thomas lennon has argued for an innovative “Eleatic” reading of Descartes. At its heart is the thesis that Descartes is a phenomenalist about motions; with this in place, Lennon goes on to argue that Descartes is also a phenomenalist about individual material bodies. Conjuring up the ghosts of Eleatics such as Parmenides, Lennon describes a Cartesian material world in which moving, individual bodies are appearances, not realities. This paper takes issue with Lennon’s thesis that Cartesian motion is phenomenal.Section 2 of (...) the paper details Lennon’s Eleatic reading, setting out his arguments and placing them in scholarly context. Lennon is aware that his reading is radical, and he considers various passages in.. (shrink)

This essay explores two of the more neglected hypotheses that comprise, or supplement, Descartes’ relationalist doctrine of bodily motion. These criteria are of great importance, for they would appear to challenge Descartes’ principal judgment that motion is a purely reciprocal change of a body’s contiguous neighborhood. After critiquing the work of the few commentators who have previously examined these forgotten hypotheses, mainly, D. Garber and M. Gueroult, the overall strengths and weaknesses of Descartes’ supplementary criteria will be assessed. Overall, despite (...) their ingenuity, it will be demonstrated that Descartes’ criteria cannot rescue his brand of natural laws from the inherent limitations of his strong relational account of motion. (shrink)

This essay explores various problematical aspects of Descartes' conservation principle for the quantity of motion (size times speed), particularly its largely neglected "dual role" as a measure of both durational motion and instantaneous "tendencies towards motion". Overall, an underlying non-local, or "holistic", element of quantity of motion (largely derived from his statics) will be revealed as central to a full understanding of the conservation principle's conceptual development and intended operation; and this insight can be of use in responding to some (...) of the recent and traditional criticisms of Descartes' physics. (shrink)

This paper examines Descartes' problematic relational theory of motion, especially when viewed within the context of his dynamics, the Cartesian natural laws. The work of various commentators on Cartesian motion is also surveyed, with particular emphasis placed upon the recent important texts of Garber and Des Chene. In contrast to the methodology of most previous interpretations, however, this essay employs a modern "spacetime" approach to the problem. By this means, the role of dynamics in Descartes' theory, which has often been (...) neglected in favor of kinematic factors, is shown to be central to finding a solution to the puzzle of Cartesian motion. (shrink)

This essay explores the vexed issue of individual corporeal substance in Descartes' natural philosophy. Although Descartes' often referred to individual material objects as separate substances, the constraints on his definitions of matter and substance would seem to favor the opposite view; namely, that there exists only one corporeal substance, the plenum. In contrast to this standard interpretation, however, it will be demonstrated that Descartes' hypotheses make a fairly convincing case for the existence of individual material substances; and the key to (...) this new found individuation is a long neglected set of passages in the Principles of Philosophy. (shrink)

I consider a somewhat obscure but important feature of Descartes’s physics that concerns the notion of the “force of rest.” Contrary to a prominent occasionalist interpretation of Descartes’s physics, I argue that Descartes himself attributes real forces to resting bodies. I also take his account of rest to conflict with the view that God conserves the world by “re-creating” it anew at each moment. I turn next to the role of rest in Malebranche. Malebranche takes Descartes to endorse his own (...) occasionalist version of physics. However, he nonetheless rejects Descartes’s account of rest by appealing to the fact that whereas God’s production of motion requires a power beyond the mere power to create, his production of rest requires only the latter power. It turns out that this argument in Malebranche is incompatible with the sort of “re-creationist” account of divine conservation that he is widely thought to have inherited from Descartes. (shrink)

This paper is the first of a two-part reexamination of causation in Descartes's physics. Some scholars ? including Gary Hatfield and Daniel Garber ? take Descartes to be a `partial' Occasionalist, who thinks that God alone is the cause of all natural motion. Contra this interpretation, I agree with literature that links Descartes to the Thomistic theory of divine concurrence. This paper surveys this literature, and argues that it has failed to provide an interpretation of Descartes's view that both distinguishes (...) his position from that of his later, Occasionalist followers and is consistent with his broader metaphysical commitments. I provide an analysis that tries to address these problems with earlier `Concurentist' readings of Descartes. On my analysis, Occasionalism entails that created substances do not have intrinsic active causal powers. As I read him, Descartes thinks that bodies have active causal powers that are partly grounded in their intrinsic natures. But I argue ? pace a recent account by Tad Schmaltz ? that Descartes also thinks that God immediately causes all motion in the created world. On the picture that emerges, Descartes's position is both continuous with, and a subtle departure from, the Thomisitic theory of divine concurrence. (shrink)

