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Blackwell Great Minds

Edited by Steven Nadler

The Blackwell Great Minds series gives readers a strong sense of the fundamental views of the great Western thinkers and captures the relevance of these figures to the way we think and live today.

  1. Kant by Allen W. Wood
  2. Augustine by Gareth B. Matthews
  3. Descartes by André Gombay
  4. Sartre by Katherine J. Morris
  5. Charles Darwin by Michael Ruse
  6. Schopenhauer by Robert Wicks
  7. Shakespeare’s Ideas by David Bevington
  8. Camus by David Sherman
  9. Kierkegaard by M. Jamie Ferreira
  10. Mill by Wendy Donner and Richard Fumerton
  11. Socrates by George H. Rudebusch
  12. Maimonides by T.M. Rudavsky
  13. Wittgenstein by Hans Sluga
  14. Locke by Samuel Rickless
  15. Newton by Andrew Janiak

blackwell great minds


Andrew Janiak

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I dedicate this book to my two philosophical sons
Isaac and Saul
who will never think an apple is just an apple


Since this book has been 7 years in the making, I have incurred many debts along the way. First and foremost, I thank Steve Nadler for his foresight and patience: foresight for asking me to write this text in my own way and patience for giving me plenty of time to figure out how to do so. Second, I thank Karen Detlefsen, Niccolò Guicciardini, Hylarie Kochiras, and Tad Schmaltz for reading an earlier draft, along with four anonymous referees for Wiley-Blackwell, all of whose comments made this a far better book. Third, over the past decade, many colleagues and friends have discussed Newton with me, including especially Zvi Biener, Katherine Brading, Patrick Connolly, Graciela DePierris, Rob DiSalle, Mary Domski, Lisa Downing, Steffen Ducheyne, Katherine Dunlop, Moti Feingold, Michael Friedman, Alan Gabbey, Dan Garber, Geoff Gorham, Bill Harper, Gary Hatfield, Nick Huggett, Michael Hunter, Sarah Hutton, Rob Iliffe, Dana Jalobeanu, Christian Johnson, Lynn Joy, Antonia LoLordo, Ted McGuire, Scott Mandelbrote, David Marshall Miller, Christia Mercer, Alan Nelson, Carla-Rita Palmerino, Marco Panza, Alex Rosenberg, Jim Ruffner, David Sanford, Eric Schliesser, Alison Simmons, Ed Slowik, Chris Smeenk, George Smith, Steve Snobelen, Marius Stan, Richard Stein, Karin Verelst, and Catherine Wilson. Over the years, I chatted about Newton, metaphysics, and many other topics with Fred Dretske and Iris Einheuser—I sorely miss them both.

I couldn’t accomplish much of anything without the love and encouragement of Rebecca Stein, a true public intellectual.

I presented many of the ideas in this book to audiences at the Royal Society in London, the American Philosophical Association (Eastern and Western division meetings), Harvard University, the University of Pennsylvania, Ohio State University, the University of Notre Dame, the Rotman Institute at the University of Western Ontario, Hendrix College, Stanford University, the University of Ghent, Bergamo University, Leiden University, the University of King’s College in Halifax, and the Max Planck Institüt for Wissenschaftsgeschichte in Berlin. Grants from the American Philosophical Society, the Josiah Charles Trent Memorial Foundation, and the Arts and Sciences Committee on Faculty Research at Duke University, funded the archival research for this book. I am grateful for their support.


Why another book on Newton? There are innumerable works on his status as a leading mathematician of the past three hundred years, his work as a founder of modern physics, his intriguing biography, his experiments in alchemy, his deeply held religious beliefs, and also his role in fighting counterfeiters of the royal currency as Warden of the Mint. Newton fascinates us, and will continue to do so, for these and many other reasons. But this book takes a distinct angle on his life and work. Rather than placing him within the history of mathematics, the history of science, or even within modern British history generally, it places him instead within the history of modern philosophy, viewing him as a philosophical figure. Unlike his work in mathematics, physics, even alchemy and religion, thinking of Newton as a philosopher requires some explanation for the contemporary reader. In what sense was Newton a philosopher? Did he dabble in philosophy here and there, as perhaps any scientist can be expected to do, especially when scientific upheavals are prominent in his day? Or did he engage with philosophy in a more fundamental way?

