Increasingly, particularly in late Enlightenment texts, this confidence in our ability to discover and apply clear moral distinctions came into conflict with an alternative view of human nature and morality derived from philosophical materialism, which was particularly influential in France. To a materialist everything, from our nervous system and reflex actions to our innermost thoughts and most ‘mystical’ beliefs, was susceptible to examination by the physical sciences; our thoughts and actions were explicable in purely physical terms. If the universe was a kind of great machine in which everything was subject to unalterable laws, might not the same be true of human beings? Building on the foundations laid by ancient philosophers such as the Roman poet Lucretius (c.98–55 BCE) and bolstered by the Enlightenment's commitment to the physical sciences, many eighteenth-century thinkers, including Diderot, pursued the consequences of this philosophy. Materialism was based on the belief that everything we can know or experience has causes and explanations rooted in physical matter. Such ideas were seen as a threat to traditional religion with its belief in an immaterial or non-corporeal, spiritual soul able to survive the body after death, and also to the concept of free will and the capacity to choose between good and evil.
Materialism, then, had disturbing implications for Christianity and for morality conceived of as obedience to a set of divine injunctions. First, it seemed to leave little scope for God, except as a possible prime mover or great architect of the universe who, having set the universe in motion, thereafter sat back without intervening further, leaving it – and humanity – to operate like automata in accordance with the unalterable laws of nature. From this perspective divine providence, after its initial intervention, was redundant. Second, if everything is reducible to physical phenomena and processes, then physical sensations assume great importance in moral matters. Building on the well-established premise that the seeking of pleasure and the avoidance of pain are central to human happiness and well-being, eighteenth-century materialists rejected the fundamental Christian concept of original sin (humankind's innate depravity) and other guilt-inducing moral dictates in order to focus on human sensations and the relationship between physical and moral health. One consequence of this was a new legitimisation of hedonism, or the conscious pursuit of pleasure.
Summary point: in Enlightenment France philosophical materialism, the belief that everything (including the apparently spiritual) can be explained in terms of physical matter and the laws governing it, became increasingly influential. It encouraged in some thinkers a tendency to argue that all moral matters might be reduced to the maximising of physical pleasure.
The Marquis de Sade (1740–1814) twisted such ideas into perversity by rejecting as a moral criterion everything except that which conduces to the gratification of our physical desires and by defining human nature in terms of the ‘natural’ influences of physiology, environment and climate. This is an approach conducive to amoralism rather than to moral guidelines. Sade's Dialogue between a Priest and a Dying Man (1782) dramatises the encounter of such a credo with the priest's conventional but ultimately assailable Catholic beliefs. The dying man's self-seeking hedonism conflicts with the Enlightenment requirement for a clearly defined, socially controlled moral code.
Materialism was one of many threats to the status of religion. Enlightenment thinkers analysed and criticised religious beliefs in the same way as they subjected to rational scrutiny secular topics such as geological or economic theories. By refusing to treat religion as sacrosanct or the source of its own authority, they threw down a challenge to ecclesiastical institutions, especially in France, where there was a strong alliance of church and state, and the former was seen both as a support and as a beneficiary (for example, through tax exemptions) of an inherently despotic system of government. The perceived corruption and grip on privilege of the Catholic Church, as well as its role in state censorship and its overall hostility to Enlightenment ideas, provoked in the French philosophes much anti-religious and anti-clerical criticism as well as the kind of sparkling irony and provocative wit that characterised Voltaire's Candide or his Philosophical Dictionary (1764). In Candide, James Boswell noted, ‘Voltaire, I am afraid, meant only by wanton profaneness to obtain a sportive victory over religion, and to discredit our belief of a superintending Providence’ (Boswell, 1951, p. 210). In 1762 the Archbishop of Paris similarly complained about both the Enlightenment message and its tone:
Disbelief has in our time adopted a light, pleasant, frivolous style, with the aim of diverting the imagination, seducing the mind, and corrupting the heart. It puts on an air of profundity and sublimity and professes to rise to the first principles of knowledge so as to throw off a yoke it considers shameful to mankind and to the Deity itself. Now it declaims with fury against religious zeal yet preaches toleration for all; now it offers a brew of seditious ideas with badinage, of pure moral advice with obscenities, of great truths with great errors, of faith with blasphemy.
(Barzun, 2000, p. 368)
The archbishop was right: the philosophes were implying that while the morality preached by Jesus was unexceptionable, educated people would be better off if they jettisoned the age-old lumber of theology, metaphysics, rituals, priests and monks. One of the alternatives frequently recommended to the enlightened was the natural, universal religion of deism, a pure and rational system of ethics uncluttered by dubious miracles, dubious science and dubious history. The authorities took action. In 1766 two young French nobles were sentenced to death for failing to remove their hats and singing ribald songs in the presence of a religious procession honouring the Virgin Mary. Sentence was confirmed by the Supreme Court in Paris, which noted that irreligion was rife and blamed Voltaire's Philosophical Dictionary. One of the young men escaped, and obtained a commission in the Prussian army through Voltaire's intervention with Frederick the Great. The other, the Chevalier de la Barre, was put to death, his tongue excised and his body burned at the stake together with a copy of the Philosophical Dictionary.
Summary point: the French Enlightenment subjected religion and its institutions to rational, secular analysis and was often disrespectful, sceptical and subversive in its attitude to the Catholic Church.
Even Rousseau, a fervent though unorthodox believer, did not escape censure. Both the Catholic Church in France and the Calvinist Church in Geneva were outraged by his suggestion that people were naturally good and that an emotional communion with nature was as sound a basis for faith as the formal teachings of the Church. Rousseau's Emile (1762), in which he advanced these views in a section called the ‘Profession of faith of a Savoyard vicar’, was formally condemned by the authorities at Geneva and publicly burned together with his political tract Du contrat social (Of the Social Contract, 1762). The Archbishop of Paris, the Sorbonne and the high court in Paris likewise condemned Entile to be burned. Rousseau fled to asylum in Prussian territory.