Source
Hance, Rarity & Ladyman (2021, v4 2022), LIT-090, read in full in NOTE-072.
What was actually shown
The paper is a conceptual argument with no theorem.
Informal terms (§II). A term is ontic when “taken to represent how the world is independently of our knowledge of it to some extent or other”, epistemic when “taken to represent knowledge or information about the world to some extent or other”. Thermodynamic entropy and proper mixtures are offered as terms that are both.
Formal terms (§III). In the ontological-models framework, ψ-ontic means disjoint supports for distinct states (Eq. 5); ψ-epistemic means overlap for some pair (Eq. 4). These are contradictories by definition.
The gap (§IV). Overlap implies the weaker condition Eq. 6: on Λψ some response function differs from the Born probability for ψ. Models without overlap can satisfy Eq. 6 as well. So “there is nothing in Harrigan and Spekkens’ formal definition for a model being ψ-ontic … which is necessary or sufficient for a model to have wavefunctions representing reality” (p. 4).
Conclusion (§V). PBR and the other no-go theorems “do not rule out all epistemic models”. The paper accepts PBR’s result “given certain assumptions” and reads its conclusion as about overlap.
What could have come out otherwise. PBR’s own text could have tied its conclusion to a broader notion than overlap. It does not: PBR defines “mere information” as overlap (PBR p. 2) and leaves the QBist option open (PBR p. 5), which NOTE-072 records as compatible with this reading (C5).
What this does not say
- That some ψ-ontic model is epistemic in a precise sense. The only example, Schlosshauer–Fine’s ψ-dependent models, is cited, not exhibited (p. 4; NOTE-072, C4).
- That Eq. 6 captures “ψ represents knowledge”. NOTE-072 shows that Eq. 6 holds exactly when ψ is predictively incomplete, so any deterministic hidden-variable model, Bohmian mechanics included, counts as epistemic, against the paper’s own §I. Hence Proposed.
- That ontological models cannot reproduce interference or the bomb tester. Appendix A says so. THEORY-403’s explicit nonnegative, local model reproduces the bomb tester in the fragment it covers.
- Anything about PBR’s assumptions. The paper never discusses preparation independence, which is where PBR is most open to challenge.
Connections
- Presupposes THEORY-605, PBR’s theorem, taking its conclusion as given and asking what it licenses.
- Allori (LIT-200) also calls the ontic/epistemic dichotomy false, without a stated condition (NOTE-154); her view is in THEORY-669.
- Cuffaro & Hartmann make the same terminological point from the side of orthodox readings (THEORY-631).
- Spekkens’ programme keeps ψ epistemic by giving up the framework’s classical probability, a different route past PBR (LIT-007 §I.C).