Newton was not a systematic philosopher. He did not sit down one day to write a philosophical treatise, as Descartes wrote the Meditations, Spinoza the Ethics, or Locke the Essay. When he wrote a treatise, such as Principia mathematica, or the Opticks, it was not a treatise in systematic philosophy. He read and discussed many such works; but instead of adding to their number, he chose to engage with philosophical problems and topics primarily when his contemporaries challenged his ideas and methods. But as it turns out, Newton’s mature intellectual life from 1672 until his death in 1727 was marked by constant challenges, controversies, and debates, and this surprises us most. To write the history of Newton the mathematician, or Newton the scientist, is to write of a triumphant figure—one whose ideas and contributions to numerous fields were warmly embraced by future generations of mathematicians and physicists. Indeed, Newton’s profound influence on these fields was felt for two centuries. Thus the fact that Newton’s work was met with skepticism, even derision, during his lifetime is difficult to square with our standard conception of him. But if we reconceive of him as a philosopher, and acknowledge that the history of modern philosophy is a history of profound debates and lasting controversies, then we can incorporate challenges and debates into our conception of Newton’s intellectual life. All of the canonical philosophers of the modern period—from Descartes to Spinoza to Locke—engaged in philosophical disputes throughout their lives. And in that sense, Newton is in good company. He was not as systematic as these figures in his thinking, but he was just as philosophical.

The guiding thread of my narrative will therefore be the controversies throughout Newton’s life, which generate his most important philosophical reflections. His first publications, which appeared in the Philosophical Transactions of the Royal Society in the early 1670s, were characteristic because they immediately caused a substantial and lasting dispute concerning the proper methodology within optics among some of Newton’s most important contemporaries, especially Christaan Huygens in Holland and Robert Hooke in England. As Newton’s thinking matured during the 1670s and 1680s, he engaged in a systematic debate with reigning Cartesian ideas in philosophy. This engagement eventually led him to embrace a new way of thinking about questions within the philosophy of nature in his magnum opus, Principia mathematica, which first appeared in 1687. Given the fact that “Newtonian mechanics” dominated thinking about nature for generations, today’s readers may be surprised to learn that Newton’s Principia was strongly challenged by many of the leading thinkers in his day, including Huygens and the talented young Continental mathematician Gottfried Wilhelm Leibniz. Since Leibniz and Newton co-discovered the calculus, it is difficult to grasp why they would have engaged in substantial and unresolvable philosophical disputes. But those disputes, which gathered steam a few years after Principia mathematica appeared, would eventually erupt into a major confrontation early in the new century, ending only with Leibniz’s death in 1716. By then, Newton had developed a series of philosophical views in his attempt to defend and further the ideas and methods he had embraced many years earlier. The past decade of Newton’s life would continue in the same vein. Although he had many supporters in England and, increasingly, in Continental Europe, during his final years, it would be many years after his death before his ideas and methods would garner widespread support throughout the intellectual community. In this book, we try to recapture a flavor of the many debates and controversies in which Newton engaged throughout his life. And these are what made him a philosopher.

chapter 1
life and times

If one picks up a book about Isaac Newton, one expects to find a character conforming to the legend. One expects to encounter a mathematician who invented calculus and employed its great analytical power for understanding natural phenomena. One expects to find a scientist who ignored the philosophical and metaphysical preoccupations of his predecessors, focusing on experimental and mathematical questions that form the nucleus of modern physics. One expects to learn of a rational thinker who scorned the religious superstitions and practices of his day, portending a new secular age in which science would reign as the surest route to knowledge of the natural world. But historians of science have long since recognized the profound gap between Newton the legend and Newton the actual historical figure (Cohen and Smith 2002, 1–6). As biographers of Newton have often suggested, one reason for this gap is that Newton was actually much more exercised about religion, theology, philosophy, and even alchemy than one would expect (Dobbs 1975, Manuel 1968, Westfall 1980). There is, however, a deeper reason for the gap, one connected intimately to the proper methodology for studying the history of science and philosophy: Newton lived in an era that is profoundly different from our own in the way that it organized human knowledge. Like his contemporaries, Newton thought about science and religion, theology and philosophy, nature and God in ways that are strikingly peculiar to any twenty-first century reader. Indeed, his thinking was different enough that he lacked any such categories, as did his contemporaries: the early moderns organized human knowledge in a way that is profoundly foreign to us. It is always a struggle to come to understand the work of any genius. But in Newton’s case, we have the additional challenge of coming to understand his age.

