  Up to the end of Section 10, Book 1 considers forces that are directed toward geometric centers rather than bodies. As a consequence, only the first two laws of motion enter into any of the proofs until late in Book 1. Even further, as Newton develops the theory to that point, only the accelerative measure of force is employed, and hence even mass plays no role. Included in this segment are by far the most widely read parts of Book 1, then and now: Section 2, which deals with centripetal forces generally, and Section 3, which develops Newton's fundamental discovery that a body moves in a conic section, sweeping out equal areas in equal times about a focus, if and only if the motion is governed by an inverse-square centripetal force directed toward this focus. The stick-figure picture of Book 1 that results from viewing these two sections as its high point blinds the reader not only to the richness of the theory developed in it, but also to several no less important results derived in the rest of it.