Structural Closure and the Admissibility of Physical Description

We present a two-layer axiomatic architecture governing the admissibility of physical description andthe physical realization of its limits. The first layer, the Closure Admissibility Principle (CAP), specifiesstructural conditions under which physical claims are admissible, recoverable, and stable under refinement.The second layer, denoted Θ, characterizes physical boundaries at which further admissible refinement isprohibited by CAP. We show that CAP admits a strict and exhaustive closure, and that Θ-boundaries arisenecessarily as physical instantiations of admissibility constraints. The resulting CAP–Θ architecture isclosed: no third descriptive layer is possible without violating admissibility or collapsing into one of thetwo existing layers. The framework introduces no new dynamics, ontology, or interpretive postulates. Itsrole is to govern the consistency of physical description across quantum, gravitational, and informationalsettings.