 The proportionality of weight to mass was understood in its broader foundational significance, as the equivalence of gravitational and inertial mass, through Einstein’s “principle of equivalence” (cf. Einstein 1916; see also Norton 1985). In Einstein’s reasoning, the identity of inertia and gravitation helped to undermine the special status of inertial motion, and suggested the extension of the relativity principle from inertial frames to frames in any state of motion whatever. If an inertial frame, K, cannot be distinguished from another frame K′ that is uniformly accelerated with respect to K, then K′ may equally be treated as a “privileged” or “stationary” frame: “they have equal title as systems of reference for the description of physical phenomena” (Einstein 1916, p. 114). This circumstance undermines a defining characteristic of inertial frames: that with respect to a given inertial frame, every other inertial frame is in uniform rectilinear motion. Corollary VI points the way, after all, toward an extended relativity principle.