Voltaire and the science of the Enlightenment Professeur Christophe Paillard

Voltaire’s literary art and his political action are well known. Whether one likes or hates him, it is universally accepted that he is the greatest of French writers as he remains the most translated and appreciated internationally. The French National Library’s catalogue which consists of 231 volumes, two of which are entirely dedicated to him, is proof enough. No other author can boast one whole volume to himself, let alone two. Furthermore Voltaire was the first to impersonate the “intellectual” or the engaged writer in the political jests of his time, thinkers such as Victor Hugo, Emile Zola, Bertrand Russell, Jean-Paul Sartre or Noam Chomsky later imitated him. His relationship to the science of the “Lumières” (Enlightnement) is however more obscure. We all know of his hard fought battle for tolerance and freedom with works such as Candide, but few people realise that he also wrote a number of scientific treaties, the most famous of these being Elements de la philosophie de Newton. This book was a central contribution to the spreading of Newton’s physics in France and continental Europe; according to W. H. Barber, “it was one of a small number of published works which contributed significantly to the acceptance and adoption of Newtonian theory in France”. At the time Voltaire had by his side the most important woman of his life, his friend and lover, Gabrielle Emilie Le Tonnelier de Breuteuil, marquise du Chatelet, best known under the name Mme du Chatelet or Emilie du Chatelet, a genius of a woman, author of the only French translation (more precisely an adaptation) of Newton’s Philosophiae Naturalis Principia Mathematica. It is accepted that Voltaire’s works in the field of physics were fertile, however specialists sometimes criticise his involvement with natural and earth sciences with respect to which he is said to have been obscure or delusional. Thus, must one see Voltaire’s scientific enterprise as enlightened or backwards?

1 Voltaire’s English experience: the discovery of Newton

Voltaire’ talents, in poetry, were first recognised in Paris, when he was very young. His rendition of the tragedy of Oedipus was met with great appreciation in 1718, when he was only 24 years old. However, his penchant for irony and insolence were met with enmity by certain great lords; Philippe d’Orléans, Régent de France, threw him into the greatest prison of all: the Bastille, while the knight Ronan-Chabot had him beaten for some sarcastic comments. With a second stay at the Bastille imminent, Voltaire chose to leave for London in 1726. His stay in London was one of the cornerstones of his career. Having left Paris a poet, he returned a philosopher. On the other side of the Channel, Voltaire discovered an intellectual context that differed radically from the one he had been brought up in. In France he had received an education based on the principles of René Descates’ philosophy. This can be reduced to rationalism – all knowledge is derived from human reason and inherent ideas, not from the senses which are not capable of coming to terms with the essence of things – and the concepts of physics as exposed in Principes de la philoshophie, of which the main points are: -the identification of matter with extension; -the distinction of three different kinds of matter; -the rejection of the notion of void or vacuum; -a theory or circular motion based on the notion of centrifugal force. Rejected at first, Descartes’ philosophical principles were integrated to the French education system at school and university levels at the end of the 17th century. His approach of the principle of inertia, later reformulated by Newton, though interesting at first glance, were based on a speculative and metaphysical approach more than on any scientific grounds. Christian Huygens clearly realised this when he stated “that the Principes, though very persuasive, was in some respects a romance rather than a serious scientific work1”. During his exile in London, Voltaire discovered two systems that profoundly affected his vision of the world. The first of these was John Locke’s system that the young man appreciated for its epistemological and political dimensions. Epistemologically the Locke’s Essay on Human understanding condemns Descarte’s rationalism and puts empiricism forward as the basis of all knowledge, meaning that innate ideas are

