A Formal Applicability Boundary for Physical Description: Complementarity, Irreversibility, and Thermodynamic Cost

This work formalizes a precise boundary at which physical description becomes applicable. Rather than proposing new dynamics or ontologies, it specifies the minimal logical and thermodynamic conditions required for physical concepts to be meaningfully defined. Building on the framework established in Minimal Necessary and Sufficient Conditions for a Physical Universe, the present work identifies distinction, complementarity, and irreversibility as structural prerequisites for physical description. Information content and thermodynamic cost are treated not as fundamental givens, but as quantities that become well-defined only once an irreversible distinction is admitted. The framework is intentionally pre-physical. It concerns definability and irreversibility as logical conditions, independent of any assumed spacetime, fields, dynamics, cosmology, or horizon structure. Physical description is shown to be conditional rather than universal, applying if and only if information is present and irreversibility incurs cost. This work does not modify the minimal condition of distinction, nor does it elevate thermodynamics to a foundational role. Instead, it clarifies when and why physical concepts, including thermodynamic quantities, become applicable downstream of distinction. In this framework, thermodynamic quantities such as temperature are not assumed to exist prior to distinction. They do not evolve from an initial value, nor are they defined in a pre-physical regime. References to temperature and thermodynamic cost therefore indicate applicability, not a transition from an earlier thermodynamic state.