There does not seem to be a consistent way to ground the concept of “force” in Cartesian first principles. In this article, I first review the literature on the subject. Then, I offer an alternative interpretation of force—one that seems to be coherent and consistent with Descartes’ project. Not only does the new position avoid the problems of previous interpretations, but it does so in such a way as to support and justify those previous interpretations. *Received June 2007; revised June (...) 2009. †To contact the author, please write to: Department of Philosophy, University of Washington, Box 353350, Seattle, WA 98195; e‐mail: [email protected]. (shrink)

In this essay, we discuss how Descartes arrives at his mature view of material causation. Descartes’ position changes over time in some very radical ways. The last section spells out his final position as to how causation works in the world of material objects. When considering Descartes’ causal theories, it is useful to distinguish between ‘vertical’ and ‘horizontal’ causation. The vertical perspective addresses God’s relation to creation. God is essential being, and every being other than God depends upon God in (...) order to exist and to continue in existence.Thus, from the vertical perspective, the act of creating and fact of coming into existence are co-extensive notions. This metaphysical/theological framework is the basis of Descartes’ commitment to three interrelated notions: that genuine causes and effects occur simultaneously; that causing is appropriately the case only when the cause is acting; and the view that God is the efficient, total, and continuous cause of everything that exists and every action that occurs. So from the vertical perspective, things are nothing without God’s continuous creation, and there is a problem in articulating how they are said to have independent being and causal efficacy. It is in terms of these commitments that Descartes’ views on horizontal, or material, causation must be approached. We will make apparent the radical extent to which his account of intra-worldly causation abandons his earlier and more traditional views about material causation. To this end we discuss Descartes’ journey to his mature position by emphasizing the growing epistemic limitations of his philosophy, which culminate in what we call his epistemic stance. (shrink)

: In this paper I address the vexed question of secondary causation in René Descartes' physics, and examine several influential interpretations, especially the one recently proposed by Dennis Des Chene. I argue that interpreters who regard Cartesian bodies as real secondary causes, on the grounds that the modes of body include real forces, contradict Descartes' account of modes. On the other hand, those who deny that Descartes affirms secondary causation, on the grounds that forces cannot be modes of extension, commit (...) Descartes to the problematic view that the undetermined and immutable will of God is the sole cause of determinate and variable motions. Des Chene's contextualist approach to Descartes' texts leads him to an intermediate position that combines elements of both interpretations. However, in my view, Des Chene's interpretation likewise fails to resolve apparent tensions in Descartes' claims. I thus propose to separate the issue of secondary causation from what Des Chene dubs 'the problem of force'. On the basis of this approach, and a study of Jesuit commentaries familiar to Descartes, I develop a new interpretation which takes seriously Descartes' claim that the laws of nature are the secondary and particular causes of particular motions and changes in motion. (shrink)

Essay review of Daniel Garber, 1992, Descartes' Metaphysical Physics, University of Chicago Press, Chicago and London, xiv + 389 pp., and Michael Friedman,: 1992, Kant and the Exact Sciences, Harvard University Press, Cambridge, Mass., and London, xvii + 357 pp. These two books display the historical connection between science and philosophy in the writings of Descartes and Kant. They show the place of science in, or the scientific context of, these authors' central metaphysical doctrines, pertaining to substance and its properties, (...) the metaphysics of force and causal agency, the relation of geometry to matter and to space, and the structure of cognition. In so doing, they provide an image of philosophy as an intellectually and culturally engaged enterprise, an image according to which it makes little sense to make pronouncements about the foundations of knowledge or the possibilities for science without direct engagement with the living practice of scientific and other cognitive enterprises. However, in some areas they did not engage the context as fully as needed; their interpretations of particular arguments and indeed of whole philosophical programs suffered thereby. (shrink)

