Voltaire as Science Popularizer

François-Marie d’Arouet, better known to us as Voltaire, begins one of his Philosophical Letters of 1734 as follows:

The philosophers of the last age found out a new universe; and a circumstance which made its discovery more difficult was that no one had so much as suspected its existence. The most sage and judicious were of opinion that it was a frantic rashness to dare so much as to imagine that it was possible to guess the laws by which the celestial bodies move and the manner how light acts. Galileo, by his astronomical discoveries, Kepler, by his calculation, Descartes (at least, in his dioptrics), and Sir Isaac Newton, in all his works, severally saw the mechanism of the springs of the world. The geometricians have subjected infinity to the laws of calculation. The circulation of the blood in animals, and of the sap in vegetables, have changed the face of Nature with regard to us. A new kind of existence has been given to bodies in the air-pump. By the assistance of telescopes bodies have been brought nearer to one another. Finally, the several discoveries which Sir Isaac Newton has made on light are equal to the boldest things which the curiosity of man could expect after so many philosophical novelties.¹

Such excitement for the new discoveries of what we now call the Scientific Revolution is characteristic of the Enlightenment — which itself has been called the “Age of Voltaire.” However, writings on science formed a minuscule part of Voltaire’s immense output, as his primary focus was on creating works of drama, poetry, and history.

According to one biographer, Voltaire passionately held two crucial Enlightenment values: “the essential value of toleration and pluralism, in the face of the fanaticism of the rêgime; and the right of every man to think and say what he liked, in the face of censorship and repression.”²

Living in an age of strict censorship, Voltaire learned the hard way to avoid expressing his ideas too openly; he was sent to the Bastille twice and exiled numerous times. However, he became a master at using ingenious stories and satire to poke fun at his primary targets: the Catholic Church and the aristocracy. Many of Voltaire’s plays achieved immense popularity in France, and appealed to a wide range of social classes. Since at this time French was the primary language of the educated, Voltaire’s popularity extended far beyond France, with many European aristocrats among his most devoted fans.

Voltaire spent almost three years living in England, and during this time he immersed himself in English culture, including its science. When Isaac Newton died in 1727, Voltaire was in London, and he probably attended the ceremonial funeral, after which Newton was laid to rest in Westminster Abbey, along with the kings and queens of England. Voltaire then set out to understand what Newton had accomplished to earn such a distinction, and this ultimately led to several publications on the Newtonian discoveries. These consisted of a handful of letters (chapters) in his 1734 work Philosophical Letters (Lettres philosophiques), his 1738 book Elements of the Natural Philosophy of Newton (Éléments de la philosophie de Newton), and a 1740 pamphlet titled The Metaphysics of Newton (La Metaphysique de Newton) which was later added at the beginning of his Elements for the 1745 edition.

The frontispiece to this book features a dramatic illustration containing three central figures: A seated figure writing at a desk represents Voltaire. Seated high above on a cloud is Isaac Newton — the source of a divine light. This light is reflected down to Voltaire by a mirror held by a female figure. This is Émilie du Châtelet, Voltaire’s lover and intellectual partner, who translated Newton’s entire Principia into French and provided invaluable assistance with Voltaire’s research on the new science. The Marquise du Châtelet was an accomplished natural philosopher and mathematician, and her translation of the Principia is still the standard French translation.

Newton and Descartes

Voltaire’s writings on Newton frequently begin with critiques of the theories of Rene Descartes — the creator of a “mechanical philosophy” that was highly esteemed in early eighteenth century continental Europe. In his Principles of Philosophy of 1644, Descartes had propounded a system of the universe based on hard impenetrable particles (of different shapes and sizes) pushing each other around, with no empty space or void between them. The nature of these particles stemmed from Descartes’ prior metaphysical view that matter fundamentally consists of extension (i.e. length, breadth, and width).

The lack of empty space between them implied that moving particles were continually circulating in vortices. Accepting the Copernican system of the planets, Descartes used these vortices to explain the orbits of the planets around the sun, the orbits of moons around planets, and the rotation of the earth. Descartes proposed elaborate mechanisms involving these particles to explain a wide variety of physical phenomena; for example he saw gravity as caused by a constant rain of fine particles pushing objects toward the earth.

As Voltaire said of Descartes, “He did not experiment, he imagined: he did not examine this world, he created one.”³

The answer to the Cartesian system came in 1687, when Newton published his famous Mathematical Principles of Natural Philosophy (Philosophiae Naturalis Principia Mathematica). The Principia presented Newton’s new understanding of force, acceleration, and gravity, and pointed out the problems with Cartesian mechanisms.

Newton’s other major scientific project was his work in optics, light, and colors, based on his extensive experimenting with prisms and lenses. After writing a paper titled “On Optics” and circulating it informally for decades, Newton finally published the much larger work Optics in English in 1704 and in a Latin version in 1719.

By the early 1700s, the Newtonian theories had found a few converts on the Continent; but overall, reception was slow due to their incompatibility with the popular Cartesian “mechanical” approach. Voltaire and Émilie du Châtelet agreed that a French-language defense of Newton was needed, and they set to work on the project.

Elements of the Natural Philosophy of Newton

Voltaire published his book Elements of the Natural Philosophy of Newton (Éléments de la philosophie de Newton) in 1738. Supplemented by over seventy figures, the chapters provide summaries of Newton’s science along with arguments against the competing explanations of Descartes. The chapters are divided into two parts, the first covering the nature of light, and the second covering motion, gravity, and celestial objects.

