The story of the changing forms of explanation adopted in the early modern sciences is too often told as a story of the wholesale rejection of the systematic Aristotelian treatment of causal questions that flourished in medieval as well as ancient science. Narratives of this sort have ignored a promising alternative way of understanding the multifaceted transformation that occurred in early modern natural philosophers’ beliefs about causality. By focusing instead on the Aristotelian tradition’s contributions to the development of rival forms of explanation, it becomes possible to characterize these new sorts of explanations against a rich conceptual background. Of course, scientific innovators in the period 1500–1800 did widely reject Aristotle’s account of the four kinds of causes as a source of acceptable theories in the specific sciences. But a more tempered view of this rejection may better reveal how the new sorts of explanations were actually conceived by their originators.

THREE NOTABLE CHANGES IN EARLY MODERN SCIENTIFIC EXPLANATIONS

This chapter considers three notable changes in early modern scientific explanations. The first was a change in the overall purpose of scientific research that was initiated by those critics of Aristotelianism who relinquished Aristotle’s goal of understanding the form of each natural substance. Rather than trying to elucidate each substance’s form, early modern innovators in the specific sciences, as well as natural philosophy, sought to determine the fundamental constituent parts – whether elements or atoms – of each kind of material body and also to identify the lawlike regularities exhibited in the organization and motions of these fundamental elements or atoms.

Philosophy at the beginning of the seventeenth century was in many ways continuous with the philosophy of the sixteenth century. Aristotle and Aristotelianism continued to be taught in the schools, and thrived there. Furthermore, Renaissance naturalism, Neoplatonic thought, and the occult tradition continued to exert influence (see Chapters 15 and 16). However, quite striking in the late years of the sixteenth century and the first years of the seventeenth is a new interest in another non-Aristotelian tradition, that of atomism, and, more generally, in what were later to be called mechanist views of the world. Mechanists tended to see the world as a great machine, on an analogy with a clock, for example, and tried to explain the manifest properties of things in terms of the size, shape, and motion of the insensible particles that were taken to compose them. With those new natural philosophies came new conceptions of body and the contents of the physical world. Although the so-called new philosophers agreed that the form and matter of their teachers must go, they disagreed about what these were to be replaced by, what the physical world was to contain, what the nature of body was, whether bodies were active or passive, the nature of the place or space in which they are found, among many other questions. The views of the new philosophers can best be understood by examining first their view of the physical world in the early part of the century; second the view of the physical world held by three of the important mechanist system-builders in mid-century, Gassendi, Descartes, and Hobbes; and third later views on body and the physical world, including reactions to earlier mechanist conceptions of body, and attempts to escape the bounds of the new mechanist orthodoxy.

The sources of Gassendi's conviction that his historical studies of Epicurus' philosophy were relevant not only to the writing of history but to the actual practice of science itself were many and varied. These sources were generally not of his own making, and Gassendi at first perceived them as shortcomings of the ongoing scientific discussions in which he participated during the 1630s and 1640s. Although he had conducted regular astronomical investigations during the 1620s in Provence, in the company of Peiresc and under the tutelage of the astronomer Joseph Gaultier, Gassendi's participation in Europeanwide discussions of physical and astronomical problems began to be notable in the following decade with activities such as his observation of the transit of Mercury and his contribution to the solution of Poysson's problem. The latter problem was typical of the kind of loosely structured, open-ended inquiries which comprised a significant part of natural philosophy during the first half of the seventeenth century. Gassendi's discussion of it shows us certain of his early attempts to define the legitimate scope of natural philosophical arguments and to question the manner in which mathematics was being used by several influential contemporaries. However, it is equally significant because it was one of Gassendi's first encounters with the incoherencies of ongoing debates about the nature of atoms. He received the problem in 1635 at a critical time in the development of his atomist ideas, just a year after completing his biography of Epicurus.

The writing of a book on Gassendi is necessarily a synthetic task. Gassendi not only expounded many subjects, but he did so in a manner which can only be fully understood by recognizing his own synthetic aims as a scholar. The variety of contemporary academic fields which must be brought together to explain his work includes the history of science, the history of philosophy, the history of historiography, classical scholarship, and French history. Moreover, this task is made even more daunting by the challenge of deciphering Gassendi's formidable texts, composed in a difficult late-Renaissance Latin style interlaced with Greek, and by the challenge of stating their significance for the modern reader. Anyone who follows Gassendi down the labyrinthine trails of his work would be unable to progress very far without the signposts and advice offered by fellow scholars from diverse academic fields concerning the intellectual geography of his writings. I have thus incurred many debts in the preparation of this study.

Several sections of the present book were begun as chapters of my Ph.D. dissertation in the Department of the History of Science at Harvard University. At Harvard I had the immense good fortune of reading Gassendi for the first time while I was a member of John Murdoch's seminar on ancient Greek and medieval atomism. Professor Murdoch's handling of these abstruse topics convinced me that perhaps even Gassendi's apparently unintelligible passages might be rendered intelligible through patient application. I am also grateful to my other teacher, I. Bernard Cohen.

