Emergent Inertia and Mach's Principle (Revisited XIX.E, LXCVI.856): Theories where inertia emerges from the interaction with the rest of the universe's mass distribution, or from the structure of the vacuum or spacetime itself, are examples of emergent gravity ideas applied specifically to the origin of inertial mass. This would resolve the mystery of why inertial mass and gravitational mass are equivalent (the Equivalence Principle V.A) by making both arise from the same underlying mechanism.