Stable Distinguishability as a Minimal Criterion for Physical Entity: A Constraint-Primacy Formal Framework

This paper proposes a minimal formal criterion for physical entity. It does not attempt to derive quantum mechanics, general relativity, quantum gravity, or any empirical prediction. Its aim is narrower: to ask what formal conditions must be satisfied before a structure may be treated as a physical entity at all. The starting point is the principle of stable distinguishability: a physical theory is possible only where distinctions can be made and re-identified under admissible change. From this requirement, the paper develops a constraint-primacy framework in which no particle, field, spacetime point, state, measurement, or observable is assumed as primitive. The formal layer consists of tokens, admissibility, and a closure operator defined on the full power set of tokens. Identity is reconstructed as closure-equivalence. Boundary is defined non-spatially as first-entry dependency, and core is defined non-circularly as a closed non-boundary substructure. A physical entity is then defined as a boundary-stable closure-core equivalence class: a closed admissible structure whose core closure-identity is preserved under relevant boundary-mediated extensions. The paper uses familiar mathematical tools, including closure, equivalence, isomorphism, and invariance; its proposed contribution is not those tools themselves, but the ontological criterion they support.