Irreversibility from Finite Causal Representation

Irreversibility is typically explained through probabilistic typicality, coarse-graining, or domainspecific dynamical hypotheses. This note isolates a structural source independent of statisticalassumptions. Any coherently evolving system that participates causally through finitely distinguishable internal state must merge distinct histories, and once merged, those distinctionscannot be recovered through further coherent evolution. The result is monotonic growth of indistinguishable pasts and therefore directional ordering. The argument is bidirectional: forwardevolution compresses histories into state, while reconstruction from state yields only equivalenceclasses of pasts. Thermodynamic entropy increase is identified as one domain instantiation ofthis structural constraint.