The combination of Mach’s hypothesis concerning the material origin of inertia and Einstein’s equivalence hypothesis on the unity of inertia and gravity raises the intriguing possibility that gravity may be explained entirely as the dynamical part of a relativized law of inertia. Shortly after the development of general relativity, it became clear that Einstein’s general covariance based approach does not reduce inertia to mass interactions. While Einstein subsequently treated “Mach’s principle” as a selection criterion for models of his theory, there is an alternative research program that implements Mach’s idea explicitly in its foundations. The aim of my talk is to bring attention to some of the key 20th Century papers that (retrospectively) form part of this research program, focussing in particular on Reissner (1915) and Sciama (1953).  Although the approaches of Reissner and Sciama are quite different, they are unified insofar as they both embody this intriguing hypothesis. By looking at the common feature of such models, I draw out some of the key implications of the hypothesis, including the epochal variability of the gravitational constant and a potential guarantee of the critical density condition in cosmology. I will be open to talking at more length about the philosophical interpretation of these ideas in Q&A.