physical systems taking mutual actions on each other. We argue that accounts of intersubjectivity that preserve the possibility of privileged observation, vantage points for solipsistic super-observers, undermine the epistemic benefits we look for when we insist on intersubjectivity agreement as a criterion for objectivity. In its place, the QBist notion of reciprocity leads to sharper characterization of what it means to objectify quantum systems with a quantum state assignment. Drawing on phenomenological resources, we argue that state assignments for quantum systems, including those for Wigner and friend, are thematizations. To assign a quantum state is to thematize a phenomenon as a quantum system, to treat something as the sort of object to which the formalism applies. Our argument accounts for why the quantum formalism does not radically change in application for different systems because the systems themselves exceed their formalization.