Ontological Evasions in Physics, Part 6 Quantum Bayesianism (QBism): Retreat into the Observer

Quantum mechanics repeatedly confronts physics with the tension between formalism and ontology. One response to this tension is Quantum Bayesianism, or QBism, which recasts the quantum state not as a property of a system, but as a reflection of an agent’s personal beliefs about possible outcomes.

At first glance, QBism appears to solve the measurement problem elegantly: collapse is nothing more than the updating of an observer’s probabilities. But beneath this apparent clarity lies a familiar ontological evasion.

The evasive manoeuvre

QBism relocates reality into the mind of the observer. The formalism is preserved, but at the cost of dissolving the system itself into an epistemic shadow. Outcomes are not actualised in the world; they are experiences registered by agents. The world becomes an extension of belief, not a domain of relational actuality.

This manoeuvre allows physicists to sidestep deep questions: What is the ontology of the quantum system? How does possibility actualise? Instead of addressing these issues, QBism effectively says: “Don’t worry about reality; focus on your expectations.”

The ontological cost

The world is no longer a network of relational events; it is a projection of subjective credences. Possibility and actualisation are flattened into the agent’s updates. Collective alignment, interaction, and emergent structure—the core of relational ontology—vanish from the picture. Reality is now dependent on consciousness, which means relationality is subordinated to belief.

The epistemic collapse

If outcomes exist only in the agent’s experience, then experiment no longer constrains reality; it constrains belief. Science risks becoming an internal bookkeeping exercise: probabilities adjusted in minds, rather than structures revealed in the world. Evidence cannot falsify or confirm theory in any robust sense, because it is inseparable from the observer’s prior expectations.

The theological return

As with other evasions, QBism smuggles in the theological impulse, albeit subtly. By placing the observer at the centre of actualisation, it effectively installs a local, personal “creator” for each measurement event. Each agent becomes a tiny deity, and the universe itself is fragmented into countless subjective realms.

A relational reframing

Relational ontology dissolves the paradox without retreating into subjectivity. Outcomes are actualised not in isolated observers, but through the collective alignment of relational construals. Possibility is not private; it emerges perspectivally across interacting systems. Measurement is a moment of relational actualisation, not the conjuring of reality by an agent’s mind.

By reframing the problem relationally, QBism’s retreat into subjectivity is unnecessary. One can preserve the predictive power of the formalism while keeping ontology intact. Possibility, actuality, and alignment remain real, but they are emergent and collective, not the product of isolated belief.

Conclusion

QBism exemplifies an evasion that preserves formalism by privatising reality. Measurement and collapse are recast as epistemic updates, but in doing so, physics abandons relational ontology. Relational thinking restores coherence: actuality emerges in relation, not in the mind, and science remains a probe into the world rather than a meditation on belief.