The System of Nature or, the Laws of the Moral and Physical World (Système de la Nature ou Des Loix du Monde Physique et du Monde Moral) is a work of philosophy by Paul-Henri Thiry, Baron d'Holbach (1723–1789).

It is originally published under the name of Jean-Baptiste de Mirabaud, a deceased member of the French Academy of Science.

D'Holbach writes and publishes this book—possibly with the assistance of Denis Diderot but with the support of Jacques-André Naigeon—anonymously in 1770, describing the universe in terms of the principles of philosophical materialism: The mind is identified with brain, there is no "soul" without a living body, the world is governed by strict deterministic laws, free will is an illusion, there are no final causes, and whatever happens takes place because it inexorably must.

Most notoriously, the work explicitly denies the existence of God, arguing that belief in a higher being is the product of fear, lack of understanding, and anthropomorphism.

Though not a scientist himself, d'Holbach is scientifically literate and he tries to develop his philosophy in accordance with the known facts of nature and the scientific knowledge of the day, citing, for example, the experiments of John Needham as proof that life could develop autonomously without the intervention of a deity.

It makes a critical distinction between mythology as a more or less benign way of bringing law ordered thought on society, nature and their powers to the masses and theology.

Theology which, when it separates from mythology raises the power of nature above nature itself and thus alienates the two (i.e. "nature", all that actually exists, from its power, now personified in a being outside nature), is by contrast a pernicious force in human affairs without parallel.

Its principles are summed up in a more popular form in d'Holbach's Bon Sens, ou idées naturelles opposees aux idées surnaturelles.