Newton lived in tumultuous times. When he was born on Christmas Day in 1642, the historically stable British monarchy with centuries of tradition underpinning it was in crisis. A civil war had broken out, one that would lead to fundamental political and social change within the next two decades. By the time Newton was a young man, England had undergone not only civil war and a political revolution, but also the beheading of the King, the bloody rule of Oliver Cromwell, and the Restoration of the monarchy in 1660. These events had a deep impact on the most important philosophers working while Newton was a young man: in 1659, in a preface to one of his most important works on experiments with his air pump, Robert Boyle wrote of “the strange confusions of this unhappy nation” (Boyle 1999, vol. 1: 145). The revolution in politics and society was accompanied by equally profound developments in intellectual life. When Newton enrolled in Trinity College, Cambridge, in 1660, a traditional curriculum was still in place, but adventurous and talented students were directed to read the great moderns of the day, who were overthrowing the Scholastic tradition that had reigned for centuries in Europe’s great universities. Many of the moderns were Newton’s countrymen, including Thomas Hobbes, Robert Boyle, and Henry More, the latter being a friend of Newton’s family and a key leader of the Cambridge Platonist movement in midcentury. Newton avidly read their works, along with the latest writings from philosophers on the Continent, especially Descartes and the Dutch mathematician Christiaan Huygens (McGuire and Tamny 1983). The political and intellectual spheres came together early in Newton’s life with the founding of the Royal Society in London in 1662, which had the imprimatur of King Charles II. It would become the model for scientific societies throughout Europe in the next generation. Newton’s career would parallel the rise of the Royal Society: he sent his earliest papers, on optics, to the Society for publication in its Philosophical Transactions in the 1670s; published his magnum opus, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) through the Society in 1687; and would eventually become its president in 1703. By the time of Newton’s death in 1727, England and its political and intellectual life were fundamentally different than they were at the time of his youth, and Newton played as central a role in transforming the intellectual and cultural life of his country as any other figure. Although he is usually mentioned along with Robert Boyle and Charles Darwin as one of England’s three greatest scientists, he might also be listed along with Milton and Shakespeare, for his impact on British intellectual life in the early modern period was deep and lasting. Scientists, philosophers, mathematicians and even theologians spent much of the eighteenth century digesting, furthering, and criticizing Newton’s work. He played the kind of role that Kant played in nineteenth-century German-speaking philosophy: everyone worked in his wake.

Isaac Newton himself lived a somewhat tumultuous life. The tumult resulted from early tragedies in his family: his father died a few months before he was born and he spent much of his childhood away from his mother. Contradictions emerged later: an intensely private person who shunned public controversies from his earliest days, he was also a recognized public intellectual, a knight of the British Empire who not only presided over the Royal Society but also sought a seat in Parliament (representing Cambridge) and served in London in the politically influential post of Warden of the Mint. A founder of what we now call modern mathematical physics, he was also a deeply religious man with numerous—often deeply heterodox—opinions on biblical chronology, the history of Christianity, and theological doctrine, including the nature and status of the Trinity.1 Although anti-Trinitarianism was illegal due to the Blasphemy Act of 1648, Newton embraced it privately, providing hints to friends like the philosopher John Locke in the early 1690s and the philosopher–theologian Samuel Clarke in his later years. He agonized over finding the proper interpretation of scripture and of Christianity’s most complex doctrines. Newton’s Nachlass2 includes not only hundreds of pages of work in mathematics, optics, and natural philosophy but also numerous manuscripts dealing with alchemical experiments and the history of alchemical thought and thousands of pages of theological materials from every decade of Newton’s life3: a complex man, indeed.

