 Berkeley thus made clear that the older understanding of inertia, unlike that expressed in the Principia, did not respect the principle of relativity. Second, Newton’s explication implicitly invokes all three laws of motion (cf. Stein 2002). Newton’s first law alone came to be identified as “the principle of inertia.” Newton himself, however, understood that inertia has three inseparable aspects: the tendency to persist in motion, the resistance to change in motion, and the power to react against an impressed force. All are essential to the explication of inertial mass as a measurable theoretical quantity. To many later commentators, Newton’s use of the phrase “force of inertia” suggested a conceptual confusion. On the contrary, it was Newton’s way of drawing attention to the precise role of inertial mass as an invariant quantity in physical interactions, underlying the various ways in which its manifestations had been previously conceived.