Mach's principle is incorporated into classical General Relativity by writing Einstein's equations as explicitly covariant integral equations involving retarded bi-tensor Green's functions. These Green's functions must be found for the Robertson–Walker metrics in order to discover which, if any, of those universes are Machian. In this theory all empty universes are non-Machian and the cosmical constant is zero. It is argued that in any truly Machian theory space-time itself must be caused by the matter and in this theory it may be considered as propagating over itself out from the matter. Our Mach's principle may also be expressed as a boundary condition on the past singularities of the universe in a way that corresponds to the idea that all space-time and therefore all inertial and gravitational influences come from matter.