Admissibility Before Modelling: A Pre-Theoretical Constraint Layer for Scientific Systems

Scientific models typically describe system behaviour after valid system states are already assumed. However, the structural conditions that permit states to exist within a system are rarely formalised explicitly. This paper introduces a minimal pre-theoretical constraint layer governing system membership and continuation. Two conditions are identified as necessary and sufficient for system membership: admissibility and reachability. A state belongs to a system if and only if it satisfies governing constraints and lies on at least one admissible trajectory from a permitted origin. If either condition fails, continuation becomes impossible regardless of the governing equations applied. This framework clarifies that many scientific models implicitly assume admissibility conditions but do not define them explicitly. The Paton System is therefore positioned as a domain-neutral structural framework operating prior to domain-specific modelling across physics, computation, biological systems, and complex systems theory.