The Labyrinth of Quantum Facts: When Relational Events Acquire Intersubjective Status

Relational Quantum Mechanics (RQM) has recently been extended from projective measurements to a POVM-native formulation in which relational events are identified with effects. That extension removes an unnecessary restriction to sharp measurements and gives RQM a natural way to represent weak, noisy, and continuous outcomes. A further distinction is nevertheless required. For a fixed input state, a POVM effect determines the probability of a reported outcome, whereas a general quantum instrument also determines the conditional transformation associated with that outcome. These representational levels are not equivalent. The single-Kraus update displayed in the current POVM-native formulation describes efficient outcome operations, while a general instrument permits arbitrary completely positive trace-nonincreasing operations. More fundamentally, fixing an effect leaves the compatible conditional continuation maximally underdetermined: for a fixed input state and a nonzero effect with nonzero outcome probability, every posterior density operator can arise from a quantum operation with that same effect. A noisy-detector model exhibits the distinction without exotic assumptions, and Kraus fine-graining cannot remove it unless the refinement is physically recorded. The consequence is constructive rather than eliminative. Effects can continue to individuate local relational events, but temporal continuation requires instrument-level structure, while interperspectival sharedness requires a process-level warrant appropriate to the task by which facts are compared. The resulting hierarchy preserves the central motivation of POVM-native RQM while fixing the scope of what an effect can represent.