The Ontology of Mass: Spatial Density of Constrained-State Energy — A Physical Foundation of Energy-Efficiency Theory

The ontological question of mass is a central pursuit in physics, yet existing theories have not provided a unified answer rooted in a first‑principles energy ontology. This paper develops an interpretation within Energy-Efficiency Theory (EET). Starting from Yang's three axioms, we define mass as the spatial density of constrained-state energy: m=Ec/c2, where Ec is the constrained-state energy—energy localized in stable structures. This definition is not circular: for composite systems (atomic nuclei, molecules, macroscopic objects), Ec can be independently calculated from first principles (e.g., quantum chromodynamics, density functional theory); for elementary particles, the definition is interpretive,…