This chapter locates Spinoza’s scientific interests and contributions in the context of the disciplinary categories of the seventeenth century, investigates the authorship of two small treatises (on the rainbow, and on the calculation of chances) often attributed to him, describes his scientific correspondence, evaluates his strengths and weaknesses as an expositor of Cartesian physics, assesses the role of Cartesian physics in his own philosophy, and explores his conception of methodology in the natural sciences.

Descartes’ Three Dreams happened on the night of November 10, 1619, the culmination of days of fevered concern with the search for truth. In the First Dream, Descartes is walking through the streets haunted by terrifying phantoms. A severe weakness in his right side forces him to bend over to his left. He tries to straighten up, but a whirlwind spins him three or four times on his left foot. More frightening is the thought that he is falling at every step he takes. Then he enters through the open gates of a college, seeking relief and intending to pray in the college chapel. But he has passed someone he knows without hailing him. He tries to turn back to greet him, but the wind hurls him against the chapel. At the same time he sees in the college courtyard another person who greets him and tells him that “Monsieur N.” has something to give him. Descartes imagines it is a melon brought from a foreign country. The people standing around him are firm on their feet, whereas he is staggering about, even though the wind has abated. Then he awakes with a pain making him fear that an evil demon is leading him astray. He rolls over from being on his left side (where he had fallen asleep) and prays to God for protection from the dream's evil effects and from the misfortunes that might invite divine retribution for his sins, though others might have thought that hitherto he has led an upright life.

Falling asleep after a couple of hours thinking about the good and evil in the world, Descartes has the Second Dream. A piercing noise, he thinks a thunderclap, frightens him back to the waking state, and on opening his eyes he sees about him large numbers of fiery sparks. This is not the first time he has woken up to find his eyes emitting sparks by which he could see objects close by. Finding a satisfactory natural explanation of the strange phenomenon, he falls asleep again.

A moment later there is the peaceful Third Dream. Descartes finds a book on his table without knowing who put it there. It is a “Dictionary” (Dictionnaire, as in Baillet), and he expects to find it useful.

Physical force is as puzzling as Saint Augustine found time to be. We know what force is if no one asks us, but if we want to explain it to someone, we don't know. We know force as a bodily sensation, and we believe it is implicated in efficient causation; we have ways of measuring it, but we cannot explain its essence. That alone would account for the absence of a systematic metaphysics of force in Descartes’ writings, which in turn would account for it being the most intractable problem posed by his physics. He never defines force per se, yet he has a concept of extension and of motion and therefore a conception of physical force expressible as a mathematical quantity. Forces are measurable in terms of the same independent variables, which enables Descartes to compare forces of the same category. For the ontological ground of forces he appeals to God's continuous creative and conserving activity in the physical world (see concurrence versus conservation, divine). It is no surprise that complete agreement among Descartes scholars on the topic of force is in short supply.

Descartes categorizes force in two ways. First, in statics or traditional mechanics, the key notion is the force or effort required to raise a body against gravity, either vertically or along lines inclined to the vertical. The measure of such a force is the product of the body's weight and the vertical distance through which it is raised. It follows that the same force will raise bodies through vertical distances inversely proportional to their weights, a rule that in various forms goes back to antiquity. Descartes’ statement of this rule may be called his “General Statical Principle” (GSP) (Gabbey 1993; Slowik 2002, 113–17).

The second category of force applies in domains outside traditional mechanics: collisions between bodies, stones in slings, rigid-body rotation, planetary motion, ballistic phenomena, and free fall under gravity. By the first law of nature, a body in motion has a force to remain in motion; at rest it has a force to remain at rest (see inertia). The quantity of motion, given by the speed times the body's volume or corporeal quantity, measures the motive force. The body's “rest force,” peculiar to Descartes, is the force with which a body at rest resists any action that would dislodge it from its state of rest.

The term persona appears only once in the original Latin text of the Meditations, in the Fourth Meditation (AT VII 61, CSM II 43). There Descartes employs the term to mean “one's role in life,” deriving from persona in the original theatrical sense of “a mask,” as in the early Private Thoughts (AT X 213, CSMK I 2). Nowhere in the Meditations and Objections and Replies, or elsewhere in the published writings, does Descartes explain his conception of personhood or address the problem of “personal identity” that has exercised philosophers since Locke. It is all the more important therefore to determine what he took a “person” to be.

One suggestion that many commentators find appealing is that a Cartesian person is a mind or soul and thus that personal identity consists in sameness of soul (see, e.g., Thiel 2011, 270). In the Second Replies, on the question of whether God could be a deceiver, Descartes notes that in the Meditations he “had a special obligation” to talk about God in a philosophical way, “since there my supposition was that no other human beings were yet known to me, and moreover I was considering myself not as consisting of mind and body but solely as a mind” (AT VII 142–43, CSM II 102). The Meditations are primarily those of a res cogitans, not of a Cartesian human being, whose nature emerges in a letter to Mesland (February 9, 1645). When we talk about a human body, we do not mean just a determinate part of matter with a determinate size, but that part as united to the soul (l’âme). We believe that we have had the same body since infancy, despite its lifelong changes in quantity, shape, and composition. Our body is the same numerically (idem numero) because it is informed [informé] by the same soul (AT IV 166–67, CSMK 243). So is a Cartesian person a human being instead?

