Origin of Rest and Relativistic Inertial Mass from Spacetime Exclusion and Entropy Progression

The physical origin of inertial mass, both in its Newtonian (rest) form and its relativistic growth, remains conceptually unexplained within Special and General Relativity. While these theories accurately describe kinematics and spacetime geometry, they do not provide a causal mechanism for resistance to acceleration. In this paper, we present a unified derivation of inertial mass based on the Spacetime Exclusion Principle (SEP) and the Entropy Progression Principle. Matter is shown to resist accelerationdue to constrained entropy reconfiguration arising from limited spacetime availability. Rest mass emerges as baseline spacetime exclusion, while relativistic inertial mass results from entropy saturation as velocity approaches the universal speed limit. Acceleration failure is explained as a breakdown of causal distinguishability rather than an infinite-energy requirement. The framework reproduces Special Relativity in its domain of validity, preserves the Equivalence Principle locally, and supplies a physical ontology absent in standard formulations.