Observation Actualizes Properties: A Quantum-Grounded Theory of Observer-Dependent Reality

Part 3 of this series established that agents (observers) are not epiphenomenal in the determination of their own future states. This paper extends that argument to a fundamental metaphysical principle: observation is not merely epistemic (a way of knowing), but ontological (constitutive of what properties are actually realized). We demonstrate using quantum mechanics, information theory, and the formal structure of measurement that: (1) The universe contains a unique, unchanging material substance M; (2) M possesses a superposition of potential properties; (3) Which properties are actualized depends on the observer’s measurement choice; (4) This is not mysticism; it is the formal structure of quantum mechanics; (5) Classical physics emerges as the special case where measurement choice appears collapsed. Conclusion: Reality is observer-dependent in a rigorous, physically grounded, and non-arbitrary way.