Motion of a body with respect to absolute space or to the inertial structure of spacetime is ruled out on epistemological grounds.

There are essentially two ways to understand Mach’s Principle: (1) Mach’s Principle rejects the absolute character of the inertial structure of spacetime, and (2) Mach’s Principle rejects the inertial structure of spacetime per se. Version (2) might be characterized as Leibnizian relativity or body relationalism; that is, one understands by relative motion the motion of bodies with respect only to other observable bodies or observable bodily reference frames. The relative motion of a body