For a long time, mechanics based upon the principle of inertia and the law of gravitation is considered to be a reliable basis of science. In the 19th century, doubts about the explanatory potential of these principles arise, since fundamental questions continue to remain unanswered. Why do all bodies fall equally fast? How can gravitation propagate through empty space and at what speed?

Ernst Mach criticizes Newton’s principle of inertia. He contends that the tendency of a body to continue its movement in a straight line is not a property of the body, but an effect exerted on it by all other bodies in the universe. Starting with this criticism, Einstein develops his General Theory of Relativity.