Newton’s immense impact on science and society, and his immensely complicated personality, transcends the capacity of any one volume to capture it. Blackwell Great Minds is devoted to the history of philosophy. If we employ the history of early modern philosophy as the lens through which to view Newton’s work, what picture emerges? Newton was not a systematic philosopher: although he read a number of systematic philosophical treatises, including Descartes’s Meditations, he never wrote one. But he did in fact develop and defend a wide range of philosophical views. He did so primarily in reaction to the many controversies that his publications, ideas, and methods generated from his earliest work in the 1670s until the end of his life in the first third of the eighteenth century. Hence, the focus in this book is on Newton’s philosophical debates and discussions with other thinkers—including, especially, Descartes and his followers, Huygens, Locke, and finally Leibniz and his followers. The philosophically salient aspects of Newton’s life can be broken into three stages for simplicity’s sake: the early years, circa 1660–1680; the middle period, circa 1680–1700; and the late years, 1700–1727. Each of these periods involved substantial philosophical output in reaction to substantive debates.

The early period took place largely in Cambridge. After finishing his studies as an undergraduate at Trinity College, Cambridge in 1665, Newton did considerable work in mathematics (and other areas), rising to become the second Lucasian Professor of Mathematics at the University of Cambridge in 1669 (the first holder of the chair was his teacher, Isaac Barrow). His required course of lectures as Lucasian Professor includes some important reflections on mathematics and natural philosophy. During the 1670s, he also made lasting contributions to experimental optics that helped solidify his reputation in England and abroad. The debates about methodology prompted by his optical writings prefigured an important aspect of Newton’s lifelong philosophical work, especially the issue of whether hypotheses or other kinds of speculation should be embraced in philosophy.

In the middle period, the 1680s–1690s, Newton made huge advances in natural philosophy, culminating in the publication of his magnum opus, Principia Mathematica, in 1687. During this period, Newton befriended John Locke and had several important philosophical exchanges with various figures, including G.W. Leibniz (the German philosopher and mathematician) and Richard Bentley (a London theologian and later the long-serving Master of Trinity College). By the turn of the new century, Newtonianism was a powerful intellectual force in England and was soon to be one on the Continent. But it remained highly controversial.

The final period of Newton’s life was focused on developing his philosophy and on defending it against its numerous critics. The most dramatic moment in this period came in 1715, when Leibniz articulated several powerful philosophical objections to Newtonianism in a series of letters sent to Princess Caroline of Wales and through her to Samuel Clarke (who was then a member of Newton’s circle in London). This ignited a major cross-Channel debate that precipitated the Leibniz–Clarke correspondence, perhaps the most influential philosophical exchange of the entire eighteenth century. Controversy did not subside with Newton’s death: in the following decades, the Leibnizian and Newtonian systems had displaced Cartesianism as the major philosophical orientations of Europe and the debate between them continued throughout the rest of the century. Émilie du Châtelet wrote an important treatise, Institutions de Physique (1740), which attempted to mediate the dispute between Leibniz and Newton, and in his Critique of Pure Reason (1781), Immanuel Kant continued that general project with his groundbreaking work. The shadow of Isaac Newton was long indeed.

1.1 Background and Childhood

In a small town in Lincolnshire, England, Hannah Newton gave birth to her son Isaac early on Christmas morning in 1642.4 The fact that he was born on Christmas obviously became the stuff of legend. Isaac was apparently in a very weak condition when he was born—it took a full week before he was baptized, on the first day of the New Year. Although he would eventually rise to become the world’s greatest natural philosopher, his early years were marked by tragedy. Isaac’s father and namesake had died while Hannah was 6 months pregnant, so the two never met. What kind of effect this fact had on Newton’s upbringing has been the subject of intense speculation over the years.5 It may have helped shape Isaac’s lifelong inability to maintain friendships he had formed. Luckily for Hannah and the young Isaac, his father left a modest estate including a manor house and a flock of roughly 200 sheep, which was above average for this part of the English countryside. The Newtons therefore were far from wealthy, and were certainly not aristocracy, but the family was in a stable financial position during Isaac’s youth. This stability, however, was soon torn by another event: when he was just 3, his mother, Hannah, remarried and left him in the care of his maternal grandmother. He would not live with his mother again until he was 11, so his formative years were spent without either of his parents. Nonetheless, Hannah was a resourceful person, for her second marriage, to the Reverend Barnabas Smith, greatly increased the family’s wealth and helped to secure Isaac’s future. We can only speculate about the challenge she faced in helping her family at the cost of living apart from her young son. For his part, the elder Newton recalled feeling great anger during his youth.

