Abstract
Phase Differential Theory (PDT) provides the physical interpretation of the geometric structures developed in Phase Transport Geometry (PTG). This first paper in the PDT series introduces Axiom 0, which asserts that coherent physical systems possess a finite coherence capacity. When the total curvature loading exceeds this capacity, coherence collapses and the system transitions to a new configuration. Combined with compact transport (PTG-2), the closure-compatible metric (PTG-3), and the sector structure (PTG-4), Axiom 0 implies the existence of a positive stability threshold that is interpreted as a positive mass gap within the PTG/PDT framework. PDT-I therefore establishes the physical foundation for the mass hierarchy developed in PDT-II and the collapse dynamics formalised in PDT-III.