A powerful repercussion of the interest in natural philosophy's mechanical and material explanations of the world was the rise of strictly materialist, anti-religious philosophies during the 1700s. Still politically dangerous and outside the mainstream even for critics of the Catholic Church, atheism entered the public discussion through the salons and through publications. The French philosophe Paul Henri Thiry, Baron d'Holbach (1723-1789) was a noted patron and host of salons, friends with many of the notable intellectuals of the Enlightenment. He was also a prolific author, and became notorious for his The System of Nature; Or, the Laws of the Physical and Moral World (1770). It is sometimes called the "Atheist's Bible" because it was an early and highly provocative statement of the case for materialism and against belief in gods. Following the mechanists' idea that all nature is matter and motion, d'Holbach left no room for spirit of any kind; thus the supernatural cannot exist. Religion is a mythology built from imagination, maintained by leaders.

He starts the treatise with a typical appeal to avoiding bias and ancient authority while exploring true knowledge:

"The source of man's unhappiness is his ignorance of Nature. The pertinacity with which he clings to blind opinions imbibed in his infancy, which interweave themselves with his existence, the consequent prejudice that warps his mind, that prevents its expansion, that renders him the slave of fiction, appears to doom him to continual error. He resembles a child destitute of experience, full of ideal notions: a dangerous leaven mixes itself with all his knowledge: it is of necessity obscure, it is vacillating and false:--He takes the tone of his ideas on the authority of others, who are themselves in error, or else have an interest in deceiving him."

Natural philosophy will lead the way out:

"The most important of our duties, then, is to seek means by which we may destroy delusions that can never do more than mislead us. The remedies for these evils must be sought for in Nature herself; it is only in the abundance of her resources, that we can rationally expect to find antidotes to the mischiefs brought upon us by an ill directed, by an overpowering enthusiasm. It is time these remedies were sought; it is time to look the evil boldly in the face, to examine its foundations, to scrutinize its superstructure: reason, with its faithful guide experience, must attack in their entrenchments those prejudices, to which the human race has but too long been the victim. For this purpose reason must be restored to its proper rank,--it must be rescued from the evil company with which it is associated. It has been too long degraded --too long neglected--cowardice has rendered it subservient to delirium, the slave to falsehood. It must no longer be held down by the massive claims of ignorant prejudice.

Truth is invariable--it is requisite to man--it can never harm him--his very necessities, sooner or later, make him sensible of this; oblige him to acknowledge it. Let us then discover it to mortals--let us exhibit its charms--let us shed it effulgence over the darkened road; it is the only mode by which man can become disgusted with that disgraceful superstition which leads him into error, and which but too often usurps his homage by treacherously covering itself with the mask of truth--its lustre can wound none but those enemies to the human race whose power is bottomed solely on the ignorance, on the darkness in which they have in almost every claimed contrived to involve the mind of man."

It was bold writing, attractive in its appeal to truth and the progress of knowledge. For the skeptic, natural philosophy was the truer path:

"Therefore, instead of seeking out of the world he inhabits for beings who can procure him a happiness denied to him by Nature, let him study this Nature, learn her laws, contemplate her energies, observe the immutable rules by which she acts.--Let him apply these discoveries to his own felicity, and submit in silence to her precepts, which nothing can alter.--Let him cheerfully consent to be ignorant of causes hid from him under the most impenetrable veil.--Let him yield to the decrees of a universal power, which can never be brought within his comprehension, nor ever emancipate him from those laws imposed on him by his essence."

As d'Holbach wrote of the laws of nature, "Man has always deceived himself when he abandoned experience to follow imaginary systems." Although it may be a "poetry" of "beautiful allegories," religion is ultimately both political and a fictional personification of nature. Mythology might be more or less harmless, but theology invented nonexistent powers, untrue to nature. Obviously controversial, d'Holbach's book was nevertheless printed widely, translated, and reprinted frequently.