2 inexistent and that knowledge derives from experiment and experience. From a political point of view Locke justifies, in his two treaties On Civil Government, the institutions that sprouted from the Glorious Revolution of 1688 and the Bill of Rights of 1689. Voltaire actively took these ideas on board as he became critical of Cartesian rationalism, stating that there is no knowledge without mediation and sensitivity, and a defender of constitutional monarchy within which the king’s power is bounded by the law. The second intellectual discovery was to be yet more decisive: that of Newton’s physics. The two men never met: when Voltaire arrived in London, Newton was terminally ill, but Voltaire was very affected by the funeral at Westminster Abbey on the 4th of April 1727. In London, Voltaire was welcomed by Caroline of Ansbach, Princess of Wales, and her social circle. “Here he met Newton’s friend and confident Samuel Clarke, and through him came into contact with others close to Newton […] Voltaire was much impressed by Clarke. He seems to have regarded himself for a time as his disciple, and his initiation into the world of Newtonian ideas probably came primarily not through Newton’s scientific work but through the lively discussions in which the princess delighted, and in which Clarke played a prominent part, on such topics as the philosophical proofs of God’s existence, the nature of time and space […], and the problems of perception2”. I will not go into the details of Newton’s critical influence on physics with you, but in the eyes of the Enlightenment he is the genius who, by discovering the principle of universal attraction, managed to unify the laws of terrestrial mechanics and celestial mechanics by establishing a single law to describe the fall of a body as well as the gravitation of planets around the sun. Newton rejected most of Descartes’ theories, such as the geometrical reduction of matter under extension, the notion of vortex, the in-existence of vacuum and the theory of circular movement. It must however be noted that the French philosopher inspired Newton. In Notebooks he kept when younger “he initially entertained the notion of a vortex system of planetary motion”. However Newton, similarly to Huyghens, swiftly realised that Cartesian physics was more akin to a novel than to science, he even composed a voluminous treaty to refute it: De gravitation et aequipondio fluidorum which was re-printed in 1962 alongside an English translation in Unpublished scientific papers of Isaac Newton (p. 89-156). Newton was right, Descartes was wrong; this is an accepted fact. However it is necessary to highlight two points on which Newton’s theories differed from today scientific understanding.

3 The first difference is that Newton opposed another universal thinker, Leibniz, on the question of what is nowadays called kinetic energy and was then known as vis viva or “force vive’ in French. This debate is generally referred to as the “vis viva controversy”. The problem, which was delicate at the time, was to define the ambiguous concept of the force of a body in motion; is this force only the momentum or is it also the energy of the moving body? “On the one side Leibniz and his followers maintained that the ‘force’ of moving bodies should be measured by the product of mass and velocity squared (mv2); on the other, the Cartesians and Newtonians contended that is should be measured by the simple product of mass and velocity3”. We shall see that this debate – or more precisely this uniquely violent conflict – between the Newton’s and Leibniz’s followers will unfold into a divergence between Voltaire, who will take Newton’s side, and Mme du Chatelet who, accurately, will defend Leibniz’s positions. The second difference that can be noted is that Newton, despite his genius, adopted a number of scientific positions far from what is today’s thinking. You probably have in mind his conception of absolute space and time which was discredited by Einstein’s theory of relativity elaborated in the 20th century. However it is not this conception I have in mind. Newtonian physics – in our minds – does not constitute a purely scientific system. It lies on speculative metaphysical foundations. Newton’s physics is entirely dependant of the idea of God the Creator. Newton believed that if the planets were only ab4andoned to a simple attractive force their orbits would undergo growing perturbations and the whole planetary system would be submitted to gravitational collapse. Thus the planetary system's steadiness relies on it being periodically rearranged by the manum emendatricem, the repairing hand of God2. In other words God must intervene periodically in order to sustain planetary stability. Another example of metaphysics within Newton’s thinking is Newton’s definition of space: the Sensorium Dei; God’s sensory organ. Space is the means by which God exists in all things. This vision is eccentric to us today but is central to Newton’s system. A third example of speculation within his thinking is found in the translator’s notes of John Locke’s Essay concerning Human understanding. Newton described God’s creation as ex nihilo: to create matter he believed that God simply needed to render some portions of space impenetrable. Furthermore we know that Newton’s thinking could take on a mystical or occult slant. He credited the prophecies the New Testaments with a certain level of truth as proven by his Observations upon the Prophecies of Daniel and the Apocalypse of St John, published posthumously in 1733. Finally Newton dedicated some time to alchemy, this was kept quiet by his heirs but was revealed during the first half of the 20th century by the economist John Maynard Keynes.

