  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 with respect to absolute space or to the  inertial structure of space (Newton) or spacetime is ruled out on  epistemological and/or metaphysical grounds. 