Introduction Conatus is a specific concept within Descartes’s physics. In particular, it assumes a crucial importance in the purely mechanistic description of the nature of light – an issue that Des- cartes considered one of the most crucial challenges, and major achievements, of his natural phil- osophy. According to Descartes’s cosmology, the universe – understood as a material continuum in which there is no vacuum – is composed of a number of separate yet interconnected vortices. Each of these vortices consists (...) in a set of bands rotating around their centres. The bands are com- posed of corpuscles of the three elements, each distinguished on the basis of their different shapes and sizes. The small globules of the second element, although impeded by the other parts of heaven, strive to move away from the centre of the vortex around which they revolve, thus exerting a certain force against the surrounding bodies. This striving or conatus, though a mere force rather than a genuine motion, is transmitted instan- taneously and along straight lines from body to body. According to Descartes, then, the nature of light consists in this striving alone. This account must be understood as strictly connected to the fundamental laws that regulate the Cartesian world. It is particularly important to recall that for Descartes the centrifugal force exerted by a rotating body is understood to be a consequence of the intrinsic rectilinearity of every motion. As the second of the three Laws of Nature set forth in Book II of the Principles of Philosophy establishes, “all motion is in itself rectilinear; and hence any body moving in a circle tends to move away from the centre of the circle which it describes as proved by the fact that a stone rotating in a sling tends to move centrifugally along the tangent of each point described by a circle.1 Similarly, each of the globules of the second element “strives to recede with a great force from the centre of the vortex in which it rotates; it is in fact prevented by the other globules placed all around, not differently than a stone in a sling”. Within this framework, as I show later, conatus occurs when a body’s intrinsic tendency to rectilinear motion is impeded by an external constraint. As Stephen Gaukroger notes, the use of the term conatus in the Principles – like “vis” and “ action ” – shows that Descartes “cannot avoid dynamic terminology”, despite his declared intent “to construe motion in a purely kinematic way”. Indeed, Gaukroger observes that the notions of force action and striving are systematically employed in the Principles – appearing 290, 59 and 8 times, respectively. Also noteworthy, conatus was – together with the more familiar concepts of actio and vis – a significant element of the conceptual apparatus of Scholastic natural philosophy. As will be seen later – and this is the first goal of this paper – here the concept of conatus had a very specific function. Indeed, it was a central part of the Aristotelian- Scholastic account of gravitation. Conatus was used to refer to the striving of a body to move towards its natural place – especially in cases where its natural motion was hindered or impeded by an external mover. This specific use of the concept of conatus can be found in texts on natural philosophy from the end of the sixteenth through the late seventeenth century. Notably, it occurs in some of the late -Scho- lastic texts which held special significance for Descartes’s thought. Therefore, Descartes introduces conatus in a context in which this concept already had a very specific meaning, and one of which he was very probably aware. The possible relations between these two apparently very different con- cepts have therefore to be scrutinized. I thereby propose to undertake a comparison between the Scho- lastic and the Cartesian conceptions of conatus. I hope to show that certain traits of the former are indeed echoed by the latter, although adapted to the much changed physical paradigm of Cartesian physics. In fact, Descartes’s conatus, though sharing some important similarities with the old one, underwent significant transformations in terms of both its meaning and its application. However, I hope to show that these concepts are both used to describe the behaviour of bodies in relation to their intrinsic motive tendency, and in particular when this tendency is impeded or prevented. Such a reconstruction offers two main areas of interest. On an immediate level, it furthers the general efforts of scholars over the past decades to reconstruct the full extent of the relations between Descartes’s thought and Scholastic philosophy. However, there is another significant reason for taking an interest in the ties between the Scholastic and the Cartesian conceptions of conatus. Indeed, the Cartesian conatus must be seen as crucial to a proper understanding of the broader, diverse conceptualization that this idea enjoyed in the thought of many of the most important philosophers of the early modern period, namely Spinoza, Hobbes and Leibniz. In particular, Spinoza, were directly influenced by Descartes in their unique formulations of conatus. Thus a reconstruction of the context in which Descartes formulated his conatus and of its relations to a pre-existing Scholastic conception will enable us to better reconstruct the history of this idea in the early modern period. The paper is structured as follows: in the first part, I provide a thorough review of the Scho- lastic employment of the concept conatus – both its meaning and the extent of its usage. I then show that it occurs in some of the most prominent Cartesian sources. Finally, I shall provide an account of Descartes’s theory of circular motion and conatus with the specific aim of empha- sizing the elements of both continuity and discontinuity that justify the claim that Descartes’s con- ception of conatus is reminiscent of the Scholastic one. (shrink)