4 Fascinated by his time in London, Voltaire wrote two major pieces which greatly contributed to the diffusion of English ideas in Europe: the Lettres Philosophiques or Lettres anglaises in 1734 and Elements concernant la philosophie de Newton in 1738. The first of these presents the English way of thinking in a series of letters about Quakers, Anglicans, Presbyterians, the government, Locke, Newton, etc. Newton is presented there in a general manner, no details are given and the laws that we know as “Newton’s laws” are not precisely described. It would be hard to overstate the importance of this piece of literature which will nourish the thoughts of younger philosophers such as Diderot or Rousseau. Les Lettres Anglaises were immediately banned in France and the author threatened with a pending prison sentence. Every cloud as a silver lining: he fled to Cirey in Champagne, now a part of France but at the time under the rule of the court of Lorraine. In Cirey stood the residence of Mr and Mrs du Chatelet, Emilie being the greatest love in Voltaire’s life. He spent 15 years in Cirey and these were without a doubt the happiest days of his life. His time there played a central role in the evolution of his thinking, it was described by Owen Wade as his “intellectual re-education”. He reminisces in his Mémoires: “ Nous ne cherchions qu’à nous instruire dans cette délicieuse retraite, sans nous informer de ce qui se passait dans le reste du monde. Notre plus grande attention se tourna longtemps du côté de Leibnitz et de Newton. Mme du Châtelet s'attacha d'abord à Leibnitz, et développa une partie de son système dans un livre très-bien écrit, intitulé Institutions de physique. Elle ne chercha point à parer cette philosophie d'ornements étrangers: cette afféterie n'entrait point dans son caractère mâle et vrai. La clarté, la précision et l'élégance, composaient son style. Si jamais on a pu donner quelque vraisemblance aux idées de Leibnitz, c'est dans ce livre qu'il la faut chercher. Mais on commence aujourd'hui à ne plus s'embarrasser de ce que Leibnitz a pensé. Née pour la vérité, elle abandonna bientôt les systèmes, et s'attacha aux découvertes du grand Newton. Elle traduisit en français tout le livre des Principes mathématiques; et depuis, lorsqu'elle eut fortifié ses connaissances, elle ajouta à ce livre, que si peu de gens entendent, un commentaire algébrique “ “In this beautiful place we delighted only in educating ourselves, with no interest in the events taking place in the rest of the world. Our greatest attention was, for a long time, given to Leibnitz and Newton. Mme du Chatelet concentrated on Leibnitz first and developed part of his system in a very well written book: Institutions of physics. She did not attempt to burden this philosophy with foreign flourishes; this was not part of her

5 male and honest character. Clarity, precision and elegance were the basis of her style. If ever some truth was given to Leibniz’s ideas, it is within this book that they must be sought. But today we tend to not encumber ourselves with what Leibniz thought. Born for the truth, she soon abandoned the systems, and devoted herself to the discoveries of the great Newton. She translated his book Principals of mathematics to French; and from that point on, when she had fortified her knowledge, she added to this book, unknown to too many people, an algebraic commentary.” Mme du Chatelet had at her disposal a small physics laboratory as well as a rich library of which Voltaire took full advantage. By dropping marbles from varying heights into wax and measuring the impact depth she proved, against Newton’s beliefs, the truth in Leibniz’s intuition that kinetic energy is a function of speed squared. By relying on the resources available at Cirey and on Mme du Chatelet’s extraordinary intelligence, Voltaire wrote his voluminous treaty Elements de la philosophie de Newton contenant la métaphysique, la théorie de la lumière et celle du monde. This popularisation of Newton’s science planted his physics into French minds. To tell the truth, Voltaire’s enthusiasm for Newton led him to accept the latter’s most speculative principles. In his exposition of the Sensorium Dei theory for the creation of space and of God’s Manum emendatricem meant to oppose gravitational collapse he took Newton’s side against Leibniz. Regarding the debate on vis viva or kinetic energy, Voltaire hesitated a lot. He at first joined Leibniz and Mme du Chatelet, opposing Newton’s vision: “the effect that is produced by the force of a moving body […] is the product of its mass by the square of its speed” (p. 275). He then refused to give his opinion of the question and it became a simple nominal controversy. Finally he retracted his initial statement and embraced Newton’s vision: “two types of force do not exist, one of them inert (i.e.. Momentum) and the other live (i.e.. Kinetic energy), one differing infinitely from the other […] force is nothing less than the product of a mass by its velocity”. Voltaire will never refute Newton’s physics; such was his respect for it. His intellectual evolution will led him however to question its metaphysical grounds. Maybe you know that between 1770 and 1773, Voltaire produced, basically alone, an encyclopaedia containing 9 volumes and 450 articles titled Questions sur l’encyclopédie. The Voltaire Foundation at the University of Oxford is currently editing this encyclopaedia for the first time since 18th century. I myself contribute to this project by being in charge of a number of philosophical and scientific articles. It is intriguing

