 The Galilean relativity principle thus contained, in Newton’s conception, a broader insight: that different states of uniform motion, or different uniformly-moving frames of reference, determine only different points of view on the same physically objective quantities, namely force, mass, and acceleration. We can see this insight expressed more explicitly in the evolution of Newton’s concept of inertia. The term had been introduced by Kepler, and played a central role in his physical conception of planetary motion. Rejecting the Aristotelian idea that the planets are carried by rotating crystalline spheres, Kepler held that the planets have a natural tendency to rest in space—what he called their natural inertia—and argued that they must be moved by active powers that overcome their natural inertia. Newton, well before working on Principia, had based his conception of inertia on the idea of Galileo and Huygens, that bodies tend to persist in uniform motion: Inertia, on this new conception, was a resistance to change in motion. Even so, Newton’s early understanding of inertia was essentially pre-relativistic, because it implied a conceptual distinction between a body’s power to resist external forces, and the power of a moving body to change the motion of another. The manuscript De gravitatione et aequipondio fluidorum (1684a), for example, was evidently written before Newton fully recognized the importance of the relativity principle; here Newton’s Definitions distinguish “conatus,” “impetus,” and “inertia” as conceptually separate properties: