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Abstract Physical theories repeatedly encounter foundational crises at the boundary of invariant causal propagation, a feature that fixes causality and enables distinction, persistence, and measurement while resisting reduction to dynamical mechanism, probabilistic ensemble, or material substrate. Light, understood here not as electromagnetic radiation but as the structural invariant of propagation itself, has repeatedly forced theoretical revision across classical mechanics, relativity, quantum theory, and thermodynamics, where quantization, irreversibility, horizon formation, and finite cosmic extent emerge as adaptations to this constraint rather than derivations from it. This recurrent pattern is diagnostic of an underlying ordering. A Light Ordering Theory follows from treating invariant causal propagation as logically prior, thereby fixing the constraints under which physical description remains admissible. These constraints close onto a minimal analytic kernel governed by three unavoidable conditions: propagation, identity through admissible continuation, and accessibility bounded by irreversibility and global extent. Joint enforcement of these conditions fixes the admissible domain of physical histories. Within this ordering, discrete mode structure, spin, entropy growth, geometric response, horizon thermodynamics, and vacuum saturation arise as constrained consequences rather than independent postulates. The cosmological constant emerges not as vacuum energy residue or thermodynamic artifact but as the geometric regulator fixing admissible extent itself. Apparent fractures between quantum mechanics, general relativity, and thermodynamics are traced to misordered primitives rather than incompatible principles. When invariant propagation is treated as prior and global admissibility is fixed by Λ, these domains align without modification of their local equations. Numerical protocols certify admissibility boundaries and structural consistency across corridors of apparently unrelated problems, providing falsifiable tests of the ordering. The framework does not claim closure but establishes light as the ordering constraint that delineates where structure must adapt and where it must terminate.