Born Rule as a Closure Fixed Point: { Uniqueness of Quantum Probability in Perfectly Self-Contained Theories}

We prove that the Born rule is the unique consistent probability assignment for any perfectly self-contained (PSC) theory whose records carry quantum effect structure. The argument has two interlocking stages. Formal stage. We machine-check in Lean 4 that any normalized, POVM-additive probability assignment \mu on n-dimensional quantum effects has at most one density operator \rho representing it: [ \mu(E) = \re\bigl(\Tr(\rho E)\bigr) \quad\textfor all effects E. ] What is machine-checked here is the uniqueness of the density-operator representation under the closure constraints. Existence of such a representation is imported from known Busch/Gleason-type results. The uniqueness theorem busch_gleason_unique is fully proved with zero sorrys, using three families of test effects that together span \Herm(n) and extract every matrix entry of \rho. Physical stage. PSC forces any record-level probability assignment to be noncontextual and POVM-additive, because probabilities that depend on external measurement context introduce semantic externality that PSC excludes. Therefore, if a Born-rule density operator exists (Busch/Gleason existence, acknowledged as a known result), it is provably unique by our machine-checked argument. The Born rule is thus not a postulate but the only fixed point of closure. The approach generalizes standard Gleason-type derivations: it applies to POVMs (not only projectors), requires no dimension lower bound, and is interlocked with the NEMS record-closure framework. The Lean artifact constitutes the first machine-verified uniqueness proof for quantum probability in an operationally grounded formalization. This overview presents the core NEMS theorem engine and selected applications; stronger domain-specific derivation and ontological synthesis claims belong to separate release surfaces with their own premise bundles and formal artifacts. Trust boundary. Machine-checked content is the density-operator uniqueness lemma under stated effect constraints (nems-lean ); Busch/Gleason existence is imported as classical mathematics. PSC⇒noncontextual semantics links are premise-sensitive. See .