 Mach’s remark roughly corresponds to Newton’s actual procedure. Even though, for Newton, absolute space was the implicit reference-frame for stating the laws of motion, the frame for their application was the standard one for most of the history of astronomy: the fixed stars. This seemingly arbitrary starting-point did not undermine Newton’s procedure as an account of the “true motions.” For the framework of the fixed stars, initially taken for granted, turns out to be justified in the course of Newton’s dynamical analysis. If all the accelerations relative to the fixed stars can be analyzed into action-reaction pairs involving bodies within the system—leaving no “leftover” accelerations that need to be traced to some yet-unknown influence—then we can conclude that the stars are a suitable (sufficiently inertial) frame of reference after all. Newton could appeal to a particular case to test this general point: the orbits of the outer planets were stable with respect to the fixed stars, their perihelia showing no measurable precession (unlike the perihelion of Mercury, for a famous example). Newton argued, then, that a relative space in which these apsides are stable is a sufficient approximation to a space at rest or in uniform motion (cf. Book III, Proposition XIV, 1687b, p. 420). 