First published over the course of more than twenty years (1751-1777), the 32 volumes of the Encyclop?die include 21 volumes of text with more than 70,000 articles on subjects ranging from asparagus to zodiac. The remaining 11 volumes contain beautifully engraved plates illustrating many of the articles. The Encyclop?die was the major achievement of the French Enlightenment whose aim, in Diderot's words, was to "change the common way of thinking" through the expansion of knowledge and the development of critical modes of thought.

The Encyclop?die was a collaborative project, the work of a "society of men of letters," as its title page declared. By the time the last volume was published, more than 140 people had contributed articles to its pages.

The preface of the 1751 Encyclopedia of Diderot and d'Alembert states :
The general system of the sciences and the arts is a sort of labyrinth, a tortuous road which the intellect enters without quite knowing what direction to take. Impelled, first of all, by its needs and by those of the body to which it is united, the intelligence studies the first objects that present themselves to it. It delves as far as it can into the knowledge of these objects, soon meets difficulties that obstruct it, and whether through hope or even through despair of surmounting them, plunges on to a new route . . .

Denis Diderot and Jean d'Alembert were neither paleontologists nor biologists. They did not make any earth-shattering discoveries about the history of life. But it can be argued that they fostered an environment in which earth-shattering discoveries could be made. What Diderot and d'Alembert did was work with an army of experts and writers to publish an irreverant set of encyclopedias about science, art and trades in pre-Revolutionary France.

The encyclopedia started as a simple, shrewd venture: a translation of a successful English work by Ephraim Chambers. Diderot and d'Alembert initially joined the project as subordinates, then advanced to editors. Once in control, they expanded the project's scope to try to encompass all knowledge. No one could really succeed at that goal, but they probably came close.

Although their works had plenty of errors, the encyclopedists stressed observation over faith in traditional beliefs, and admitted quite openly that their new way of thinking was not new at all ? they traced it back to Francis Bacon, Lord Chancellor of England. Despite their open acknowledgment of Bacon's influence, the encyclopedists soon came under attack, ostensibly for plagiarizing Bacon's work. The accusation came from a Jesuit journal. (The Jesuits' real concerns about the encyclopedias probably had little to do with Bacon and much to do with the fact that the fathers of the Society of Jesus hadn't been asked to author any theological articles in the reference books.) Not only did the encyclpedists overturn the notion that boy babies first emit the sound "A" (the root of masculinus) and girl babies first emit the sound "E" (the root of feminina), they also devoted as much attention to the manufacture of stockings as to the human soul. Needless to say, their books regularly came under fire. Diderot had to spend some time in jail in 1749, two years before the first edition's release.

Though they both ranked among the sharpest intellects of the day, the encyclopedists' temperaments weren't perfectly matched. D'Alembert was the son of a well-born soldier and a nun-turned-socialite, who promptly abandoned her inconvenient son. Raised by a humble nurse and supported financially by his father, he soon showed remarkable mathematical talent ? and a sunny temperament. The stunning conversationalist Diderot was a bit of a bohemian, and the mathematician d'Alembert found some of his colleague's friends, such as Rousseau and Buffon, a little unpalatable. Worn down by the constant controversy surrounding the Encyclopedia, the thin-skinned D'Alembert left the effort after the French government banned the work in the late 1750s. His departure caused a lasting break with Diderot.

Diderot and d'Alembert lived and worked in a time when printed material was anything but fixed. Rather than faithfully adhering to the original text, publishers often changed the content of the encyclopedias depending on where they were published and for what audience. Diderot later felt disgust at some of the finished volumes.

Some have argued that in ushering in an era of free thought, the encyclopedists also ushered in the French Revolution, complete with its Reign of Terror and guillotine. Others have countered that they fostered freedom of the press and everyone's right to vote. Either way, their emphasis on facts advanced the cause of science.