In his accounts of plant and animal generation Pierre Gassendi offers a mechanist story of how organisms create offspring to whom they pass on their traits. Development of the new organism is directed by a material “soul” or animula bearing ontogenetic information. Where reproduction is sexual, two sets of material semina and corresponding animulae meet and jointly determine the division, differentiation, and development of matter in the new organism. The determination of inherited traits requires a means of combining or choosing (...) among each parent's contributions, and towards this end, Gassendi outlines the nature of competition and dominance among animulae. Unlike his fellow mechanists, Gassendi can extend his mechanism to his heredity account, because his proposed vehicle for ontogenetic transmission is material. That proposal in turn relies on his atomist hypothesis. The relative uniformity of atoms allows animulae to operate equivalently across different modes of generation, spontaneous or otherwise. Further, his molecular model of atomic structures allows a material means of storing ontogenetic information received from the souls of parent organisms. These accounts—flawed and sketchy—unsurprisingly fail to specify how hereditary information might be borne physically, and in any case do not meet Gassendi's own empiricist standards. Yet this generation theory with pretensions to a materialist mechanism establishes Gassendi's firm commitment to a unity of the sciences through an atomist ontology that underlies all physical phenomena, including the organic. (shrink)

Este artigo explora a relação que Descartes estabelece com as noções de forma e de matéria do aristotelismo escolástico ao tratar da interação entre as substâncias nas correspondências com Elisabeth, Gassendi e Arnauld. Partindo do exemplo do peso utilizado por Descartes em diversas cartas, traça-se uma relação de identidade entre a noção de forma em Tomás de Aquino e de pensamento em Descartes, assim como se traça a crítica à noção de forma, entendida como qualidade oculta, para os corpos inanimados. (...) Por fim, explora-se a interpretação ocasionalista do sistema cartesiano e a relevância dos movimentos celestes para a explicação dos movimentos naturais em Tomás de Aquino.This paper deals with the relation that Descartes points between his system and concepts of matter and form from Aristotelian scholasticism. The resemblance arises when Descartes treats interaction among substances in correspondences with Elisabeth, Gassendi and Arnauld. Beginning with the example of the scholastic concept of weight, used many times by Descartes, this article traces an identity between concepts of form in Thomas Aquinas and of thought in Descartes, as well as traces his criticism on the concept of form, understood as hidden qualities, to inanimate bodies. Finally, this paper investigates Cartesian occasionalism as well as the importance of celestial movements for a better comprehension of nature in Thomas Aquinas. (shrink)

It is common to assume that Descartes did not have a conception of an object's matter density independently of its size, but this is a rather incomplete assessment of the early modern natural philosopher's theory. Key to our understanding of Descartes's physics is a consideration of the ratios between the quantities of the different types of matter in which an object consists. As these ratios determine the degree of an object's porosity and elasticity, they also affect in Descartes's theory the (...) phenomena of gravity and weight. (shrink)

En el presente artículo se muestra que en la cosmología cartesiana no resulta claro cómo se puede hablar o referir a cuerpos individuales en el marco de un universo absolutamente compacto. Se defiende que el concepto de cuerpo individual en el interior de dicha cosmología se puede construir como una ficción del espíritu. Un cuerpo individual se puede construir como la extensión que se encuentra encerrada en una superficie maximal cuyas subdivisiones carecen de movimiento relativo entre sí. Se muestra, también, (...) que la cosmología cartesiana puede presentarse como un modelo trivial de la Teoría M. (shrink)

Philosophy and Memory Traces defends two theories of autobiographical memory. One is a bewildering historical view of memories as dynamic patterns in fleeting animal spirits, nervous fluids which rummaged through the pores of brain and body. The other is new connectionism, in which memories are 'stored' only superpositionally, and reconstructed rather than reproduced. Both models, argues John Sutton, depart from static archival metaphors by employing distributed representation, which brings interference and confusion between memory traces. Both raise urgent issues about control (...) of the personal past, and about relations between self and body. Sutton demonstrates the role of bizarre body fluids in moral physiology, as philosophers from Descartes and Locke to Coleridge struggled to control their own innards and impose cognitive discipline on 'the phantasmal chaos of association'. Going on to defend connectionism against Fodor and critics of passive mental representations, he shows how problems of the self are implicated in cognitive science. (shrink)

