Materialism in Eighteenth-Century European Thought

François Marie Arouet, better known as Voltaire (1694–1778), endorsed Locke's sensationalism and argued against Spinoza and Nicolas Malebranche (1638–1715) in his Dictionnaire philosophique (1764; Philosophical dictionary) and Lettres anglaises ou philosophiques (1734; English or philosophical letters). He also became very popular as an apologist for Newton, with his Éléments de la philosophie de Newton (1738; Elements of Newton's philosphy). Voltaire opposed institutionalized religion and intolerance based on superstition, and, although not an atheist, he supported the rationalistic deism. His famous Candide (1759) is a witty and ironic, though simplistic, lampoon of Leibniz's metaphysics of the best of all possible worlds.

His mistress and patron of the great French salon, Madame Émilie du Châtelet, who, with his aid, translated Newton's Principia mathematica into French, was also a prominent supporter of Newtonianism and did much to publicize these new scientific views. They were both forced to flee Paris in 1747. Once in Berlin, Voltaire entered a dispute with his old friend and protégé Pierre-Louis Moreau de Maupertuis (1698–1759), the president of the Berlin Academy and author of Venus physique (1745; The physical Venus) and Système de la nature (1751; System of nature).

Paul-Henri-Dietrich d'Holbach's (1723–1789) most famous book, Système de la nature, ou des loix du monde physique et du monde moral (1770; The system of nature, or the laws of the physical and moral world), was published under the name of J. B. Mirabaud. Following the tenets of Epicurean atomism, the book derided religion and espoused an atheistic, deterministic materialism: all causation was reduced to patterns of motion, man became a machine devoid of free will, and religion was excoriated as not just untrue, but dangerous. Système social (1773; Social system) placed morality and politics in a utilitarian framework: duty was reduced to prudent self-interest. D'Holbach used his inherited fortune to support Diderot's Encyclopédie project, writing many articles for these volumes himself and translating from German.

George Louis Leclerc, comte de Buffon (1707–1788) was appointed to the French Academy of Science in 1734; in 1739 he became superintendent of the Royal Botanical Garden (Jardin du Roi; present-day Jardin des Plantes). Buffon was given the task of completing a catalog of the royal collections in natural history, which he transformed into a project to produce an account of the whole of nature. This became his great work, Histoire naturelle, générale et particulière (1749–1804; General and particular natural history), which was the first modern attempt to systematically present all existing knowledge in the fields of natural history, geology, and anthropology in a single publication. Buffon criticized the Swedish naturalist Linnaeus for the artificiality of his taxonomy of plants and animals. Buffon opposed metaphysics in science, denied divine intervention in nature, and was Newtonian in believing that knowledge should be derived from observations of natural phenomena. He held that what separated humans from animals was reason alone, through their use of language. He was the first to reconstruct a geological history, including a discussion of nature and a theory of the age of the earth, in a series of stages in Histoire et théorie de la terre (1749; History and theory of the Earth), and in Époques de la nature (1778; Epochs of nature), he proposed the theory that the planets had been created in a collision between the sun and a comet. His theory of the age of the Earth incensed religious opponents. Buffon divided matter into vital and nonvital, balancing an overall Newtonian physical framework with vitalistic tendencies (though rejecting nonmaterial substances). So, he opposed the idea that life was a form of organization of matter. On the other hand, life being a property of matter (of "organic molecules"), it does not require any explanatory principle external to matter. Buffon believed in spontaneous generation as resulting from aggregation of organic molecules present in the environment; his theory was criticized by Lazzaro Spallanzani (1729–1799), whose experiments confirmed Francesco Redi's (1626–1697) rejection of spontaneous generation.

The outcry following publication of Julien Offroy de La Mettrie's (1709–1751) materialistic views in Histoire naturelle de l'âme (1745; Natural history of the soul) forced his departure from Paris. In Holland, in 1747, he published L'homme machine (The man-machine), which was publicly burned, even in that notoriously liberal environment. La Mettrie then fled to Berlin to ask for the protection of Frederick II of Prussia (r. 1740–1786). La Mettrie, in opposition to Descartes, held that matter was not only extended, but also endowed with an inherent principle of motion and that it could have sensations. Humans were infinitely more complex than lower machines, yet they were still machines. La Mettrie suggested that atheism and an ethics of hedonism were the only proper paths toward human happiness.

Claude Adrien Helvétius (1715–1771) in his De l'esprit (1758; On the spirit) explains all of human reason as being based on sensation (like Locke and Condillac). Like La Mettrie, Helvétius placed humans on a continuum with animals. Helvétius also adumbrated a somewhat Epicurean ethical system based on self-interest, pleasure, and pain. His book provoked an outraged reaction both in the court and in the schools; both the Sorbonne and the Parlement of Paris condemned it. Despite Helvétius's public recantation, it was burned publicly (along with the works of other philosophes like Voltaire). Helvétius, like La Mettrie, was welcomed at the Prussian court in Berlin.

