"Mach's profound critique of the foundations of Newtonian mechanics played a key role in Einstein's development of . Mach's principle has also guided other developments in gravitation theory such as the ... It has inspired interesting experiments, such as the ... and continues to be of current interest ... Mach identified the essential epistemological shortcoming of the Newtonian foundations of physics, namely, that the intrinsic state of a particle in Newtonian mechanics, i.e. its mass, has no immediate connection with its extrinsic state in space and time, i.e. its position and velocity. Mach's observation can be re-stated in terms of the a priori independence of position (x) and momentum (p) of a particle in Newtonian mechanics."