What was the philosophical import of Gassendi's presentation of Epicurus' atomist principles in the form of a history of philosophy? In his presentation of these principles, he had endorsed two apparently incompatible methods of argument. On the one hand, he had held with Epicurus that atomist principles should be adopted in physics because only an atomist physics did not contradict the facts about ordinary bodies and motions which are evident in sense perception. He, too, espoused Epicurus' belief in the veracity of the senses. Nonetheless he had also seen the need to buttress Epicurus' proofs of various atomist principles not with further appeals to sense perception but with historical arguments by means of which he could show that Epicurus' arguments concerning indivisibles had been either exempt from or superior to those of his critics. Gassendi thus supplemented his own and Epicurus' physical and philosophical arguments with a historical account of the debates between the proponents of atomism and their opponents which was designed to show that, in those debates, atomism had been able both to defeat the objections raised against it and to develop crucial objections to opposing positions. He seems to have believed that a combination of empirical and historical arguments was necessary to advance his predecessor's conclusions. Even when he described his own appeals to experience during the testing of particular theoretical statements about atoms, he sometimes could not furnish adequate reasons for concluding that his observations confirmed the truth of the theoretical statement being tested.

Gassendi's success as a historian, a scientist, and a philosopher was well recognized during his lifetime. At the height of his powers and influence, he enjoyed in the last decade of his life many of the comforts which a successful érudit might have considered to be the just rewards for years of patient scholarship. From his appointment as professor of mathematics in the Collège royal in 1645 until his death in 1655, he managed despite intermittent illnesses and the changing political fortunes of his principal patron to focus his remaining energies on the completion of the Epicurean project as well as on an impressive assortment of other writings, which included a quartet of short biographies of recent astronomers (1654), a textbook on the principles of astronomy (1647), a study of the Roman calendar (1654), and a series of historical notes on the Catholic Church in Digne (1654). The Epicurean project yielded his De vita et moribus Epicuri, his Animadversiones, and his posthumous Syntagma philosophicum, the publications of which were described in Chapter 4. It also drew him into an extended debate over the merits of his atomist principles with another professor of mathematics in the Collège royal, the astronomer Jean-Baptiste Morin. Gassendi's defense in this debate was largely handled by his disciple, the physician François Bernier, who published two polemics attacking Morin. Morin's antipathy towards Gassendi's views was certainly the exception rather than the rule among contemporary readers of his work.

The discussion of the nature of atoms which grew out of Poysson's problem suggested to Gassendi that physical atomism would be an indispensable key to the future development of physics. However, this relatively brief scholarly exchange was one among many similar exchanges in which Gassendi had observed that incompatible definitions of concepts and incommensurable rival solutions to the problems at hand had rendered the exchanges incoherent. The ambiguities which characterized Poysson's problem were also endemic to the interpretation of several major scientific investigations in astronomy and optics which Gassendi undertook during the 1630s and 1640s. Even when these investigations were based on a specific set of observations or experiments, at least four sorts of disagreements usually arose in the subsequent scholarly exchange. (1) Different observers could not always agree about what exactly they saw when they observed celestial phenomena through a telescope or when they made naked-eye observations, for example, of the apparent magnitude of the sun at different elevations above the horizon. (2) Their explanations of what they saw were often expressed in terms of widely divergent optical theories about the constitution and behavior of light.

Gassendi's early career epitomizes the bewildering juxtaposition of possibilities and limitations which shaped the intellectual interests of French scholars of his generation. At his disposal, even in the provincial city of Aix, were texts, manuscripts, and artifacts representing the classical cultures of Greece and Rome. His further access in Paris to sources of Egyptian, Hebrew, and Arabic thought was also ample enough to encourage him to try to master the latter two languages in addition to the Latin and Greek which he had learned as a schoolboy. Gassendi's awareness of what intellectual possibilities lay open to an inquisitive philosopher must surely have rankled him whenever he considered that his first appointment as a professor of philosophy in the University of Aix (1616–22) limited his teaching to the basic principles of Aristotle's logic, metaphysics, physics, and ethics. The standard philosophy curriculum at Aix had assimilated, in the Greek texts of Aristotle, only a fraction of the ancient philosophical works which Italian, Dutch, and French humanists had spent the better part of two centuries making available to their contemporaries. As a young professor, Gassendi became aware of the extent of these editions, translations, and commentaries largely through his friendship with two Provençal scholars, Nicolas-Claude Fabri de Peiresc and Joseph Gaultier. However, Gassendi himself could not yet have been counted as a humanist in any meaningful sense, since he had not perfected any of the technical skills practiced by scholars like Peiresc.