Despite Hannah’s finances, it seems that few in her family had received any formal education. Indeed, it appears that Isaac’s father could not write his own name.6 But things would be different for the young Isaac. His mother arranged for him to attend the Grantham School, where the influential Cambridge Platonist Henry More had also gone as a boy (one of Newton’s teachers was More’s former student). He enrolled at age 12 and lived with the nearby apothecary—he thus lived apart from his mother again after just a year together. He presumably studied mostly Latin and almost certainly no mathematics at Grantham, and he would have devoted substantial time to Bible study. Although he learned no mathematics and no natural philosophy at this time, both of which were flourishing in England and on the Continent in this period, his knowledge of Latin and of scripture would serve him throughout the rest of his life. More immediately, they also prepared him for future university study. But he would not continue with his studies directly: when he graduated from Grantham at 17, his mother called him back to Woolsthorpe and tried to encourage him to engage with the upkeep of the family farm. Apparently, these efforts came to naught. Isaac was ill suited to farm life and did little to help with the daily work on his mother’s land. Her small staff thought he was useless; one of them noted that he was fit “for nothing but the ‘Versity.’” That turned out to be a prescient remark: in June 1660, when he was not yet 18, Isaac would set out on a journey for Cambridge. In a sense, he would never return.

On June 5, 1660, the young Isaac passed his entrance examination and enrolled in Trinity College. The colleges were technically separate from the University of Cambridge: students who sought degrees would enroll in the university as well. Newton did so 1 month after arriving in Cambridge. Although he had traveled just over a 100 kilometers, he had obviously entered a completely new world. The son of functionally illiterate parents, he was now a full-fledged member of one of the oldest institutions of learning in all of Europe.7 He would eventually become one of its most famous graduates.

1.2 Early Years in Cambridge

The university as an institution had existed in England since the twelfth century, and in Newton’s day, the curriculum at Cambridge’s colleges had not changed substantially in many years. Indeed, it seems that much of the formal study at Trinity College—in fields such as logic, ethics, and rhetoric—had little influence on Newton’s thinking and on his future work. But the curriculum was just one aspect of the flourishing intellectual community that the young Newton had joined, and it is this wider community that enables us to understand how Newton received his true education. There are two crucial features of the community that profoundly influenced Newton in his first years in Cambridge. First, he began to learn substantial amounts of mathematics, both through his own reading of texts like Euclid’s Elements and van Schooten’s extended Latin edition of Descartes’s Geometry and especially through the instruction of Isaac Barrow, a Cambridge don who was the first person to hold the now famed Lucasian Professorship of Mathematics. Newton attended Barrow’s lectures on mathematics and worked with him personally. It was Barrow more than any other figure at Cambridge who came to recognize Newton’s intellectual gifts, not to say his genius, and who quickly facilitated his rapid rise at Trinity College. Given that the 17-year-old Newton knew little if any mathematics after his training at Grantham, it is astonishing that he would become the second holder of the Lucasian chair a mere 9 years after arriving in Cambridge, discovering the generalization of the binomial theorem and fundamental aspects of what we now call the integral and differential calculus in the intervening period. Such facts present us with the temptation to picture Newton as a lone genius, finding and solving complex mathematical and scientific problems completely on his own through sheer intelligence. Despite his undeniable and prodigious gifts, the young Newton did not exist in an intellectual vacuum: if it were not for Barrow’s instruction and encouragement, we do not know what Newton might have accomplished on his own. Second, and just as importantly, although the curriculum was of little use to Newton, like many enterprising and talented young students, he was instructed to read the great moderns on his own. His notebook from his early years at Trinity College records his voluminous reading of More, Galileo, Descartes, Charleton (who passed on the views of Gassendi and of certain atomists), Hobbes, and Boyle, among others (McGuire and Tamny 1983). Because of these two facts, Newton quickly became aware of the very latest and most advanced work in both mathematics and natural philosophy. It would not be long before he himself had made major advancements in each of these areas.