A human being, qua substantial union of a body and a mind, is still not a rounded person, because persons have moral and social commitments. Even in the Passions of the Soul (1649), Descartes explains that his purpose is “not to explain the passions as an orator, nor even as a moral philosopher, but solely as a physicist [physicien]” (AT XI 326, CSM I 7).

Descartes’ views on explanation are best approached against the background of traditional and contemporary views on the topic. There are no entries on explanatio or explicatio in the philosophical dictionaries of the early modern period. Normally Scholastic writers talked not about explanations of natural phenomena but about their demonstrations, by way of syllogisms yielding apodictic knowledge of causal dependence (Jardine 1976; Dear 1995, 26–30). By contrast, the etymology of explanation and explication (in English, Latin, and French) reveals related senses of clarifying, removing ambiguities, and making intelligible that do not implicate the notion of demonstration. This range of senses is in Cicero, who refers to physics as explicatio naturae, a label borrowed for the same purpose by some writers in the early modern period. Explicationes appear in Scholastic manuals as commentaries or philosophical explications de texte, not typically as causal accounts of natural phenomena. A Scholastic demonstration is an explicatio to the extent that it interprets and clarifies phenomena in a certain way; an explanation in the new physics is a demonstration to the extent that it yields causal knowledge, whether or not it be scientia strictly understood. Yet demonstration and explanation remain different notions, and the differences between them spotlight important contrasts between Peripatetic physics and the new mechanical philosophies. Proponents of these new philosophies sought mechanical explanations of all kinds of properties, including those manifesting the supposed forms and qualities that for the Scholastics were matters of definition and of specification of essences (see form, substantial and quality, real).

Many proponents of the new natural philosophy, including Descartes, retained the ideal of constructing explanations that would count as scientia, especially when the problem could be mathematized and the solution derived from assured principles. But they also appreciated the difficulties in constructing unqualified scientiae of nature, notably of the biological world (Fouke 1989). Claims to apodictic security became less characteristic of the new mechanical philosophies, where the terms explanation and explication multiplied in tandem with increasing concerns about whether causal intelligibility could ever be strictly demonstrative. Explanations became common coin in the sciences, with demonstrations remaining common coin in mathematics, pure and mixed, though in the latter, unqualified demonstrability depended on the likelihood of causal demonstrations within the physical world. In the empirical sciences, other explanatory values began to displace strict demonstrability, such as intelligibility, simplicity and minimalization of theoretical principles, unification of laws, and high informational content.

In Scholastic manuals, natural philosophy (physics) was typically defined as “the science of natural bodies, in so far as they are natural.” This means that natural philosophy in the Peripatetic tradition did not deal with artifacts qua artificial. It did not share the concerns of the mechanical arts in general or of mechanics in particular. Up to and including Descartes’ time, mechanics in one sense was concerned with the construction and operation of machines and other artifacts designed to move terrestrial things against nature (contra naturam) and for human ends. However, from the time of the Mechanica, written by a member of the Peripatetic school (Strato of Lampsacus?), and of Pappus's Collections (book 8), mechanics was also the theoretical discipline that dealt mathematically with problems relating to the construction and use of machines. Mechanics in this sense was often also called the science of weights, or statics. Before a mechanical device begins to act on something, there is stasis, a state of rest in which opposing forces are in equilibrium before being unbalanced by an external force. In statics, one calculated the relative strengths of such forces to account for states of equilibrium (equal weights on a balance) and of disequilibrium (budging a boulder with a lever). In the Renaissance, which saw the rediscovery of the Mechanica, mechanics became a “middle science” operating between mathematics and physics, in which the operations of machines were treated in a mathematical way (Laird 1986). In the Collegio Romano there arose the important question of whether this middle science could be fully integrated into the natural philosophy of the Scholastics to produce a true middle science that was not subalternated to mathematics (Wallace 1984, 202–7). The question is reflected in a professional contrast of the period: treatises on mechanics were written by mathematicians or engineers, not by philosophers within the Scholastic tradition. Not until the Newtonian eighteenth century did the question receive a positive answer, though one that was seriously qualified by considerations that cannot be addressed here.

Among the notable eighteenth-century expositions of Newton's achievements were Henry Pemberton's A View of Sir Isaac Newton's Philosophy (1728), Willem Jacob 's Gravesande's Mathematical Elements of Natural Philosophy confirm'd by experiments: or, an introduction to Sir Isaac Newton's Philosophy (6th edn, 1747), and Colin Maclaurin's posthumous An Account of Sir Isaac Newton's Philosophical Discoveries (1748). To the modern eye, there is something puzzling about these titles. We note the terms “philosophy,” “natural philosophy,” and “philosophical,” and we wonder what they mean in this setting. Take Maclaurin's Account, the best of the genre, and written by one of the leading Newtonians of the day. Newton made great scientific discoveries, and we can learn what most of them are from reading An Account, but what philosophical discoveries did he make? Maclaurin describes Newton's work in mechanics, rational and celestial, and in physics, theoretical and experimental (though not optics). But Newton the philosopher? To answer these questions requires a preliminary disentanglement of the disciplinary classifications that clustered around the business of “philosophy” in the seventeenth and eighteenth centuries.

