**1. Introduction**

Before Einstein's creation of general relativity theory, the conception of inertia was that captured in Newton's laws of mechanics, Newton's elaboration of the idea of inertia, first introduced by Galileo some years earlier. Inertia, properly *vis inertiae* or inert force, was taken to be an inherent property of "matter" conferred on it by its existence in absolute space, that only ceases to be inert when external forces act on matter to produce proper accelerations, rising to produce the reaction force the matter exerts on the accelerating agent to resist the impressed force. Already in Newton's day, this conception of inertia as due to absolute space was seriously called into question by, among others, Bishop Berkeley who argued that a body in an otherwise empty universe would have no inertia since there would be no other matter to refer motion of the body to. From this point of view, absolute space's action on matter is not the origin of inertia, the action of other matter in space is the cause of inertia. In the 17th and 18th, and most of the 19th centuries, Newton's view prevailed.

Berkeley's conjecture was revisited in the late 19th century by Ernst Mach, who noted that local rotation coincided with rotation relative to the "fixed stars",

suggesting that local inertial frames of reference were determined by some long-range action of matter at cosmological distances. Einstein took Mach's insight to mean that in any properly constituted theory of gravity, inertia would emerge as an "inductive" gravitational effect of cosmic matter since gravity was/is the only known long-range force that might cause such effects. His first explicit attempt in this direction appeared in his, "Is There a Gravitational Effect Which is Analogous to Electrodynamic Induction?" in 1912 [1]. Einstein noted that Newtonian gravity was not sufficient to correctly encompass the induction of inertia – that is, the generation of the mass of matter by the gravitational interaction with chiefly cosmological matter – and inertial reaction forces – that is, Newton's third law forces on accelerating agents. Induction requires vector or tensor interactions. Several years later, general relativity was Einstein's theory that he was convinced accomplished this task. Indeed, that's why he called it "general relativity" because he, as we would say today, "unified" inertia and gravity by making inertia an inductive gravitational effect. Analogous to Maxwell's "unification" of electricity and magnetism in his electrodynamics.