Newton’s early years are now the stuff of legend. As with many legends, some details are apparently apocryphal, but there may be a kernel of truth in them. For instance, during the great plague of the summer of 1665, Newton returned home to Woolsthorpe to stay with his mother, as Cambridge was evacuated. During his stay in the countryside, the famed episode of the falling apple is said to have happened. The story is compelling and fits with our conception of the lone genius grappling with a nearly unsolvable problem, like the question of how to understand free fall on earth and the motions of the planetary bodies in a unified way. But it simply is not true that Newton discovered his complete theory of universal gravity at this time: we can document the emergence of that theory during the period from 1684 to 1686, the time when Newton wrote successive drafts of a manuscript entitled De motu corporum (On the motion of bodies). This ultimately became Principia Mathematica (1687, first edition), but only after many drafts and many crucial developments. It also seems that Newton had not yet conceived of his crucial concept of impressed force (vis impressa), which sits at the center of his mature physics (Westfall 1980, 148). But 1664–1666 has been called Newton’s anni mirabiles, or miracle years, for a good reason: he did make substantial advances in mathematics at this time; in particular, he claimed years later that he had worked out his fundamental conception of the calculus during these 2 years. This is obviously a remarkable feat for a young man who had just a few years of college under his belt, having had a rather paltry education before then, especially in the relevant areas. When Newton returned to Cambridge in 1667, Isaac Barrow eventually learned of these great leaps in mathematics and was wise to promote Newton to the Lucasian chair a mere 2 years later. Perhaps no embellished legend is needed: it is astonishing to think that basic aspects of the calculus—which remains so fundamental to modern science and mathematics to this day—were worked out by a young college student on a farm in the English countryside.8

Another legend of Newton’s early years is that he worked in an intellectual vacuum, holed up for hours at his mother’s house or in his room at Trinity College, a solitary figure single-handedly discovering many fundamental underpinnings of modern mathematical physics. The kernel of truth here, of course, is that Newton did in fact grow up largely on his own—at least without his parents—and his 2 years in Woolsthorpe during the plague were spent largely alone, at least intellectually speaking. His achievements during those years are obviously awesome by any measure. But as we have seen, Newton was not alone in Cambridge: he had the important influence of senior figures and his records from the time record celebrations with friends in the tavern when he received welcome academic news. So the legend must be tempered.

Newton’s actual writings during his early years as Lucasian Professor indicate a prodigious output. In the period 1670–1675, Newton created a telescope, celebrated by the newly formed Royal Society, and made fundamental contributions to optics, which were published in the Philosophical Transactions (see Chapter 3). As it turned out, Newton’s claim that he had used a prism in order to determine experimentally that ordinary sunlight contains a series of embedded rays of various colors and that the colors of the rainbow are not created through a modification of light embroiled him in a substantial controversy. Numerous important figures from this time, including the famous Continental mathematician Huygens and the London experimental philosopher Robert Hooke, objected to Newton’s methodology. Newton found the ensuing extensive debate extremely taxing, and he developed a lifelong aversion to intellectual controversy. But his reputation, both in England and, increasingly, on the Continent, had been established. It was soon to increase dramatically.

1.3 Mature Years in Cambridge and London

Despite these early achievements, no one among Newton’s contemporaries was prepared for his work in the next decade. In this period, the stuff of legend is not apocryphal: it is well documented. As has been told countless times, in August of 1684, Edmond Halley—for whom Halley’s comet is named—came to visit Newton in Cambridge in order to discover his opinion about a subject of much dispute in celestial mechanics. At this time, many in the Royal Society and elsewhere were at work on a cluster of problems that might be described as follows: how can one take Kepler’s Laws, which were then considered among the very best descriptions of the planetary orbits, and understand them in the context of dynamical or causal principles? What kind of cause—for some, what kind of force—would lead to planetary orbits of the kind described by Kepler? In particular, Halley asked Newton the following question: what kind of curve would a planet describe in its orbit around the sun if it were acted upon by an attractive force that was inversely proportional to the square of its distance from the sun? Newton immediately replied that the curve would be an ellipse, rather than a circle.9 Halley was amazed that Newton had the answer at the ready. But Newton also said that he had mislaid the paper on which the relevant calculations had been made, so Halley left empty handed. He would not be disappointed for long. In November of that year, Newton sent Halley a nine-page paper, entitled De Motu, that presented the sought-after demonstration, along with several other advances in celestial mechanics. Halley was delighted, and immediately returned to Cambridge for further discussion. It was these events that precipitated the many drafts of De Motu that eventually became Principia Mathematica. It is shocking to think that if Halley had not visited Newton and then persisted in talking with him, one of the founding texts of what we now call modern science might not exist.10

