  Equally, the contention that the empirical reasoning in the Principia does not presuppose an unbridled form of absolute space should not be taken as denying that Newton invoked absolute space as his means for conceptualizing true deviations from inertial motion. Corollary 5 to the Laws of Motion, quoted above, put him in a position to introduce the notion of an inertial frame, but he did not do so, perhaps in part because Corollary 6 showed that even using an inertial frame to define deviations from inertial motion would not suffice. Empirically, nevertheless, the Principia follows astronomical practice in treating celestial motions relative to the fixed stars, and one of its key empirical conclusions (Book 3, Prop. 14, Corol. 1) is that the fixed stars are at rest with respect to the center of gravity of our planetary system.