power. This he brought to bear against the only living person who could even hope to challenge his intellectual supremacy: Gottfried Wilhelm Leibniz, who despite being a foreigner (he was Hanoverian) had been made a Fellow of the Royal Society in 1673, largely in recognition for his invention of the Stepped Reckoner, a mechanical computer.
The contrasts between Newton and Leibniz were lavish. Newton seems to have had an entirely accurate sense of just how he compared to his contemporaries, and acted accordingly without concern for dusty precedents or the personal feelings of those who clung to them. When confronted with anything less than uncritical acceptance of his work, he lashed out and then secluded himself. He published rarely but ex cathedra, handing down nearly flawless treatises over which he had toiled for years or decades, perfectly organised into definitions, axioms, lemmas and laws, framing a mathematical physics that could be used to explain past observations and to make verifiable predictions.
Isaac Newton’s signature.
Leibniz was an accomplished courtier who maintained long friendships with the Electress of Hanover, the first Queen of Prussia, the sister-in-law of Louis XIV, and the future Queen Consort of England, while moonlighting, late in his career, for Peter the Great. He corresponded so heavily that scholars are still sorting through his unpublished papers. In his philosophy he practised an ecumenicism that in a lesser mind would strike us as suspicious or even craven. Leibniz seems never to have met a philosopher or a theologian he didn’t like, and his metaphysics developed out of an effort to harmonise the ancient thinking of (both) Plato and Aristotle with tenets of Christian and Jewish theology and with the ‘mechanical philosophy’ the Royal Society had been created to champion. It is impossible to know precisely what he was thinking without perusing his vast legacy of papers. In effect, Leibniz’s philosophy ceased to exist at the moment he died. Since then, anyone who has wanted to know it has first had to reconstruct it, which is only possible for forensically inclined scholars, fluent in Latin, French and German, and well versed in the history of Western philosophy, Christian theology and Enlightenment science.
Given Leibniz’s stature as one of the great thinkers of Western history, one might expect that, as of the 350th anniversary of the founding of the Royal Society, all of his writings would long since have been published, and that everything would be known about his philosophy. But the question of ‘what did Leibniz believe, and when did he believe it?’ is unsettled and is the topic of current research and debate.
A squalid row over the origins of the calculus, which these two men had independently invented decades earlier, became the public face of the conflict, which is regrettable since it is not very interesting and since it reflects dreadfully on the combatants. Much more significant in the long run was a debate on topics that reach so deeply into the foundations of science that they are still discussed in our times. This broke the surface in the last year of Leibniz’s life, in an exchange of letters that has come to be known as the Leibniz–Clarke correspondence.
The year was 1715, and because of two royal deaths (in England, Queen Anne; in Hanover, Electress Sophie), Princess Caroline of Brandenburg-Ansbach had just become the Princess of Wales. To the modern reader, Caroline seems less like a real historical personage than a plucky, clever, independent-minded heroine from some post-feminist historical novel. A noble but poor orphan, raised as a ward of the Prussian court, she was conversant with scientific topics of the day, largely because she had been tutored in them by Leibniz. She had married into the Hanoverian dynasty and had moved with it to London, where her father-in-law
Portrait of Gottfried Wilhelm Leibniz, by an unknown artist.
Portrait of Isaac Newton by Charles Jervas.
had been crowned King George I. The sixty-nine-year-old Leibniz, who had become unfashionable and, because of the dispute over the calculus, something of a political problem, had been left behind in Germany. He wrote a short letter to Caroline, warning her that religion was declining in England; that John Locke did not believe in the immortality of the soul; and that Sir Isaac Newton held to some strange views about the relationship between God and the physical universe.
Anyone who has blithely forwarded a private email to a corporate mailing list, with incalculable consequences, will recognise what happened next: Caroline made Leibniz’s letter known, and one Samuel Clarke stepped forward to rebut Leibniz’s charges. The result was a series of letters (five each by Leibniz and Clarke) over the course of a year, at which point Leibniz died. Clarke, though he had serious credentials in his own right both as theologian and scientist, was acting as a spokesman for Newton, and so the correspondence can fairly be read as a debate between Leibniz and Newton.
In the opening round, the combatants practically trip over each other in their eagerness to remind the Princess that atheism is bad and that true natural philosophy in no way conflicts with religion. There is no reason to think that either of them is being disingenuous. The scientific revolution had created doubts about the existence of God, or at least the veracity of religious dogma, in the minds of many; but not Newton or Leibniz.
These concerns are dispensed with in a few paragraphs. The bulk of the correspondence, which runs to about eighty pages, resembles an email exchange that devolves, as it goes on, into several distinct threads, each concerning a specific sub-topic. The correspondents begin to number their paragraphs (Leibniz’s fifth letter contains 130 of them), the better to keep track of all the rebuttals and counter-rebuttals. The over-arching theme is the relationship of God to the universe, and more specifically the universe as perceived, measured and understood by scientists. Leibniz, in the universal manner of authors promoting their latest work, finds frequent occasion to mention his books Theodicy and Monadology. Even when he isn’t mentioning them by name, he is presenting arguments, and using terminology, derived from them.
My theme is the legacy of Leibniz’s metaphysics from the time of his death down to the present day, and so a direct summary of that system, based on the scholarship of latter-day researchers, will do better service than any attempt to untangle the points and counter-points in the correspondence. The account presented below is patterned after the work of Christia Mercer of Columbia University. Her book Leibniz’s Metaphysics: Its Origins and Development, published in 2001 by Cambridge University Press, is a formidable work of forensic scholarship that can in no way be improved by my attempts to summarise it.
In 1661, at the age of fourteen, Leibniz had formed a resolution to embrace the new mechanical philosophy. For most natural philosophers of the era, this meant rejecting the Aristotelian worldview of the medieval schoolmen. As mentioned, though, Leibniz was an ecumenicist and a conciliator, and so for him it meant, rather, the beginning of a lifelong quest to reconcile certain select, precisely defined tenets of Aristotelian and Platonic thought with modern science.
In his metaphysical reasoning, Leibniz is at least as meticulous as is Newton in his mathematical physics. Bertrand Russell called Leibniz’s system ‘profound, coherent, largely Spinozistic, and amazingly logical’. Newton, however, can verify his results by comparing them to observations, while Leibniz is beholden to no one except Leibniz. By pure thinking, Leibniz fabricated a metaphysical system that could hardly be more at odds with that of Newton, or indeed any other person who attempts to think in a commonsensical way about how the world might work.
Where Newton’s work is grounded in Euclidean geometry, Leibniz begins with certain precepts that he takes to be axiomatic, such as the Principle of Sufficient Reason (nothing exists without a reason; there is no effect without a cause) and the Identity of Indiscernibles (two individual things cannot differ in number
