Bounded Causal Influence as a Structural Constraint Causal Physics Note

Finite propagation constraints are often introduced as dynamical or geometrical features ofspecific physical theories, most notably in relativity. We argue that a stronger, pre-dynamicalclaim holds: before introducing time parameters, metric distance, spacetime geometry, or anyspecific propagation constant, objective causal discourse already demands a nontrivial bound oncausal influence. Using a purely intervention-based formulation, we demonstrate that unboundedcausal reach—meaning every event is intervention-accessible from every other event—leads toinvariant modal collapse: no observer-independent distinction between influenceable and noninfluenceable event pairs survives representational relabelling. This no-go result establishesbounded causal influence as a minimal structural admissibility condition for any physical theorythat aims to support coherent causal explanation. The argument is model-independent andprecedes any choice of dynamical laws or spacetime structure.