Arianna Laura Savelli
University of Trento
QBism interprets quantum states as expressions of an agent’s personal degrees of belief and construes quantum theory as a normative framework governing probabilistic coherence. At the same time, it maintains that agents interact with an agent-independent reality that “kicks back” and underwrites the regularities captured by the Born rule.
I argue that this combination generates a structural tension. While QBism rejects any representational role for quantum states, it nevertheless appeals to an external reality that shapes agents’ expectations without providing an account of its nature. This results in an ontologically underdetermined framework. As a consequence, the appeal to worldly constraints cannot account for the stable and highly specific regularities captured by the formalism, such as quantum correlations. The upshot is an explanatory deficit: QBism specifies how agents should update beliefs but does not explain why the probabilistic constraints encoded in the formalism are systematically successful.
I further argue that this difficulty reflects a deeper instability. QBism does not eliminate the distinction between world and experience but preserves it in a weakened form, as a residual “world-in-itself” that conditions expectations while remaining inarticulate. I conclude that QBism faces a dilemma: either articulate a more substantive ontological account or abandon the residual world-experience distinction that sustains this tension.
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SOPhiA 2026 