Modal Actualism offers a realist, non-Everettian interpretation of quantum mechanics that preserves unitary dynamics while explaining definite outcomes without invoking collapse or branching. The central distinction is between modal structure—the full set of decoherent histories physically supported by the universal wavefunction—and actuality, the single decoherent history that is realised. Decoherence partitions modal structure into quasiclassical trajectories, but only one such trajectory constitutes the actual worldline. Probability is interpreted as modal measure: a structural feature of the wavefunction that quantifies the weight of possible histories without implying their actuality. This framework dissolves foundational paradoxes by blocking the inference from modal multiplicity to ontological multiplicity, thereby avoiding Everettian inflation while retaining the explanatory power of decoherence. Modal Actualism reconciles quantum correlations with relativistic locality and reframes cosmological fine-tuning as a feature of modal structure rather than evidence for multiverse ensembles
An outline for a modal interpretation in terms of possible worlds is presented. The so-called Schmidt histories are taken to correspond to the physically possible worlds. The decoherence function defined in the histories formulation of quantum theory is taken to prescribe a non-classical probability measure over the set of the possible worlds. This is shown to yield dynamics in the form of transition probabilities for occurrent events in each world. The role of the consistency condition is discussed.