Mach's Principle and Mach's Hypotheses

We argue that the driving thought behind Mach’s critique of Newton’s first law consists in the assertion that inertial motion is not motion in the absence of causes; rather, it is motion whose cause lies in some homogeneous aspect of the environment. We distinguish this formal requirement (Mach’s principle) from two hypotheses which Mach considers concerning the origin of inertia: that the distant stars play a (1) merely “collateral” or (2) “fundamental” role in the causal determination of inertial motion. This interpretation is made possible by close attention to some of Mach’s earliest writings. We propose that much of the controversy in secondary literature concerning the definition of Mach’s principle stems from Mach’s deliberate avoidance of explicitly referring to the concept of causation in subsequent writings