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.

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.

Suarez places key elements of Aquinas' account of the substantial form, most notably his view that the immortal soul is both the form of the human body and a subsisting thing, on center stage and develops them. However, he also diverges significantly from Aquinas' arguments when he pries apart several concepts that, for Aquinas, are closely bound together. Suarez explicitly develops these distinctions in response to refinements and criticisms introduced by later Scholastic discussions of the substantial form. However, as I will argue in the next chapter, Suarez's explicit replies to criticisms that arose within the Scholastic tradition itself can also be seen as an implicit response to the devastating attack that had just been leveled against Scholastic philosophy by Renaissance humanists. Faced with the intellectual and political turmoil of a modern world, to many European universities, Protestant and Catholic alike, Suarez's innovative works provided a major resource for a revamped Aristotelianism that could meet the threat of skeptical humanism and other destabilizing intellectual movements. I will say more about the relationship between intellectual controversies in the Netherlands and Descartes' intellectual trajectory in Part III. For now, I limit myself to a discussion of Suarez's key arguments regarding material substantial forms and their relevance to Descartes' a priori argument against them.

I argued in Part I that Descartes' arguments based on the obscurity of substantial forms fail unless one presupposes his dualist metaphysics, but that his a priori argument against substantial forms is successful. In particular, I showed that it does not create a straw man if one reads it as targeting Suarez's doctrine of the substantial form and that, read this way, it reveals that, by this stage of his career, Descartes had a good grip on the strategy Suarez employed to argue for the existence of substantial forms. However, Descartes introduces this argument for polemical purposes after having re-acquainted himself with Scholastic philosophy in replying to the Objections to his Meditations. His own route to the rejection of substantial forms stemmed from scientific concerns, not from a critical engagement with Scholastic metaphysics. In Part II, I traced Descartes' arguments based on his identification of natural objects with machines, and on the superiority of mechanical explanations, back to developments in Aristotelian mechanics, and interpreted the nature of the scientific demonstrations he employs in the Discourse in this light. There, Descartes' scientific explanations take the form of demonstrations encountered in the newly founded science of mechanics. Mechanical demonstrations were considered mathematical in nature because they were based on the principles of geometry, and this is the sense in which Descartes takes his scientific explanations to be mathematical. Finally, I proposed a reading distinguishing Descartes' early theory of matter from his mature metaphysics.

In this chapter I examine the metaphysical basis for Gorlaeus' elimination of substantial forms before unpacking Descartes' argument against material substantial forms based on the substance/mode distinction (argument 5 among the ones he offers to Regius). Gorlaeus firmly rejects the Aristotelian substance/accident distinction in favor of a substance/mode ontology, and hence anticipates Descartes' later metaphysics. Little is known about David Gorlaeus. Most of the scanty biographical details we have were uncovered by F. M. Jaeger in 1918. Christoph Lüthy, in an article on Gorlaeus' atomism, supplements Jaeger's findings with a few more recent discoveries. Two works by Gorlaeus survive: the Exercitationes Philosophicae (Philosophical Exercises) published in 1620, and the Idea Physicae (Physical Idea), which did not appear in print until 1651. I will focus on the Exercitationes, both because of its earlier publication date and because it contains the metaphysical foundations of Gorlaeus' physics, whereas the Idea Physicae is a summary of his physics. Both works were published posthumously, since Gorlaeus died in 1612, at the young age of twenty-one. From the fact that Gorlaeus refers to Galileo's telescopic discoveries, Lüthy establishes that these works were written sometime between 1610 and 1612. That someone so young could have authored them is remarkable. Lüthy attributes the innovative ideas they contain in part to Gorlaeus' educational trajectory. In 1606 Gorlaeus enrolled as a student at the Frisian University of Franeker, where he would have been exposed to the natural philosophy of Italian naturalist philosophers, like Girolamo Cardano, through the teachings of Professor Henrico de Veno. In 1611, he enrolled as a theology student at the University of Leiden just before the controversy over the hire of Vorstius broke out.

Descartes' most sustained arguments against substantial forms occur in the correspondence of January 1642 with Henricus Regius, where Descartes instructs his Dutch disciple how to defend himself against Voetius' attacks on the Cartesian natural philosophy Regius taught at the University of Utrecht. For Regius' benefit, Descartes collects several objections against substantial forms one finds throughout his writings and adds a few more. He organizes his discussion around seven theses, following the structure and order of Voetius' objections, and urges Regius to employ and elaborate on his suggested replies in answering Voetius. In the course of countering some of Voetius' specific points, Descartes offers several arguments to defend the superiority of Cartesian principles over the Scholastic notions of substantial forms and real qualities. These can be divided into two broad classes: scientific arguments and metaphysical arguments. The scientific arguments can be further subdivided into three distinct ones. (1) In the first and fourth theses, Descartes argues from the use of substantial forms in physics, claiming they are unnecessary and pointing to the explanatory success of his scientific principles. (2) In the second and third theses he employs an analogy between natural objects and machines to show that substantial forms are no more necessary to explain the actions of the former than the latter. (3) In the fifth thesis he charges that scientific explanations in terms of substantial forms explain the obscure by the more obscure.

Descartes likewise offers three distinct metaphysical arguments.

The arguments discussed in the previous chapter illustrate the intense dissatisfaction among practical men of science, like Sanchez, with the foundations of Aristotelian scientific explanations, as well as the impotence of Suarez's empirical defense of the substantial form in the face of global skepticism. However, since Sanchez provides no positive solution, only negative skeptical arguments, it is not surprising that, in the absence of better alternatives, Aristotelian scientific explanations prevailed. Still, the parallel Sanchez draws between the difficulty of discovering the clock's underlying mechanisms from the motions of the decorative clock-face, and the difficulty of uncovering the causes behind sensible natural phenomena, suggests a way out. If the universe were just like the clock, we could come to explain natural phenomena by positing their underlying mechanisms. Indeed, Descartes highlights the irrelevance of substantial forms when explaining the actions of mechanical devices in the second argument against substantial forms he proposes to Regius: “All the reasons for proving substantial forms could be applied to the form of a watch, which nevertheless, no one calls substantial.” In the second thesis he justifies the replacement of substantial forms by the kinds of principles appealed to in the case of a watch by denying the distinction between natural and mechanical phenomena. Automata, he insists, are also works of nature. When building them we simply apply active things to passive things as we do when we sow grain or breed a mule.