  What does follow are two books of propositions that provide means for inferring forces from motions and motions from forces and a final book that illustrates how these propositions can be applied to the system of the world first to identify the forces governing motion in our planetary system and then to use them to differentiate between certain true and apparent motions of particular interest. In this respect, the empirical content of the theoretical concepts that Newton has explicated in the section called “Definitions” is inextricably linked with the physical theory presented in the rest of the Principia.