Constraint Closure and the Quantum-to-Classical Transition: Decoherence, the Measurement Problem, and the Ontology of Macroscopic Determinacy

The quantum-to-classical transition — the disappearance of quantum superposition, interference, and entanglement at macroscopic scales — is standardly explained through decoherence: environmental entanglement suppresses interference terms, making macroscopic superpositions effectively unobservable. Decoherence is mathematically rigorous and empirically well-confirmed, but it faces a persistent philosophical limitation: it explains the appearance of classicality without fully accounting for its ontology. The measurement problem survives decoherence because decoherence does not explain why quantum systems produce definite outcomes, why the pointer basis is selected, or what it means for a macroscopic object to have a determinate state rather than merely appearing to have one. This paper argues that Constraint Theory (CT) — the thesis, established by transcendental argument, that constraint is the ontological primitive constitutive of determinate existence as such — provides the ontological grounding that decoherence theory presupposes but does not supply. On the CT account, quantum superposition is open constraint configuration: the genuine ontological openness of a possibility space that has not been closed by the system’s constraint profile or its environmental interactions. Macroscopic determinacy is closed constraint configuration: the progressive closure of macroscopic possibility spaces by the cumulative propagation of environmental constraint into the system’s profile at a rate that overwhelms quantum coherence. Decoherence is the process by which environmental constraint propagation closes possibility spaces; CT explains why this closure has the ontological consequences it does rather than merely the epistemic consequences that the standard account describes. The pointer basis selection problem receives a principled CT resolution: the pointer basis is the constraint-stable basis, the set of states whose constraint pro files are robust under the specific pattern of environmental constraint propagation. The measurement problem is reframed as the question of which branch of an open constraint configuration is closed by a measurement interaction, a question that CT narrows to its irreducible residual without dissolving prematurely. The mesoscopic regime — where quantum behaviour persists at scales larger than individual particles — is explained as the regime of partial constraint closure: systems whose environmental coupling is sufficient to close some but not all dimensions of their possibility space. The paper engages the principal interpretations of quantum mechanics and the philosophical literature on decoherence, situating the CT account as an ontological foundation compatible with multiple interpretive positions.