Gravitation is interpreted to be an “ontomathematical” force or interaction rather than an only physical one. That approach restores Newton’s original design of universal gravitation in the framework of “The Mathematical Principles of Natural Philosophy”, which allows for Einstein’s special and general relativity to be also reinterpreted ontomathematically. The entanglement theory of quantum gravitation is inherently involved also ontomathematically by virtue of the consideration of the qubit Hilbert space after entanglement as the Fourier counterpart of pseudo-Riemannian space. Gravitation can be (...) also interpreted as purely mathematical or logical “force” or “interaction” as a corollary from its ontomathematical (rather than physical) realization. The ontomathematical approach to gravitation is implicit in general relativity equating it to operators in pseudo-Riemannian space obeying the Einstein field equation and also well-known by the “geometrization of physics”. Quantum mechanics shares the same by the separable complex Hilbert space and defining “physical quantity” by the Hermitian operators on it. One can interpret special Minkowski space involved by special relativity and the qubit Hilbert space of quantum information as Fourier counterparts immediately noticing that general relativity means gravitation as the Fourier counterpart of non-Hermitian operators implying non-unitarity and the violation of energy conservation and thus destroying Pauli’s particle paradigm. Since the Standard model obeys it, this explains the impossibility of “quantum gravitation” in any framework conservatively generalizing the Standard model so that it would include gravitation along with electromagnetic, weak, and strong interactions. Einstein’s geometrization of gravitation can be continued into a purely mathematical theory of it following Euclid’s realization for geometry to be exhaustively built in a deductive and axiomatic way as well as Riemann’s parametrization of all the class of Euclidean and non-Euclidean geometries by “space curvature”, then being generalized to Minkowski space as the operators on pseudo-Riemannian space as the Einstein field equation means gravitation. The transition from mathematical gravitation to logical one can rely on the historical lesson of the pair of Lobachevski’s and Riemann’s approaches now “reversely”, i.e., from the latter to the former. Logical gravitation is linkable to Hegel’s dialectical logic and ontological dialectics abandoning their interpretations as a new zero logic substituting classical propionyl logic. The approach of ontomathematics generalizing that of ontology, traceable even to Aristotle’s reformation of Plato’s doctrine, needs Hegel’s doctrine to be formalized as a first-order logic naturally containing Boolean algebra, isomorphic to both classical propositional logic and set theory being the class of all first-order logics, as a sub-logic along with Peano arithmetic as another sub-logic. The first-order logic at issue is called Hilbert arithmetic and elaborated in detail in other papers. It allows for both self-foundation of mathematics to be internally proved as complete and furthermore, quantum mechanics reinterpreted as quantum information to be included by the qubit Hilbert space interpretable in turn as a dual and physical counterpart of Hilbert arithmetic in a narrow sense, that is, both counterparts constitute Hilbert arithmetic in a wide sense, being mathematical and physical simultaneously and thus overcoming the Cartesian dualism of “body” gapped from “mind” by an abyss. Then, the proper philosophical interpretation of gravitation to be the fundamental ontomathematical force or interaction overcomes the ridiculous belief of the Big Bang wrongly alleged to be a scientific theory. Ontomathematical gravitation suggests an omnipresent and omnitemporal medium of “God’s” creation “ex nihilo” following only the natural necessity of quantum-information conservation particularly and locally manifested as energy conservation. (shrink)

In 1675, Burchard de Volder became the first university physics professor to introduce the demonstration of experiments into his lectures and to create a special university classroom, The Leiden Physics Theatre, for this specific purpose. This is surprising for two reasons: first, early pre-Newtonian experiment is commonly associated with Italy and England, and second, de Volder is committed to Cartesian philosophy, including the view that knowledge gathered through the senses is subject to doubt, while that deducted from first principles is (...) certain. On the surface, it is curious that such a man would bring experiments to university physics. After all, in demonstrating experiments to his students, he was teaching them through their senses, not reason. However, if we consider the institutional context of de Volder’s teaching, we come to see de Volder’s Cartesian experimentalism as representative of a certain form of pre-Newtonian Dutch Cartesian science, as well as part of a larger tradition of teaching through observation at the University of Leiden. Considering de Volder’s institutional context will help us see that de Volder’s experimentalism was not in spite of his Cartesianism, but motivated by it. This paper will describe de Volder’s physics curriculum, the Theatre in which it was taught and how his work predates similar efforts in other European Universities. The second part will consider three aspects of the culture at the University of Leiden: a tradition of teaching through observation, a particular reception of Cartesianism, and an eclectic approach to the New Philosophy. The concluding portion shows how de Volder would have understood his enterprise in Cartesian terms. (shrink)

Descartes is usually taken to be a strict reductionist, and he frequently describes his work in reductionist terms. This dissertation, however, makes the case that he is a nonreductionist in certain areas of his philosophy and natural philosophy. This might seem like simple inconsistency, or a mismatch between Descartes's ambitions and his achievements. I argue that here it is more than that: nonreductionism is compatible with his wider commitments, and allowing for irreducibles increases the explanatory power of his system. Moreover, (...) Descartes depends on nonreductionist knowledge and on irreducibles in order to maintain his epistemology and metaphysics. That is, there is more in the Cartesian world than mind, matter, and God. (shrink)

