  Newton originally planned a two-book work, with the first book consisting of propositions mathematically derived from the laws of motion, including a handful concerning motion under resistance forces, and the second book, written and even formatted in the manner of Descartes's Principia, applying these propositions to lay out the system of the world. By the middle of 1686 Newton had switched to a three-book structure, with the second book devoted to motion in resisting media. What appears to have convinced him that this topic required a separate book was the promise of pendulum-decay experiments to allow him to measure the variation of resistance forces with  velocity.[14] When Hooke raised a priority issue on inverse-square forces, Newton dropped the original version of the last book, switching to presenting the system of the world in a sequence of mathematically argued propositions, many of which demand far more of the reader than anything in the original version. The original “System of the World” did appear in print the year after Newton died. No complete text for the original version of Book 1 has ever been found.