  The purpose of Book 2 is to provide a conclusive refutation of the Cartesian idea, adopted as well by Leibniz, that the planets are carried around their orbits by fluid vortices. Newton's main argument, which extends from the beginning of Section 1 until the end of Section 7, occupies 80 percent of the Book. Section 9, which ends the Book, offers a further, parting argument. We best dispense with this second argument before turning to the first.