The rich connections between metaphysics and natural philosophy in the early modern period have been widely acknowledged and productively mined, thanks in no small part to the work of Margaret Wilson, whose book, Descartes, served as an inspirational example for a generation of scholars. The task of this paper is to investigate one particular such connection, namely, the relation between occasionalist metaphysics and strict mechanism. My focus will be on the work of Nicholas Malebranche, the most influential Cartesian philosopher after (...) Descartes himself. I begin with two crucial facts about Malebranche’s philosophy: (1) Malebranche was an occasionalist, that is, he held that God was the only true cause, that all modifications of bodies and of minds can be produced by God alone. (2) Malebranche adhered firmly to strict mechanism. By strict mechanism, I mean the view, found most prominently in Descartes and in Boyle’s more ideological writings, that the qualities of bodies are exhausted by a very short list (size, shape, motion, and perhaps solidity) and that, most importantly, bodies interact only at contact by impact. Another way of describing this “contact action” requirement is as the thesis that the only fundamental laws of physics are laws of inertial motion and laws of the communication of motion at impact. In.. (shrink)

Argues that Descartes's commitment to mind-body causation leads to a commitment to body-body causation.

Resumen: Propongo una crítica a la noción de serialidad en la comprensión del concepto de tiempo en el contexto de los estudios cartesianos. En el debate entre los defensores del tiempo continuo y quienes defienden un tiempo discreto, sostengo que ninguna de estas posiciones tiene en cuenta que la serialidad se enmarca en una noción de tiempo que se concibe como divisible y numerable y que no pertenece intrínsecamente a la naturaleza de la experiencia temporal del cogito. Mi propuesta consiste (...) en comenzar a desarrollar la caracterización de dicho tiempo experiencial y cogitativo.: My aim is to criticize the notion of seriality embedded in the understanding of time within the framework of Cartesian studies. I hold that neither those who defend that time is continuous nor those who think that it is discrete take into account the fact that seriality is rooted in a conception of time conceived as divisible and numerable that does not belong intrinsically to the nature of the experience of time of the cogito. I propose to advance towards a characterization of such experiential and cogitative time. (shrink)

This essay offers an interpretation of Descartes’ treatment of the concepts of place and space in the Principles of Philosophy. On the basis of that interpretation, I argue that his understanding and application of the concept of space supports a pluralist interpretation of Descartes on extended substance. I survey the Scholastic evolution of issues in the Aristotelian theory of place and clarify elements of Descartes’ appropriation and transformation thereof: the relationship between internal and external place, the precise content of the (...) claim that space and body are really identical, and the way we conceptually distinguish space from body. Descartes applies his concept of space in ways that illuminate the metaphysical structure of extension. In particular, he uses the concept to specify the degree to which finite parts of matter are independent of each other. I argue that for Descartes, conceiving extension as indivisible is an artifact of conceiving it as a space. That is, pace the monistic reading of Cartesian extended substance, regarding extension as indivisible is an artifact of conceiving it in a way more removed from its real nature. (shrink)

In the Principles of Philosophy, Descartes attempts to explicate the well-known phenomena of varying bodily size through an appeal to the concept of "solidity," a notion that roughly corresponds to our present-day concept of density. Descartes' interest in these issues can be partially traced to the need to define clearly the role of matter in his natural laws, a problem particularly acute for the application of his conservation principle. Specifically, since Descartes insists that a body's "quantity of motion," defined as (...) the product of its "size" and speed, is conserved in all material interactions, it is imperative that he explain how solidity influences the magnitude of this force. As a means of resolving this problem, Descartes postulated an idealized condition of "perfect solidity" which correlates a body's "agitation" force (a forerunner of Newton's concept of non-accelerating, or "inertial" motion) with the interplay of its volume, surface area, and composition of minute particles. This essay explores this often misunderstood aspect of Descartes' physics, as well as the special function of idealized conditions in his collision rules. Contrary to those commentators who regard "perfect solidity" as a stipulation on bodily impact, this notion, it will be argued, is primarily concerned with the internal composition of macroscopic bodies, and only indirectly with their collision characteristics. Along the way, many of Descartes' hypotheses will be shown to display a level of sophistication and intricacy that, despite their essential incompatibility, belie several of the common misconceptions of Cartesian science. (shrink)