6 to see how Voltaire, in 1770, comes back to Newtonian themes which he had defended thirty years earlier. In the “Space” article he criticises Newton’s Sensorium Dei notion: “ Newton regarde l’espace comme le sensorium de Dieu. J’ai cru entendre ce grand mot autrefois, car j’étais jeune; à présent je ne l’entends pas plus que ses explications de l’Apocalypse. L’espace sensorium de Dieu, l’organe intérieur de Dieu; je m’y perds et lui aussi. Il crut, au rapport de Locke, qu’on pouvait expliquer la création, en supposant que Dieu par un acte de sa volonté et de son pouvoir, avait rendu l’espace impénétrable. Il est triste qu’un génie tel que Newton ait dit des choses si inintelligibles”. “Newton sees space as God’s sensorium. I heard this grand word once in days past, as I was young; nowadays I do no hear it more than I hear his explanations of the Apocalypse. Space the sensorium of God, the internal organs of God; I am confused and so was he. He believed in Locke’s report, that creation could be explained by supposing that God, in an act of his own will and power, had made space impenetrable. It is sad to see that a genius such as Newton would has said such unintelligible things.” I also edited the article entitled De la fin du monde de Questions sur l’encyclopédie in which Voltaire passionately talks of Newton’s physics while condemning his mysticism. “ Bossuet et Newton ont commenté tous deux l'Apocalypse; mais à tout prendre, les déclamations éloquentes de l'un, et les sublimes découvertes de l'autre, leur ont fait plus d'honneur que leurs commentaires. Ajoutons à l'article Apocalypse, que deux grands hommes, mais d'une grandeur fort différente, ont commenté l'Apocalypse dans le dix-septième siècle. L'un est Newton, à qui une pareille étude ne convenait guère; l'autre est Bossuet, à qui cette entreprise convenait davantage. L'un et l'autre donnèrent beaucoup de prise à leurs ennemis par leurs commentaires; et, comme on l'a déjà dit, le premier consola la race humaine de la supériorité qu'il avait sur elle, et l'autre réjouit ses ennemis”. “Bossuet and Newton both commented on the Apocalypse; but it must be said that the eloquent declarations of the first and the sublime discoveries of the other, gave them more honour than their commentaries. To the Apocalypse article it must be added that that, two great men, of a very different greatness, commented the apocalypse in the course of the 17th century. The first is Newton, for whom such a study was not suitable; the other is Bossuet, for whom it was more suitable. One and the other presented a lot of weaknesses to their enemies through their

7 commentaries; and, as was stated earlier, the first treated the human race with his own superiority over her, and the second treated his enemies” These two articles from Questions sur l’Encyclopédie, Space and Force in Physics, reveal Voltaire’s scepticism as well as his profound attachment to scientific rationality. He distinguishes Newtonian physics, irrefutable in its positivity, from the underlying metaphysics which are purely speculative and thus open to criticism. Newton’s laws are one thing, God’s repairing hand and Sensorium Dei something completely different….

Voltaire and natural sciences

This leads me to the second part of this intervention which will be briefer. Voltaire may be credited with important services to the world of physics through his vulgarisation of Newton’s works; he is nevertheless criticised for his backward looking in the field of natural and earth sciences. He is said to have been insensitive to the evolution of geology and biology. His faith in God the Creator may have prevented him from approving the revolutions brought about by the science of the “Enlightenment”. It has been written, provocatively I think, that Voltaire “defended his faith” more so than a scientific position. I will try and show that this point of view is exaggerated. It must however be noted that Voltaire did show some signs of a certain “stationary” attitude, which stopped him from admitting to the idea of a history of life and earth, that is a progressive creation of life and earth via certain mechanisms. Voltaire refused to accept the idea that planet earth had undergone important geological modification, that our globe was formed through astronomical mechanisms and that animal species were a product of evolution. I will only cover the first of these aspects, the one concerning the geological modifications of the earth. Voltaire believed in the relative stability of the globe since its creation. Planet earth may have undergone certain marginal modification but Voltaire completely excludes all possibility that its geography may have been subject to radical change: « Il n’y a […] aucun système qui puisse donner la moindre vraisemblance à cette idée si généralement répandue que notre globe a changé de face, que l'océan a été très-longtemps sur la terre habitée, et que les hommes ont vécu autrefois où sont aujourd'hui les marsouins et les baleines. Rien de ce qui végète et de ce qui est animé n'a changé, toutes les espèces sont demeurées invariablement les mêmes »