While he was writing the text that was to become the Principia, Newton corresponded with Halley. One of his letters in particular indicates how Newton understood the discipline of natural philosophy, to which he intended to contribute with his new book, even while warning Halley of his aversion to intellectual debate. On June 20, 1686, he wrote to Halley as follows:

I designed the whole to consist of three books…. The third I now design to suppress. Philosophy is such an impertinently litigious lady that a man had as good be engaged in law suits as have to do with her. I found it so formerly [he presumably means the 1670s optics disputes] & now I no sooner come near her again but she gives me warning. The two first books without the third will not so well bear the title of Philosophiae naturalis Principia Mathematica & therefore I had altered it to this De Motu corporum libri duo: but upon second thoughts I retain the former title. Twill help the sale of the book which I ought not to diminish now tis yours. (Correspondence II: 437)

This letter was sent in response to Halley, who had written to Newton 2 weeks earlier that he ought to include Book III because “the application of this mathematical part, to the system of the world; is what will render it acceptable to all naturalists, as well as mathematicians, and much advance the sale of the book” (Correspondence II: 434). Without the third book, in which Newton discusses what he calls “the system of the world,” the first two books of Principia Mathematica could not very accurately be called natural philosophy; rather, they would be better described as “two books on the motion of bodies.” For Newton, as for Halley, natural philosophy was not principally concerned with just any motion of bodies that was tractable through mathematical analysis; rather, it was concerned with the mathematical analysis of the actual motions of the bodies within our solar system—within nature as we perceive it in our vicinity of the universe. That coheres with a long tradition in natural philosophy. Newton’s innovation is to provide a mathematical analysis of the motions of these bodies (Chapter 5 describes why this maneuver is innovative). It is no surprise that for Newton, as for many natural philosophers in this period, one of the great outstanding questions is how to understand the earth–sun system, including the question of whether the earth or the sun is at its center. Just a year after this letter, the first edition of what many would now regard as the first true text of modern mathematical physics appeared. For his part, Halley wrote an anonymous piece in the Philosophical Transactions announcing the arrival of the text: “it may be justly said, that so many and so Valuable Philosophical Truths, as are herein discovered and put past Dispute, were never yet owing to the Capacity and Industry of any one Man.”11 And so at the age of 44, Newton was quickly propelled into the first rank of mathematicians and philosophers in England.12

The decade of the 1690s brought Newton considerable attention for his magnum opus, along with several new and significant friendships. When John Locke returned from political exile in 1688, he made a point of meeting Newton soon after and the two struck up a friendship. Their correspondence indicates that they held similar unorthodox religious beliefs, each finding reason, for instance, to question the official Anglican doctrine of the Trinity. These beliefs were closely held by Newton throughout his adult life, and Locke may have been the first person to whom he revealed them. In the second edition of the Principia in 1713, Newton may have given some hints as to his real views, especially in the newly added General Scholium at the book’s end, but he would never publish his anti-Trinitarian conceptions openly, as they would have landed him in considerable political (and perhaps legal) trouble. For his part, Locke took Newton’s work very seriously: he wrote an anonymous, largely laudatory, review of the Principia (having famously asked Huygens to vouch for the mathematics he could not understand) in the Bibliothèque Universelle, mentioned Newton in the preface to his Essay Concerning Human Understanding (1690), and told Bishop Edward Stillingfleet that Newton’s work had convinced him that his long-held mechanist understanding of natural change required revision (see Chapters 5 and 6). During this same time, Newton quickly developed an intense and very close friendship with the Swiss mathematician Nicolas Fatio de Duillier, who reports being converted from Cartesianism to Newtonianism in the process (Westfall 1980, 493). Yet Newton’s tumultuous personal life did not come to an end with these particular friendships: as his correspondence amply demonstrates, he often found himself passionately devoted to a friend, only to have a significant fight with him later. Some scholars have speculated that the constant tumult of Newton’s life can be traced back to the absences of his childhood.

