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, equating mass with the energy locked in the particle's field configuration. Using this definition, we unify four fundamental physical phenomena: (1) inertia as the stability of constrained-state energy; (2) the mass-energy equivalence as a conversion formula between free and constrained states; (3) nuclear reaction mass defect as the release of constrained-state energy; (4) the Higgs mechanism as the conversion of free-state background into constrained-state energy. We show that the equality of inertial and gravitational mass follows from the fact that both measure the same constrained-state energy, consistent with the equivalence principle. The framework yields two testable predictions: (1) the mass defect in nuclear reactions satisfies Δm=ΔEc/c2 with ΔEc calculable from nuclear binding energy; (2) in strongly constrained environments (e.g., neutron star cores), changes in the constraint barrier lead to measurable mass corrections. This paper is classified as a mixed-type paper in EET: the definition and derivations (Sections 2–4) are rule-consequent, while the predictions (Section 8) are natural-causal. It provides an energy-ontological foundation for mass, inertia, and gravity, bridging microphysics and cosmology.
Hongpu Yang (2026) studied this question.