René Descartes gives few philosophical arguments to directly support his rejection of forms in favor of mechanisms. Moreover, the scattered reasons he offers in his corpus are cryptic and hard to unpack. Hence I will draw on Descartes' intellectual context to reconstruct his reasoning and shed light on his historic elimination of Scholastic Aristotelian substantial forms from the physical world. Given that Descartes continues to call the soul a substantial form, my focus will be on his rejection of material substantial forms employed in Aristotelian physics (for lack of a better term I will refer to all substantial forms that exist only in matter, i.e., all except the rational soul, as ‘material substantial forms’). I will not, therefore, examine the viability of his claim that the soul is the substantial form of a human being and instead refer the reader to the body of literature that already exists on this subject. Unlike the rational soul, which was thought to be directly created by God and to survive the body, material substantial forms were widely held to be educed from pre-existing matter, and to exist only in matter. It is only by familiarizing ourselves with contemporaneous arguments for and against such forms and the philosophical issues at stake in this debate that we can fully understand and appreciate Descartes' contribution to their ultimate elimination from physics. We are all familiar with the Cartesian rhetoric against substantial forms.