Diderot's Materialism: The Rejection of the Soul in Enlightenment Thought

Denis Diderot was the most thoroughgoing materialist of the major French philosophers of the Enlightenment. His philosophical dialogues, especially the Entretien entre d’Alembert et Diderot and Le Rêve de d’Alembert, presented a comprehensive materialist account of the natural world and of human nature. This article examines the central arguments of Diderot’s materialism and their significance for the development of modern thought.

The Central Argument

Diderot’s materialism is based on the principle that the natural world, including human beings, can be explained in terms of matter in motion, without any appeal to immaterial substances, souls, or divine intervention. The universe, in Diderot’s view, is a continuous system of matter in motion, governed by natural laws that can be investigated by the methods of natural science.

This position was even more radical than the deism of Voltaire or the natural religion of the English tradition, and the broader account of Diderot’s place in the French Enlightenment is set out in Diderot. Diderot did not believe in a creator God, in the immortality of the soul, or in the freedom of the will. He believed that the world, including the human world, was a vast system of natural causes, and that the proper role of philosophy was to investigate this system with the same rigor that natural science applied to the physical world.

The Theory of Living Matter

One of the most original features of Diderot’s materialism is his theory of living matter. In the Rêve de d’Alembert, Diderot develops the idea that matter itself has a kind of sensitivity or irritability, and that life is a property of matter in certain complex organizations. The distinction between living and non-living matter, on this view, is not a difference in kind but a difference in the complexity of organization.

This idea anticipates some of the developments of modern biology, and it represents an important contribution to the philosophical foundations of the life sciences. The idea that life is a natural property of matter, rather than the result of some special vital principle, is a basic assumption of contemporary biology, and it is an important part of the modern naturalistic worldview.

The Theory of the Mind

Diderot’s theory of the mind is closely connected to his theory of living matter. The mind, for Diderot, is not an immaterial substance; it is a property of the organized body. The Entretien entre d’Alembert et Diderot develops the idea that the mind is a kind of “brain event,” produced by the operations of the bodily organism.

The theory of the mind has important implications for the traditional problems of philosophy. The immortality of the soul, for example, is rejected on the grounds that the mind is not a separable substance that could survive the death of the body. The freedom of the will is rejected on the grounds that all human actions are determined by the natural causes that operate on the bodily organism.

These conclusions were controversial in Diderot’s time, and they remain controversial today. But the basic position — that the mind is a natural property of the body — has become a basic assumption of contemporary philosophy of mind, and it is an important part of the modern naturalistic worldview.

The Theory of Knowledge

Diderot’s theory of knowledge is grounded in his materialism. The mind, for Diderot, is essentially a system for the processing of information about the world, and the proper method for the acquisition of knowledge is the empirical method of natural science.

The Letter on the Blind (1749) develops an important argument for the empiricist theory of knowledge. Diderot argues that a blind person, having no experience of sight, cannot have the idea of color that is presupposed by the proofs for God’s existence from the design of the visible world. The argument is an early statement of the empiricist principle that all knowledge derives from experience, and it was one of the reasons that the work was condemned by the French authorities.

The Reception of Diderot’s Materialism

Diderot’s materialism was controversial in his own time, and it was widely attacked by religious and philosophical authorities. The Letter on the Blind was condemned by the authorities, and Diderot was briefly imprisoned at Vincennes in 1749. The Entretien entre d’Alembert et Diderot and the Rêve de d’Alembert were not published in Diderot’s lifetime, and they circulated in manuscript form among his friends.

Despite the controversy, Diderot’s materialism had an important influence on the subsequent development of modern thought. The German materialists of the late eighteenth and early nineteenth centuries, including Ludwig Büchner, Jakob Moleschott, and Karl Vogt, drew on Diderot’s pioneering work. The contemporary naturalism of the modern scientific worldview is, a continuation of the project that Diderot began.

The Legacy

Diderot’s materialism remains an important part of the philosophical legacy of the Enlightenment. The basic insight of Diderot — that the natural world, including human beings, can be explained in terms of matter in motion — has been an important feature of the modern scientific worldview, and it has been an important source of the contemporary naturalist tradition.