  In the early stages of his work on the Principia Newton had identified three logically equivalent alternatives for the third law: the action-reaction principle he ultimately chose, the principle we call conservation of linear momentum (Corollary 3 in the Principia), and the principle that “the common center of gravity of two or more bodies does not change its state whether of motion or of rest as a result of the actions of the bodies upon one another” (Corollary 4). Huygens had stated that both of these principles follow from his solution for spheres in collision, and the center of gravity principle, as Newton emphasizes, amounts to nothing more than a generalization of the principle of inertia. Even though his third law was novel in comparison with these other  two,[23] Newton nevertheless chose it and relegated the other two to corollaries. Two things can be said about this choice. First, the third law is a local principle, while the two alternatives to it are global principles, and Newton, unlike those working in mechanics on the Continent at the time, generally preferred fundamental principles to be local, perhaps because they pose less of an evidence burden. Second, with the choice of the third law, the three laws all expressly concern impressed forces: the first law authorizes inferences to the presence of an impressed force on a body, the second, to its magnitude and direction, and the third to the correlative force on the body producing it. In this regard, Newton's three laws of motion are indeed axioms characterizing impressed force. Real forces, in contrast to such apparent forces as Coriolis forces (of which Newton was entirely aware, though of course not under this name), are forces for which the third law, as well as the first two, hold, for only by means of this law can real forces and hence changes of motion be distinguished from apparent ones.