Of the gross defects in the execution of the Encyclopaedia nobody was so sensible as Diderot himself.  He drew up a truly formidable list of the departments where the work was badly done.[171] But when the blunders and omissions in each subject were all counted, the value of the vast grouping of the subjects was hardly diminished.  The union of all these secular acquisitions in a single colossal work invested them with something imposing.  Secular knowledge was made to present a massive and sumptuous front.  It was pictured before the curious eyes of that generation as a great city of glittering palaces and stately mansions; or else as an immense landscape, with mountains, plains, rocks, waters, forests, animals, and a thousand objects, glorious and beautiful in the sunlight.  Theology became visibly a shrivelled thing.  Men grew to be conscious of the vastness of the universe.  At the same time and by the same process the Encyclopaedia gave them a key to the plan, a guiding thread in the immense labyrinth.  The genealogical tree, or classification of arts and sciences, which with a few modifications was borrowed from Bacon and appeared at the end of the Prospectus, is seen to be faulty and inadequate.  It distributes the various branches of knowledge with reference to faculties of the human understanding, instead of grouping them according to their objective relations to one another.  This led to many awkward results, as when the art of printing is placed by the side of orthography as a subdivision of Logic, to which also is given the art of heraldry or emblazonment.  There is awkwardness too in dividing architecture into three heads, and then placing civil architecture under national jurisprudence, and naval architecture under social jurisprudence, while under fine arts no kind of architecture has any place.  But when we have multiplied these objections to the uttermost, the effect of the magnificence and vastness of the scheme remains exactly what it was.

Even more important than the exposition of human knowledge was the exposition of the degrees by which it had been slowly reared.  The Preliminary Discourse to the Encyclopaedia, of which by far the greater and more valuable portion was written by D’Alembert, contains a fine survey of the progress of science, thought, and letters since the revival of learning.  It is a generous canonisation of the great heroes of secular knowledge.  It is rapid, but the contributions of Bacon, Descartes, Newton, Locke, Leibnitz are thrown into a series that penetrates the reader’s mind with the idea of ordered growth and measured progress.  This excited a vivid hopefulness of interest, which insensibly but most effectually pressed the sterile propositions of dogmatic theology into a dim and squalid background.  Nor was this all.  The Preliminary Discourse and the host of articles marshalled behind it, showed that the triumphs of knowledge and true opinion had all been gained on two conditions.