In another sense the GTR is in direct opposition to Mach s principle In Mach s universe you can not have a complete description of a body if you do not know the dynamics of everything else in the universe In contrast the GTR is a gauge theory which deals with a strictly local field Fundamentally it is hopeless to do physics with Mach s principle The GTR saves the science of physics because you can give a complete description from local observations In that sense the principle of relativity is a theory ABOUT physics rather than a theory OF physics