8 “There is no system that could give any truthfulness to this idea that our globe’s face has changed, that the ocean covered inhabited land, and that where humans once lived now live porpoises and whales. None of what vegetates or what is lively has changed, all species have state invariably the same.” At the time, the maritime fossils found in the mountains led geologists from the Enlightenment to believe that water used to cover those parts of the world in ages past. This idea disturbed Voltaire deeply: « Je sais bien qu'il se trouvera toujours des gens sur l'esprit desquels un brochet pétrifié sur le mont Cenis, et un turbot trouvé dans le pays de Hesse, auront plus de pouvoir que tous les raisonnements de la saine physique; ils se plairont toujours à imaginer que la cime des montagnes a été autrefois le lit d'une rivière ou de l'océan, quoique la chose paraisse incompatible; et d'autres penseront, en voyant de prétendues coquilles de Syrie en Allemagne, que la mer de Syrie est venue à Francfort. Le goût du merveilleux enfante les systèmes; mais la nature paraît se plaire dans l'uniformité et dans la constance autant que notre imagination aime les grands changements (…) » “I know that there will always be people who’s mind is more impressed by a petrified pike on mount Cenis, and a turbot found in the country of Hesse than by all the reasonings and sane physics; they will always enjoy imagining that the top of mountains was once a river bed or an ocean, though this appears incompatible; and others will think, when they see shells from Syria in Germany, that the Syrian sea came to Frankfurt. A taste for the marvellous leads to great systems; but nature seems to pride itself in being uniform and constant, as much as our imagination loves great changes […]” How to explain the existence of maritime fossils in the heart of the land? In a Dissertation from 1748 Voltaire tries to explain this, he describes them as the remnants of pilgrims’ picnics: « On a vu aussi dans des provinces d'Italie, de France, etc., de petits coquillages qu'on assure être originaires de la mer de Syrie. Je ne veux pas contester leur origine; mais ne pourrait-on pas se souvenir que cette foule innombrable de pèlerins et de croisés, qui porta son argent dans la Terre Sainte, en rapporta des coquilles? » “In some provinces of Italy, France, etc., shells have been seen which are sworn to originate from the Syrian sea. I do not wish to contest their origins; but could one not remember the great number of pilgrims and crusaders who taking their money to the holy land, brought back shells?”

9 Buffon, the famous naturalist, pretending not to know the author of this Dissertation mocks Voltaire in his Histoire naturelle: « les poissons pétrifiés ne sont, à son avis, que des poissons rares rejetées de la table des Romains parce qu’ils n’étaient pas frais ; et à l’égard des coquilles ce sont, dit-il, les pèlerins de Syrie qui ont rapporté dans les temps des croisades celles des mers du Levant qu’on trouve actuellement pétrifiées en France, en Italie et dans les autres états de la chrétienté ; pourquoi n’a-t-il pas ajouté que ce sont les singes qui ont transporté les coquilles au sommet des hautes montagnes et dans tous les lieux où les hommes ne peuvent habiter, cela n’eut rien gâté et eût rendu son explication encore plus vraisemblable. Comment se peut-il que des personnes éclairées et qui se piquent même de philosophie, aient des idées aussi fausses sur ce sujet ? » “petrified fish are, in his opinion, simply rare fish which have been refused at the Romans’ table because they were not fresh; and, with respect to the shells from the eastern seas found in France, Italy and the other countries of Christianity, he says that the Pilgrims from Syria brought them back; why did he not add that monkeys carried the shells to the top of high mountains and in the places where men cannot live, that could not have made things worse, and would have rendered his explanation yet mot truthful looking. How is it possible that such intelligent people, who even pride themselves as philosophers, could have such wrong ideas on the subject?” This text led to a lasting opposition between Buffon and Voltaire. The latter would constantly oppose Buffon’s theories, who nevertheless played a decisive part in the development of natural history. Let us not criticise Voltaire too much however. His opposition to the idea of transformations taking place within the earth is certainly based on metaphysics but it also has scientific backing. The metaphysical reasoning sprouts from his deism. The singularity of the French Enlightenment (as opposed to the European Enlightenment) is their division in two wings. On one side the deists, in accordance with the European Enlightenment, remain attached to the idea of God the Creator. This movement is followed in France by Montesquieu, D’Alembert, Rousseau and Voltaire. On the other hand, the materialist wing represents what Jonathan Israel calls “radical enlightenment”; this wing believes in reality without the need for a God. If energy and movement are inherent properties of matter, then there is no need for a great architect to shape it; matter has the power to organise itself and that suffices in ensuring the constitution of reality. Voltaire may have been an opponent of the catholic party, he nonetheless also strongly opposed atheistic materialism. In his eyes the advancement of natural

