Despite earlier attacks by naturalists like Telesio, humanists like Bruno, and eclectic proto-atomists like Basso, Aristotelian physics and its cornerstone doctrine of the substantial form underwent something of a revival in the early seventeenth century and managed to survive late into the century. Its ultimate demise had to await a second wave of attacks by figures we now hail as the great philosophers and scientists of the early modern era: Francis Bacon, René Descartes, Pierre Gassendi, Thomas Hobbes, Robert Boyle, and John Locke, to name just a few. But what frequently gets neglected in the story they tell, and which we continue to regurgitate, is the role that innovations within Scholastic Aristotelianism played in the shift from hylomorphism to mechanism. While the larger story yet to be told lies beyond the scope of this book, I will begin, in Part I, to make up for this neglect by showing that Descartes' metaphysical arguments against the substantial form are best understood against the background of Suarez's definition of the substantial form as an incomplete substance. Indeed, Descartes' arguments would fail against Aquinas' account of the substantial form. I will demonstrate that the post-Suarezian Scholastic doctrine of the substantial form, targeted by anti-Aristotelians like Gorlaeus and Descartes, had key features that facilitated its ultimate replacement, whether by atomism or mechanism. But first I examine Descartes' arguments against the substantial form in order to then make sense of them in light of the relevant arguments in its favor advanced by the premier authorities of the Jesuit educational system: St. Thomas Aquinas and Francisco Suarez.