This preprint introduces the Coherence Principle (TCP), a variational and action-based foundation for the Sympathetic Harmonic Resonance Framework (SHRF). The work formalizes SHRF as a dynamical theory by specifying a single action from which mechanical, gravitational, and cosmological behavior emerge as limiting cases. Within TCP, physical dynamics arise from the evolution of a coherence-bearing field characterized by density and phase. Apparent forces, inertial response, gravitational attraction, and cosmological expansion are not treated as fundamental inputs, but as effective descriptions of how matter and spacetime respond to coherence structure and phase evolution. The paper derives the governing field equations from the action, identifies conserved quantities associated with phase symmetry, and demonstrates how classical mechanics is recovered in the point-particle limit. It further shows how gravitational behavior emerges from spatial variation of coherence density and how cosmological expansion and redshift arise from temporal coherence evolution in homogeneous settings. TCP is constructed to recover established Newtonian and relativistic limits while admitting clear falsification criteria. Deviations from standard behavior are predicted only in regimes where coherence gradients or phase dynamics become significant, and all coupling parameters are subject to experimental and observational constraint. This work serves as the foundational mechanics paper for SHRF. Prior SHRF studies addressing astrophysical anomalies, cosmological observations, information flow, and coherence-driven phenomena across domains are reinterpreted as applications of specific limits of the same underlying variational structure. No prior results are modified; their dynamical basis is made explicit. The Coherence Principle emphasizes conceptual economy, internal consistency, and testability. It provides a unified dynamical language intended to support further theoretical development, quantitative constraint, and observational comparison within the SHRF program. coherence principle, resonant coherence, variational principle, action-based dynamics, phase dynamics, emergent mechanics, emergent gravity, cosmological dynamics, coherence fields, phase locking, inertial emergence, gravitational emergence, cosmological expansion, redshift dynamics, conserved currents, symmetry and conservation, coherence gradients, sympathetic harmonic resonance framework, SHRF, theoretical foundations