Agency Beyond Pure Unitarity: Closure-Stable Decision, Probability Rigidity, and the Limits of Purely Quantum Models

What are the physical conditions required for agency?

Recent work has rigorously shown that purely unitary, coherence-preserving quantum dynamics are insufficient to support deliberation, comparison of counterfactual actions, and reliable action selection. These results establish a genuine no-go theorem for agency in a strictly quantum-coherent regime.

This paper provides a structural completion of that no-go result. Rather than challenging its correctness, the analysis identifies the underlying assumption on which it rests: that unitarity alone is sufficient as an admissibility criterion for decision endpoints. We show that this assumption is incomplete.

Working within the Tier-0 closure framework, physical lawfulness is characterized by closure-stable fixed points selected by structural constraints rather than by unconstrained dynamical evolution. Within this framework, coherent quantum states may function as intermediate computational or representational states but are excluded as lawful endpoints for records, decisions, and control.

Applying closure stability to quantum systems yields a classical decision subspace without introducing new dynamics, observer-dependent postulates, or interpretation-specific assumptions. The paper further shows that agency requires a closure-stable internal selection process capable of consistent expected-utility comparison. Imposing closure compatibility rigidly fixes the standard quadratic Born probability rule as the only weighting compatible with stability, representation-independence, and admissibility-preserving re-expression.

Together, these results integrate agency into the same structural framework that accounts for quantum measurement and probability. The absence of agency in purely quantum-coherent models is shown to reflect an under-constrained admissibility regime rather than a fundamental limitation of quantum theory. Lawful agency emerges as a closure-stable property of coupled agent–world dynamics, fully consistent with standard quantum mechanics.

This work clarifies the physical limits of purely quantum models of decision, sharpens existing no-go arguments, and provides a mathematically explicit foundation for agency grounded in structural admissibility rather than interpretation.