Étienne Bonnot de Condillac (1715–1780) maintained an empirical sensationalism based on the principle that observations made by sense perception are the foundation for all human knowledge. The ideas of his L'essai sur l'origine des connaissances humaines (1746; Essay on the origins of human knowledge) are close to Locke's, though on certain points, as in rejecting what he took to be Locke's innate ideas of reflection, Condillac modified Locke's position. In his most significant work, the Traité des sensations (1754; Treatise on sensations), Condillac denied, for example, that the human mind makes inferences about the shapes, sizes, positions, and distances of objects. Examining each sense separately and the knowledge thereby obtained, he concluded that all human knowledge is sensation transformed by language. Modeled on algebra, language was the underlying principle used to form all higher cognition. In this work he famously analogized man to a statue who gains complex reasoning from its underlying sensations. The importance of language to thought and rational progress is one of the major themes of the Enlightenment. Despite Condillac's materialistic psychology, he believed his views about the nature of religion, especially the reality of the soul, to be consistent with his sensationalism. Condillac's own brand of sensationalism had many followers in Italy. In particular, the jurists Giandomenico Romagnosi (1761–1835), Cesare Beccaria (1738–1794), and Melchiorre Gioia (1767–1829) used his epistemology as a basis for a rationalistic study of jurisprudence and political and social theory.

In 1745 the publisher André Le Breton (1708–1779) approached Denis Diderot (1713–1784) with a proposal to produce a French translation of Ephraim Chambers's Cyclopaedia, (first edition, 1728). Together with the mathematician and philosopher Jean Le Rond d'Alembert, Diderot transformed the project into the great Encyclopédie. By adhering to strictly rationalistic and materialistic principles, Diderot and d'Alembert saw themselves as forging a weapon against the hold of religion on science and human knowledge. In 1749, Diderot published the Lettre sur les aveugles à la usage de ceux qui voient (An essay on the blind for those who can see), remarkable for introducing the first sketch of Diderot's "evolutionary theory" of survival by adaptation. The existence of diversity and monstrosities in nature is explained by a theory of evolution from chaos; the apparent order and adaptation is explained with a probabilistic argument, since nature has time for innumerable trials that lead to growth, increased complexity, and specialization. Only the organisms that became adapted to their environment could survive. Diderot held that our sensory organs, not ideas about essences, should determine our metaphysics. This open endorsement of radical materialism was condemned and, in 1749, led to Diderot's being incarcerated for three months.

Another ugly moment for Diderot came when Helvétius's De l'esprit, was condemned to be burned by the Parlement of Paris in 1758, and the Encyclopédie itself was formally suppressed. Diderot published the remaining volumes semiclandestinely. Among his philosophical works, special mention may be made of L'entretien entre d'Alembert et Diderot (written 1769, published 1830; Conversation between d'Alembert and Diderot), Le rêve de d'Alembert (written 1769, published 1830; D'Alembert's dream), and the Éléments de physiologie (1774–1780; Elements of physiology). In these works Diderot developed his materialist philosophy and concluded that simple, reductive mechanical explanations are not sufficient to explain sentient life, without assuming that all matter is potentially sentient and that life and sentiency are specialized functions that arise from a higher level of complexity. In Jacques le fataliste (1771; Jacques, the fatalist) Diderot gave a common sense solution of the problem of free will: while there are no rational arguments to support free will, he argued that extreme determinism is ultimately self-defeating.

Jean Le Rond d'Alembert (1717–1783) attended a prestigious Jansenist school where he developed a lifelong distaste for religion. After studying law for two years and medicine for one, he finally discovered his passion for mathematics, which he mainly taught himself. In 1739 he read his first paper to the French Academy of Sciences, of which he became a member in 1741. In 1743 he published his Traité de dynamique (A treatise on dynamics), containing the famous "d'Alembert's principle," stating that Newton's third law of motion (for every action there is an equal and opposite reaction) is true for both freely moving and fixed bodies. Other mathematical works followed; in particular the development of partial differential equations. In 1745, d'Alembert joined Diderot in the Encyclopédie project as editor of the mathematical and scientific sections. In fact, his contribution went much further, including the Discours préliminaire (Preliminary discourse) that introduced the first volume in 1751. The introduction endorsed a view of science as a unified and rational enterprise.

Besides the authors mentioned above, it is useful to recall that there was a clandestine literature that offered materialistic responses to Descartes and that most likely contributed to the philosophical educations of the authors listed. Of particular interest, a collection of manuscripts at the Douai library (ms. 702), on which appears the date 1723, and contains, among others, a Dissertation sur le sentiment des bêtes, l'instinct et la raison, contre les Cartésiens (Essay on the feelings of animals, instincts and reasons, against Cartesians); L'essai philosophique sur l'âme des bêtes (Amsterdam, 1732; Philosophical essay on the soul of animals); and Principes physiques de la raison et des passions des hommes (1709; Physical principles of reason and passions of men) by a Dr. Maubec. Most of this literature, while it circulated widely in its day, is still available only in manuscript.

A late-eighteenth-century figure, Pierre-Jean-Georges Cabanis (1757–1808), was a French philosopher, physician, and physiologist who published Rapports du physique et du moral de l'homme (1802; Relations of the physical and moral of man). Cabanis, opposing Condillac's sensationalism, explained all of reality, including the mind-body relations in man, in terms of a mechanistic materialism built on the organic needs of an organism and its automatic responses (the irritable properties of tissues). For Cabanis, life was merely an organization of physical forces; "secretions" in the brain, analogous to the liver's secretion of bile, produced thoughts. The concept of soul was superfluous, since consciousness was merely an effect of mechanistic processes, and sensibility, the source of intelligence, was a property of the nervous system. Cabanis is best known, however, as a medical reformer who brought new ways of health care and medical education to France.