My excavation of the context surrounding Descartes' rejection of Aristotelian material substantial forms has laid bare the various strata involved in his eventual replacement of forms with mechanisms. By situating each of the arguments he recommends to Regius both temporally and spatially within Descartes' corpus, and by identifying his most likely interlocutors during each phase, I have offered a plausible reconstruction of the steps by which Descartes came to eliminate material substantial forms. The end result is a more nuanced and, in many ways, less romanticized portrait of the renowned father of modern philosophy. I have argued that, against radically anti-Aristotelian skeptics like Sanchez, Descartes strives to preserve the Aristotelian ideal of scientia as causal knowledge of natural phenomena founded on necessary principles and certain demonstrations. In this sense, he is a conservative rather than a radical. In seeking to ground scientific knowledge on foundations that could withstand skeptical attacks, Descartes takes his inspiration from the budding Aristotelian science of mechanics. By gradually conflating the objects of mechanics, physics, and mathematics, Aristotelian commentators on the Quaestiones Mechanicae held out the promise of providing secure mathematical demonstrations of physical phenomena. In keeping with Suarez's redefinition of the material substantial form as a concrete, physical entity justified by empirical arguments, Descartes attempts at first not to eliminate substantial forms but to cash them out in mechanical terms.