Causality Without the Classical Triad: Reichenbach's Principle as a Theorem, and the Bell Correlations as Its Regime Boundary

Wood and Spekkens showed that classical causal explanations of Bell-inequality violations require fine-tuning, sharpening a long-standing tension between quantum correlations and Reichenbach’s Common Cause Principle, and the program they opened asks for a means of causal explanation that privileges no member of the classical triad — “A causes B,” “B causes A,” “a common cause produces both.” This paper presents such a means, extracted from a framework of observer-indexed partial information states, and proves two things about it. First, in the regime of mutually exclusive alternatives, Reichenbach’s principle — demand, disjunction, and screening certificate together — is a theorem: with the two causal horns defined behaviorally, the disjunction is exhaustive by construction, and pairwise-composed joint couplings provably realize the screened horn, the certificate arriving as a one-line factorization. The classical triad is then recovered as the narration inventory of the two horns — five folk stories, two arrangements, one signature — with irreducible higher-arity couplings classified onto the direct horn, where their strained folk telling supports the analysis. Second, the Bell-type deviations are located as that theorem’s regime boundary (on the framework’s analysis of the entangled state as co-obtaining): where alternatives co-obtain rather than exclude, the certificate’s quantification loses its subject, and the correlations escape screening structurally — no parameter is tuned, because there is no parameter — while the grounding demand remains satisfied in full. The deviation is thereby the boundary of a derived theorem, not an exotic causal species. No causal arrow appears anywhere in the analysis.