One State, Many Perspectives: Everettian Quantum Mechanics Without Worlds and Without a Special Probability Problem

Everettian quantum mechanics is commonly discussed as the “many-worlds” interpretation. This article argues that this label misidentifies the ontology and obscures the probability problem. The proposed framework, perspectival quantum monism, keeps a single universal quantum state, treats branches as robust perspectival structures rather than countable worlds, and makes every empirical description relative to a descriptive reference frame: a choice of system-environment cut and grain of description. On this basis, the article separates the probability problem into two questions. The quantitative question asks why rational weights should have the Born form. That question is answered by Theorem T*, established in a companion technical article by reduction to Gleason’s theorem and Short’s theorem; its central descriptive route has been mechanically verified in Lean 4, with Gleason’s theorem isolated as the single declared axiom. The semantic and normative question asks what uncertainty and rational expectation mean when all outcomes occur. The article answers this question through internality, self-location on accessible records, and a comparison withdeterministic classical probability. The remaining bridge from objective measure to rational expectation is not derived; it is the same bridge already presupposed by ordinary probabilistic physics. The resulting claim is therefore limited but precise: Everettian quantum mechanics, formulated without worlds-as-entities, has no special probability problem beyond the general measure-to-expectation bridge used throughout physics.