 Newton understood the Galilean principle of relativity with a degree of depth and clarity that eluded most of his “relativist” contemporaries and critics. It may seem bizarre, therefore, that the notion of inertial frame did not emerge until more than a century and a half after his death. He had identified a distinguished class of dynamically equivalent “relative spaces,” in any of which true forces and masses, accelerations and rotations, would have the same objectively measured values. Yet these spaces, though dynamically equivalent and empirically indistinguishable, were yet not equivalent in principle. Evidently, Newton conceived them as moving with various velocities in absolute space, though those velocities could not be known. Why should not he, or someone, have recognized the equivalence of these spaces, and the dispensability of a distinguished resting space—“absolute” space—immediately?