Extending the Agent in QBism

Christopher A. Fuchs

Christopher A. Fuchs

This chapter expresses what a breath of fresh air it has been since a few phenomenological philosophers have started to engage with QBism. In service of the newfound discussion, the aim of this exposition is to lay out the structure of QBism as clearly as possible for that audience. In the process, we arrive at eight tenets for QBism: (1) A quantum state is an agent's personal judgment. (2) A quantum measurement is an agent's action upon its external world. (3) Quantum measurement outcomes are personal to the agent performing the action. (4) The quantum formalism is normative rather than descriptive. (5) Unitary evolution too expresses an agent's degrees of belief. (6) Even probability-one assignments are judgments without ontic content. (7) Subjective certainty about what an outcome will be does not negate that unperformed measurements have no outcomes. And, (8) quantum theory is a single-user theory for each of us. We then analyze the Wigner's friend thought experiment in light of the eight tenets and indicate that a still more extended analysis is potentially QBism's surest path to uncovering an ontology to go with quantum theory's normative structure. We conclude with a small discussion of how the philosophy of Maurice Merleau-Ponty may be relevant to this quest.

Shan Gao

Aman Chawla

Quantum Bayesianism (QBism), as articulated most prominently by Mermin, Fuchs, and Schack, reframes quantum theory as a user-centered framework in which probabilities represent an agent’s personal degrees of belief about the outcomes of interventions. While QBism successfully dissolves several foundational paradoxes by rejecting the objectivity of the quantum state, it deliberately refrains from positing an underlying ontological substrate. In this work, we propose that a theory of Nonlocal Unification (NU), grounded in a universal informational field and biologically mediated sampling processes, provides a natural completion of QBism. We argue that agents are not abstract entities but physically instantiated samplers whose structureis shaped by genetic and neural constraints. Spacetime itself emerges as a derived construct from the sampling of nonlocal informational correlations. In this sense, NU preserves QBism’s participatory ethos while extending it into a coherent ontological framework.

Guido Bacciagaluppi

Rüdiger Schack

In the QBist approach to quantum mechanics, a measurement is an action an agent takes on the world external to herself. A measurement device is an extension of the agent and both measurement outcomes and their probabilities are personal to the agent. According to QBism, nothing in the quantum formalism implies that the quantum state assignments of two agents or their respective measurement outcomes need to be mutually consistent. Recently, Khrennikov has claimed that QBism's personalist theory of quantum measurement is invalidated by Ozawa's so-called intersubjectivity theorem. Here, following Stacey, we refute Khrennikov's claim by showing that it is not Ozawa's mathematical theorem but an additional assumption made by Khrennikov that QBism is incompatible with. We then address the question of intersubjective agreement in QBism more generally. Even though there is never a necessity for two agents to agree on their respective measurement outcomes, a QBist agent can strive to create conditions under which she would expect another agent's reported measurement outcome to agree with hers. It turns out that the assumptions of Ozawa's theorem provide an example for just such a condition.