The Observer as Logon: Completing Quantum Mechanics

Quantum mechanics has operated for a century without a formal definition of the observer. Every interpretation — Copenhagen, many-worlds, pilot wave, QBism, decoherence — either presupposes the observer, brackets it, or dissolves it into environment. None defines it. This paper supplies the missing definition. The observer is a logon: a perception-action dyad ℓ = (p, π), where perception engages the model (wave function) and action produces a determinate carrier (particle). This single identification resolves the measurement problem, explains wave-function collapse, derives the Born rule, accounts for decoherence, and renders every major interpretation either a special case or an unnecessary appendage. The framework requires three definitions and no additional axioms. Collapse is made explicit as a primitive instantiation rule rather than left as an undefined "measurement" primitive. From this, everything follows.