Admissible Histories and Allowity in Constraint-Based Physical Descriptions

Physical theories constrain systems through equations of motion, symmetries, boundary conditions, and variational principles. In many cases these constraints do not uniquely specify a single trajectory or configuration, but instead permit a family of admissible histories consistent with the same governing structure. This multiplicity is often treated implicitly, absorbed into formalism, or deferred to probabilistic interpretation. In this work, I introduce allowity as a diagnostic concept that characterizes the effective extent of admissible histories implied by a set of constraints, prior to any statistical weighting, measurement, or interpretation. Allowity is not randomness, uncertainty, freedom, or ignorance; it is a structural property of constraint-based descriptions. By making admissibility explicit, this framework clarifies how physical descriptions may remain stable despite non-uniqueness, or become brittle despite apparent determinism. The analysis is conservative and framework-neutral: it introduces no new dynamics and modifies no existing formalism.