The first part begins with critiques of the theories of light of Descartes and the priest Nicolas Malebranche. Voltaire then presents astronomer Ole Roemer’s observations and reasoning showing that light takes seven or eight minutes to travel from the sun to the earth, which contradicted Descartes’ belief that light travels instantaneously.⁴

Several chapters then explain the well-established features of light rays, including the nature of reflection, refraction (the bending of light), Snell’s law of refraction, the optics of lenses, how the cornea works as a lens in the eye, the behavior of concave and convex mirrors, the operation of telescopes, and how we visually perceive distance.

Voltaire then turns to the Cartesian theory of colors:

If you ask the philosophers what produces the colors, Descartes will answer that «the globules of its elements are determined to spin on themselves, besides their tendency to move in a straight line, and that it is the different spinning that makes the different colors». But do his elements, his globules, his spinning, even need the touchstone of experience for the false to be felt?”⁵

Voltaire points out the problem of how differently-spinning particles would interact with each other; they would apparently influence each other’s spinning. Voltaire describes a simple experiment which disproves this influence, and then continues:

Finally, turn to Newton. He will tell you: Don't believe me; believe only your eyes and mathematics; put yourself in a completely dark room, where daylight only enters through an extremely small hole: the ray of light will come to give you the color of whiteness on paper.⁶

So begins a discussion of Newton’s experiments with prisms and his discovery that sunlight is made up of a mixture of different colors, which can be separated by refraction through a prism, and can be recombined back into white light.

The second part of Elements covers the discoveries presented in Newton’s Principia, which concern the nature of force, acceleration, and gravity. After an initial discussion of the Aristotelian account of gravity, Voltaire then turns to Galileo’s experimental work leading to his law of free fall. Falling bodies accelerate at a uniform rate of acceleration (neglecting the resistance of air). After the air pump had been developed by Boyle, it was seen that when the air is evacuated, a heavy coin and a light feather both fall at the same rate, further confirming Galileo’s law.

Starting with Galileo’s view of inertia and law of free fall, Newton then considered Kepler’s three laws of planetary motion, which Kepler had derived from observations of Mars. Newton concluded that an inverse-square law of attraction based on total masses would explain planetary orbits.

This led him to the realization that the moon is continually falling towards the earth — that the same laws of motion govern both terrestrial objects (like falling apples) and celestial objects (like planets and moons).

Several chapters cover the then-current state knowledge of our moon, the sun, Mercury, Venus, Mars, Jupiter (with its four moons), and Saturn (with its five moons). Other chapters address the tides, the spheroid shape of the earth, the precession of the equinoxes, and the nature of comets. The final chapter discusses the small-scale attraction seen in capillary action, water droplets, and similar phenomena.

In his concluding paragraphs, Voltaire includes a quotation from Newton’s Principia, which explains how Newton avoided the problems that plagued the mechanisms of Descartes:

I do not make hypotheses, hypotheses non fingo. For that which is not deduced from phenomena is a hypothesis; and hypotheses, whether metaphysical or physical, whether suppositions of occult qualities or suppositions of mechanics, have no place in experimental philosophy.⁷

Voltaire’s Rejection of “Metaphysics”

After publishing the Elements of the Philosophy of Newton in 1738, Voltaire developed his ideas on the nature of metaphysics as used in natural philosophy. Impressed by Newton’s comment about rejecting metaphysical hypotheses, and repulsed by the rationalist systems of both Descartes and Leibniz (both of which were based on speculative ideas on the fundamental  nature of reality), Voltaire turned against metaphysics entirely.  He also saw the rejection of metaphysics as key to the success of Newton.

This led to Voltaire’s short work The Metaphysics of Newton (La Metaphysique de Newton), published in 1740, and then added to the beginning of his Elements of the Philosophy of Newton, republished in 1745.

Conclusion

Voltaire was not the only continental intellectual to advocate for Newton’s ideas, but he was surely the most widely read such advocate, and his writings must have had an impact. Over the course of the eighteenth century, the theories of Newton gradually displaced the mechanistic theories of Descartes. Newton had indeed become, in the words of Voltaire, “the destroyer of the Cartesian system.”⁸ Most importantly, Newton’s discoveries exemplified for the Enlightenment the power of reason to understand the world.

References for Further Reading:

For a good biography of Voltaire, I recommend Voltaire Almighty: A Life in Pursuit of Freedom, by Roger Pearson (New York: Bloomsbury, 2005).

For a translation of Voltaire’s Elements, I recommend Elements of the Philosophy of Newton: A New Translation, trans. Tim Newcomb (Stuttgart, Germany: Newcomb Livraria Press, 2023). This edition does not include the plates of figures, which are often essential for following the text. However, these figures can be found online at Wikisource, at the end of the original French text:

https://fr.wikisource.org/wiki/Éléments_de_la_philosophie_de_Newton/Édition_Garnier

1. Philosophical Letters, Letter 16 (https://www.gutenberg.org/files/2445/2445-h/2445-h.htm)

2. Voltaire: A Life, by Ian Davidson (New York: Pegasus, 2010 ), p. xvii

3. Elements of the Philosophy of Newton: A New Translation, by Voltaire, trans. Tim Newcomb

(Stuttgart, Germany: Newcomb Livraria Press, 2023), p. 56 (end of part 2, ch. 1)

4. Ole Reomer did not initially set out to measure the speed of light; he was observing Jupiter’s moon Io in order to find a timekeeping aid for mariners. A good explanation of his discovery can be found in the video here:

5. Elements, p. 106 (part 2, ch. 10)

6. Elements, p. 107 (part 2, ch. 10)

7. Elements, p. 221 (part 3, ch. 14)

8. Philosophical Letters, Letter 14