Observer Instantiation and Functional Invariance

The measurement problem is commonly taken to show that unitary quantum mechanics is explanatorily incomplete, requiring collapse postulates, semantic reference rules, or observer-relative selection principles to recover determinate outcomes. This paper argues that the appearance of incompleteness arises from a category error: the misidentification of observers as primitive tensor factors rather than physically instantiated structures. We introduce a stability-based criterion of observer instantiation grounded in dynamical robustness, environmental redundancy, and effective irreversibility. Using this criterion, we prove the Unitary Closure Theorem: for any physically instantiated observer, unitary quantum mechanics alone entails determinate records relative to its environment, without collapse, hidden variables, or semantic postulates. The framework distinguishes mathematical coexistence from physical co-instantiation, rejecting Everettian ontological proliferation while preserving full unitarity. Uninstantiated branches are treated as unrealized structural possibilities rather than alternative histories. The resulting position—Dynamic Singularity—allows realism about unitary quantum mechanics without commitment to Many-Worlds. The account resolves the measurement problem by grounding determinacy in physical instantiation within a single causal history, closing the theory without altering its dynamics.