10 sciences led towards this atheistic materialism by asserting the existence of dynamic within nature as well as the vitality of matter. A fight against materialism suggests a total disregard for its epistemological basis. “From 1765 to his death, Voltaire wrote more than twenty five books, treaties, dialogues, fake letters, false questions, satires in verse and prose, mediations or short stories, all directed, partly or entirely, at atheism and its scientific basis.3” To oppose the progress of science in the name of metaphysical reasons is certainly not scientific, you will say, and you will be right. It must however be understood that Voltaire’s opposition to the theories of his time are also based on certain scientific reasons. Each of Buffon’s theory to which Voltaire opposed, were less scientific than speculative; we know today of their fallacious aspects. His fidelity to Newton’s physics led Voltaire to fight against natural sciences. Newton’s essential point in Voltaire’s eyes is the famous Hypotheses non fingo “I do not feign hypotheses”. Newton stuck to a strict “phenomenalism”. He gave a description of the laws of universal attraction without ever trying to explain the cause of this attraction. The natural sciences of the Enlightenment on the other hand presented a speculative aspect and relied on hypothesises which were impossible to prove. Thus a true paradox is established: Buffon was, fundamentally, right however he was wrong when it came to describe the mechanisms involved. Buffon was partly right when he stated “it appears certain that the land which today is dry and inhabited was, in the past, under sea water”. Buffon puts this fact forward to defend one of his favourite theories: “the mountains were created by the sea” and more precisely “by the movement and the sedimentation of the waters”. The formation of mountains is the result of a natural mechanism: in this sense Buffon is right. However Buffon is wrong regarding the mechanism: the formation of mountains is not the result of sedimentation, they appeared due to a geological phenomenon discovered at the end of the 19th century, that is plate tectonics. The same could be said of Buffon’s other theories to which Voltaire was opposed, starting with his so called evolutionism and his idea of the creation of the earth by the action of a planet hitting the Sun. To summarise, Voltaire opposed Buffon’s theories, partly because of his aversion to atheistic materialism, but mainly because of a reasonable scepticism, as inspired by Newton. The scientific spirit asks that every theory be backed by irrefutable proof. However it must recognised that though the physics of the Enlightenment was relatively well backed, biology and geology were mere speculations. W. H. Barber remarks:

11 ‘These whole areas of enquiry concerning the nature and history of living things, and the history of earth itself, were thus in Voltaire’s times largely fields for speculative controversy based on what now seem non-scientific premises, rather than firmly founded branches of human knowledge. And for Voltaire the contrast with the solid achievement of Newtonian physics was clearly an acute one (…)6”

Conclusion

To conclude, Voltaire was a Newtonian philosopher, a loyal Newtonian who remained sceptical with respect to the underlying metaphysics of the system, the Sensorium Dei and God’s manum emendatricem. In this sense he was more Newtonian than Newton himself. It was as a Newtonian that Voltaire opposed the natural and earth sciences of the time. He was never more loyal to Newton than when he doubted these speculative hypotheses: Hypotheses non fingo. Though Voltaire was never a complete physicians, he never ceased, from the time of his London visit between 1726 and 1728, to consider Newton’s physics, unburdened of its metaphysical aspects, as the paradigm of rationality which all form of philosophy should follow. His contribution to the advancement of physics was critical, not least his scepticism towards all which was speculative. Reason must only accept/admit what is proven or will be: such is Voltaire’s lesson after Descartes and Newton…

12