Today the study of the physical world and its contents is principally the concern of the physicist, chemist, engineer, or biologist, rather than that of the philosopher, even the philosopher of science. In keeping with this disciplinary demarcation, non-historical discussions of the nature of body or of the constituents of the physical world make infrequent appearances in volumes or journals devoted to contemporary philosophy. By contrast, the disciplinary demarcations of the early modern period were such that investigations and speculations on ‘body and the physical world’ were legitimate concerns not just of those one would now describe as ‘scientists’, but of most of the philosophical community, who shared a much broader conception of the scope of ‘philosophy’ than is common among philosophers today.

PERIPATETIC NATURAL PHILOSOPHY

The Peripatetic tradition was the intellectual framework within which most seventeenth-century philosophers were educated and within which many of them pursued their philosophical careers (see Chapter I). Peripateticism, in whatever propaedeutic form, was the earliest contact they had as individuals with serious philosophical and scientific concerns. However unsatisfying it became for some of them, at least it comprised a rigorously organised body of doctrine that included a systematic interpretation of the diversities of nature. It showed the thoughtful student of nature that an intelligible and comprehensive account of natural phenomena was a prima facie possibility. At the same time, and perhaps inevitably, Peripatetic natural philosophy was the principal object of criticism for many of those who participated in the philosophical and scientific revolutions of the period.

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.

INTRODUCTION

As noted in Chapter 15, Aristotle and the Peripatetics held that local motion is primary with respect to all other kinds of motion or change. Proponents of the new natural philosophies of the seventeenth century, including neo-atomists and those of Stoic inspiration, would have accepted this view and, indeed, would not have spurned the way it was presented by some of the later Peripatetics. They would not have been out of sympathy with the Aristotelian Keckermann, whose teaching on this issue in the Danzig Gymnasium in 1607 was not far removed from the new understanding of the rôle of local motion: ‘According to the order of nature, local motion is the first among motions, partly because it is common to the totality of all natural bodies, partly also because the other motions arise from it as from a cause [tanquam à causa]’. Nor would they have been seriously at odds with Chasteigner de la Rochepozay: ‘All other motions are included in local motion as in a cause [ut in causa], on account of [its being] the primary motion, because it is the cause of every corporeal motion, and without it there cannot be any other motion’. Yet it is not clear whether the causality in these texts is real or analogical or, if real (assuming it to be efficient), whether proximate, partial, or productive efficient causality is intended. The causal rôle reserved for local motion by some early seventeenth-century scholastics is still not the special rôle reserved for it by the proponents of the mechanical philosophy.

The question of Spinoza's involvement with science depends initially on the kinds of scientific pursuit in which he is thought to have been involved. To judge by the prestigious Dictionary of Scientific Biography, Spinoza was not importantly involved in any kind of science, since he does not merit an entry in its ostensibly comprehensive sixteen volumes (Gillispie 1970-80). This judgment is shared by the authors of most histories of science that cover the seventeenth century, where Spinoza's name appears, if at all, only in passing as part of the historical furnishing. A minority of historians of science takes a more constructive view. In Wolf's History of Science, Technology and Philosophy in the 16th and 17th Centuries, where one enjoys the wider vision of an older historiography of science, Spinoza joins Hobbes, Descartes, Locke, and Leibniz in the chapter on psychology, and he is accorded a chapter or section in most histories of psychology, including collections of source material. Furthermore, many articles and book-length studies deal in one way or another with Spinoza's psychology (however understood); biology, medicine and psychoanalysis also figure topically in Spinoza bibliographies.

An intriguing feature of Simon Patrick's A Brief Account of the new Sect of Latitude-Men (1662) is its conspicuous inclusion of some reflections upon the New Philosophy, as the full title puts it, meaning the new natural philosophy, the new science, that characterized the first half of the seventeenth century. A priori, there is no obvious reason why we should have expected Patrick to make a connection between characteristically latitudinarian positions and recent advances in natural philosophy. The latter seem to belong exclusively to the scientific domain, the former to areas of controversy within Anglicanism. Yet A Brief Account linked the latitude-men directly with an important feature of the “new philosophy” by crediting them with having “introduced” into England new doctrines of mechanical explanation of natural phenomena as correctives of the explanatory tautologies of peripatetic physics. Patrick illustrated the contrast between the mechanical and the peripatetic philosophies with a lengthy parable of a farmer's faulty clock, the fruitless attempts of “a certain Peripatetick artificer” to mend it, and the intervention of “an ingenuous Gentleman” who, understanding perfectly the mechanical workings of clocks, was able to show the farmer why he needed a new one, and what he should do in future with any other clock that might go out of kilter.

It is not wholly clear from Patrick's pamphlet, which contained the first appearance in print of the terms “latitude-men” and “latitudinarian,” who exactly the latitude-men were.