Having ensured that divine providence played a major role in his mechanical philosophy, Gassendi turned to the question of human freedom in Book III of the “Ethics,” the last part of the Syntagma philosophicum, entitled “On Liberty, Fortune, Fate, and Divination.” In this concluding section of his magnum opus, Gassendi cast his discussion in the form of a debate among the major classical philosophies, particularly Stoicism and Epicureanism. The main issue was freedom – human and divine. While questions about fate, fortune, and divination may, at first glance, appear rather remote from the primary concerns of seventeenth-century natural philosophy, in fact they involve metaphysical issues central to the articulation of the mechanical philosophy: the extent of contingency and necessity in the world, the nature of causality, and the role of providence and the extent of human freedom in a mechanical universe. Gassendi's treatment of these issues reflects his underlying voluntarism.

Since classical times, natural philosophers had dealt extensively with questions about fate, fortune, and divination. The concept of fate was central to Stoicism, which had explained the world as governed by a deterministic, rational ordering principle, the Logos. According to Stoic doctrine, fate is the expression of the Logos in the causal nexus of a deterministic universe.

Like many other seventeenth-century natural philosophers, Descartes shared with Gassendi the goal of replacing Aristotelianism with a mechanical philosophy of nature. Descartes presumed that there was a close relationship between the metaphysical roots of this philosophy – which he worked out in the Meditations and Part I of The Principles of Philosophy – and the mechanical explanations that grew from those roots. Articulating what he considered the conceptual framework within which natural philosophy was to develop, Descartes specified what sorts of things exist in the world and the means by which they interact. Accordingly, he described a new theory of matter and redefined causality, replacing the old scheme root and branch.

From the time Descartes began composing Le monde in 1629 until the end of his life, he consistently held certain consistent views about natural philosophy. The physical world, he thought, consists entirely of matter, the one defining property of which is extension. Matter is infinitely divisible and fills all space. There is no void. All physical causality is reduced to the motion and impact of particles of matter. From the beginning, Descartes sought a philosophy of nature that would reflect his claim that its first principles could be known a priori.

This book is about ways of understanding contingency and necessity in the world and how those ideas influenced the development of philosophies of nature in the seventeenth century. Is the world contingent on forces beyond the possibility of human understanding and control? Or does the world necessarily conform to rationally intelligible principles? The interplay between these conceptions goes back to both the Greek and the Hebrew sources of Western thought, forming an important strand in the long history of the relationship between Athens and Jerusalem.

In this study, I have examined the transformation of medieval ideas about God's relationship to the creation into seventeenth-century ideas about matter and method as embodied in early articulations of the mechanical philosophy. Medieval thinkers were primarily concerned with the theological problem of God's relationship to the world he created. They discussed questions about necessity and contingency as related to divine power. By the seventeenth century, the focus had shifted to natural philosophy and the extent and certitude of human knowledge. Underlying theological assumptions continued to be reflected in the epistemological and metaphysical orientations incorporated into different versions of the mechanical philosophy.

I have argued that the differences between Gassendi's and Descartes' versions of the mechanical philosophy directly reflected the differences in their theological presuppositions. Gassendi described a world utterly contingent on divine will. This contingency expressed itself in his conviction that empirical methods are the only way to acquire knowledge about the natural world and that the matter of which all physical things are composed possesses some properties that can be known only empirically. Descartes, on the contrary, described a world in which God had embedded necessary relations, some of which enable us to have a priori knowledge of substantial parts of the natural world. The capacity for a priori knowledge extends to the nature of matter, which, Descartes claimed to demonstrate, possesses only geometrical properties.

Despite the fact that Gassendi's thought developed in a variety of ways during his lifetime, his fundamental principles – a voluntarist theology, an empiricist epistemology, a nominalist or conceptualist account of universals, and an anti-essentialist metaphysics – remained constant throughout his writings. In making this statement, I disagree with Bloch's contention that Gassendi's philosophy does not constitute a system. Bloch regards the Syntagma philosophicum as “an assemblage of historical, scientific, and philosophical discussion.” He also denies that theology played any important role in Gassendi's thought, claiming instead that it was introduced only after 1641 to dissimulate the materialism of which Gassendi was growing increasingly aware. As I have argued in Chapters 2 and 3, whatever Gassendi's state of belief, the theological views upon which he drew are voluntarist and are reflected – directly and indirectly – throughout his work. Despite changes in his reactions to skepticism, despite his growing interest in natural philosophy, and despite his increasing focus on his Epicurean project, the same theological and philosophical views remained central to his thinking from the early Exercitationes paradoxicœ adversus Aristoteleos (1624) to the posthumously published Syntagma philosophicum (1658).

Prior to the publication of the Meditations in 1641, Descartes asked his friend and correspondent Marin Mersenne to circulate the manuscript for comment and criticism among a number of philosophers, including his fellow mechanical philosophers, Hobbes and Gassendi. These comments and Descartes' subsequent responses, were published as a lengthy appendix to the Meditations, the “Objections and Replies.” Despite the fact that Gassendi and Descartes shared a strong anti-Aristotelian sentiment, a consuming interest in natural philosophy, and a dedication to the establishment of a new, mechanical philosophy of nature, their arguments in the “Objections and Replies,” later amplified in Gassendi's Disquisitio metaphysica (1644), were heated, to say the least. This controversy signaled a major rift between the two founding fathers of the mechanical philosophy. What was really at stake between them?

Their conflict can be understood as yet one more episode in the ongoing confrontation between the Judeo-Christian belief in divine omnipotence and the classical Greek preoccupation with the intelligibility of nature.

