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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.

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