This volume explains why the standard formalism of quantum mechanics is not merely successful, but structurally inevitable. Rather than proposing a new interpretation or modifying existing equations, this work derives quantum mechanics as the only stable descriptive framework compatible with the FSPS (Foundational Structure / Projected Structure) model. Within FSPS, observable physics (PS) arises as a projection from a non-geometric foundational layer (FS), and this projection necessarily introduces non-commutativity, probabilistic structure, and unitary evolution. The volume combines the axiomatic foundation of FSPS–00 v2.2 with a focused derivation showing that classical mechanics is structurally unstable under projection, while the Hilbert-space formalism of quantum mechanics represents the minimal information-preserving description. Measurement, wavefunction collapse, and probabilistic outcomes are treated not as additional physical processes but as re-projection and stabilization events within PS. No changes are made to Schrödinger dynamics, operator algebra, or experimental predictions. This work serves as a unifying explanation for why quantum mechanics has its present form, consolidating earlier FSPS-based analyses into a single, coherent account aligned with modern quantum theory.