In Part I, I showed how the medieval discourse about divine power and its relationship to the created world was refracted in the prism of seventeenth-century natural philosophy, emerging as the two very different approaches to the epistemological status of knowledge about the natural world adopted by Gassendi and Descartes. In Part II, I turn to the mechanical philosophy itself, namely, the view that all natural phenomena can be explained solely in terms of matter and motion. Here I explore how the theories of matter these mechanical philosophers adopted reflected their respective epistemological inclinations and thereby engendered two distinct styles of science.

In its role as conceptual framework, the mechanical philosophy determined the ultimate terms of explanation in which the physical world was to be understood and the methods by which knowledge of that world could be acquired. At the most abstract level, the mechanical philosophy replaced both the Aristotelian metaphysics of matter, form, and privation and the Neoplatonically inspired animistic philosophies of the Renaissance with a new metaphysics, which populated the natural world with only one kind of entity, matter. The motions, positions, and collisions of particles of matter were thought to be sufficient to explain everything in the physical world. In this mechanistic framework, matter was the only real existent, and causality was reinterpreted in terms of the impact of material particles. Action at a distance was deemed impossible.

The concept of matter adopted by the mechanical philosophers differed in important ways from the older theories of matter.

Like Gassendi, Descartes belonged to the early-seventeenth-century community of natural philosophers committed to Galilean science and the mechanical philosophy. Educated by the Jesuits at La Flèche – where Mersenne was also a student from 1604 to about 1609 – Descartes qualified as a lawyer at the University of Poitiers in 1616, after which he served as an officer in the army of Prince Maurice of the Netherlands and then spent several years traveling in Europe. He had a significant encounter with Isaac Beeckman in 1619 and, like Gassendi, found contact with the Dutch schoolmaster extremely stimulating. Shortly after meeting Beeckman, Descartes dedicated himself to the pursuit of natural philosophy. He settled permanently in the Netherlands in 1629, remaining there until his final, fatal move to Sweden in the last year of his life. He corresponded frequently with Mersenne and many other natural philosophers, including Beeckman, Constantijn Huygens (father of the physicist Christiaan), William Cavendish, the brothers Jacques and Pierre DuPuy, Jean-Baptiste Morin, Gassendi, Hobbes, and Henry More. His letters contain a wealth of information about key issues in seventeenth-century natural philosophy.

Descartes published a number of important and widely read works in natural philosophy.

Gassendi's Epicurean project

Pierre Gassendi is most frequently remembered for introducing the philosophy of the ancient atomist Epicurus into the mainstream of European thought. His revival of Epicureanism can be understood in the context of the early-seventeenth-century search for a new philosophy of nature. The young Gassendi was an active member of the intellectual community of Provence and, eventually, all of France. He conducted experiments and discussed many scientific topics, particularly in astronomy, with his patron and correspondent, Nicolas-Claude Fabri de Peiresc (1580–1637). He was a friend of Hobbes, whom he encountered in the circle around Mersenne in Paris. He corresponded with Beeckman, Galileo, Hevelius, and other astronomers and natural philosophers. He performed the famous experiment of dropping a heavy object from the top of a moving ship's mast, an experiment previously imagined by Galileo as proving the possibility of the earth's motion. And he wrote a treatise defending Galileo's new science of motion, De motu impresso a motore translato (1642). As an active member of this community of natural philosophers, Gassendi predictably rejected the philosophy of Aristotle. There is evidence that he was interested in Epicureanism as a replacement for Aristotelianism from at least the mid-1620s.

In earlier chapters, I discussed the reasons why Gassendi considered it necessary to modify Epicureanism in order to render it theologically acceptable. I then argued that a close relationship existed between his voluntarist theology and his empiricist theory of scientific knowledge. In this chapter, I argue that his theories of matter and causality were enmeshed in his epistemology and that they thus indirectly reflected the theological assumptions that informed his entire philosophical enterprise.

Gassendi was not the first advocate of atomism in the seventeenth century, but he was surely the most systematic. His coupling of atomism with the astronomy of Copernicus and the physics of Galileo, as well as his commitment to ridding atomism of the theologically objectionable doctrines traditionally associated with Epicureanism, set him apart from earlier advocates of the atomic theory and probably account for the ultimate success of his project. Like Gassendi, one early-seventeenth-century atomist, David van Goorle (d. 1612) was also concerned to refute Aristotle. Sebastian Basso (fl. 1550–1600) advocated atomism as an alternative to Aristotelianism, which he criticized at length in Philosophiae naturalis adversus Aristotelem (1621).

If the development of science in Western Europe can be understood, at least in part, as resulting from the coupling of Greek ideas about rationality with biblical notions of God's power, one troublesome offspring of this union was the problem of reconciling two of God's primary attributes, his omnipotence and his omniscience. Although many classical thinkers had considered the cosmos to be governed by rational principles of some kind, their discussions of chance, fate, and fortune had implicitly acknowledged the fact that human life depends on forces beyond rational control. Medieval Christian theologians faced the difficult task of reconciling the Old Testament God of Abraham, Isaac, and Jacob, who created the world and rules it freely according to his own will, with Greek ideas about the self-sufficiency and rationality of cosmic principles – an endeavor memorably characterized by Lovejoy as “perhaps the most extraordinary triumph of self-contradiction.” Tensions between these classical and biblical ideas formed the intellectual context of medieval and early modern discussions about God's relationship to the creation.

In the present chapter, I explore the way God's relationship to the creation was considered in the thirteenth and fourteenth centuries, when it came to be formulated in terms of the dialectic between the absolute and ordained power of God.