In the mid-1690s, the allure of Cambridge and its intellectual environment had begun to fade for Newton. The figures who had such an immense impact on his early years, especially Isaac Barrow and Henry More, had long since died, and in some ways, the intellectual life that Newton sought had shifted to London, where figures like Richard Bentley were then living, and where the Royal Society was located.13 But Newton did not move to London merely to seek intellectual company, and the picture of Newton as the solitary scholar is difficult to square with his many years in the capital. In 1696, after 35 years in Cambridge, Newton decided to move permanently to London, taking up the post of Warden of the Mint as his new official position. Newton’s time as Warden, focused on bureaucratic and logistical issues, especially the question of how to defeat counterfeiters of the currency, is of little interest philosophically, but it does indicate that the mature Newton was a political figure and far from being a solitary scholar. Newton’s political position, however, did not end his philosophizing: it was during this period that he finally decided to publish what became his second great book, the Opticks. Newton’s research in optics was largely in completed form long before he decided to publish his work, and the results of his numerous experiments, together in a single volume. In 1704, the first edition of the Opticks, which would lead to its own powerful tradition in the experimental aspects of natural philosophy in the eighteenth century, appeared after the death of Newton’s old rival in optics, Robert Hooke. Newton’s friend, Samuel Clarke, translated the Opticks into Latin in 1706, thereby making it available to a wide Continental audience. In subsequent editions, Newton would add a series of long Queries

Although Newton did not publish another major work during his last two decades, he did make substantial revisions and additions to both the Opticks and the Principia. Many of these changes were prompted by the serious criticisms leveled against his views by leading mathematicians and philosophers on the Continent, especially Huygens and Leibniz. Indeed, the last two decades of Newton’s life are marked by an intriguing contrast: on the one hand, Newton was at the height of his intellectual and institutional power and prominence, and he held an unquestioned position as the leading intellectual figure in England at that time; on the other hand, despite his numerous followers throughout his native land, including Bentley and Clarke, his views in natural philosophy and in theology (to the extent that they were known) were the subject of vigorous debate and deep controversy throughout the Continent. Numerous figures in Europe in the early eighteenth century were still convinced of the basic truth of Cartesianism, and therefore spent considerable energy attempting to undermine Newton’s views. For those who had come to dispense with Cartesian ideas, however, there remained a powerful and influential alternative to Newton, namely, the philosophical orientation of Leibniz (Chapter 6). In the 1690s, Leibniz had published a number of important papers—in the Acta Eruditorum and elsewhere—that were critical of Descartes’s views in natural philosophy and metaphysics, and many figures in the early eighteenth century saw him as the leading metaphysician of the day.14 An extensive debate between Leibniz and his followers and Newton and his followers would erupt during this period, focused both on substantive philosophical, theological, and mathematical issues and on nonsubstantive issues connected with the political and nationalist implications of the calculus priority dispute. The many denunciations issued by both sides in the dispute over which mathematician first discovered the calculus, which became extremely heated in the 1710s and ended only with Leibniz’s death in 1716, should not overshadow the genuine and genuinely interesting philosophical issues debated by both sides.

Chapter 6PrincipiaPrincipia

Demonstration of the Being and Attributes of God15

Despite his frailty at birth, Newton outlived Leibniz and many of his other critics and friends. He spent the last years of his life in declining health. A third edition of the appeared in 1726, just a year before he died, but it did not differ substantially from the second edition. And as might be natural, Newton spent his last few years reminiscing about his past and telling stories to his friends and acquaintances. It is apparently from these days that the story of the apple originated (Westfall 1980, 862). Despite his declining health, however, Newton continued to preside over the Royal Society. And the Society’s influence continued to be felt: on Newton’s last meeting, in March of 1727, a letter from the newly formed Academy of Science in St. Petersburg was read. Just 3 weeks later, Sir Isaac died at the age of 85. The Royal Society canceled its next meeting in the wake of his death. By the end of that month, he had been interred in Westminster Abbey, at the center of London and indeed of British politics and culture, where his crypt remains to this day.