In 1749 Diderot first showed himself a thinker of original power, in his Letter on the Blind. This work, 'Lettre sur les Avengles à l'Usage de Ceux qui Voient' (Letter on the Blind for the Use of Those who See) opened the eyes of the public to Diderot's peculiar genius, and the eyes of the authorities to the menace in his principles. The result was his imprisonment, and from that the spread of his views. His offense was, that through his ingenious supposition of the mind deprived of its use of one or more of the bodily senses, he had shown the relativity of all man's conceptions, and had thence deduced the relativity, the lack of absoluteness, of all man's ethical standards--thus invalidating the foundations of civil and social order. The broad assertion that Diderot and his philosophic group caused the French Revolution has only this basis, that these men were among the omens of its advance, feeling its stir afar but not recognizing the coming earthquake. Yet it may be conceded that Diderot anticipated things great and strange; for his mind, although neither precise nor capable of sustained and systematic thought, was amazingly original in conception and powerful in grasp. The mist, blank to his brethren, seems to have wreathed itself into wonderful shapes to his eye; he was the seer whose wild enthusiasm caught the oracles from an inner shrine. A predictive power appears in his Letter on the Blind, where he imagines the blind taught to read by touch; and nineteenth-century hypotheses gleam dimly in his random guess at variability in organisms, and at survival of those best adapted to their environment. Diderot's monumental work, 'L'Encyclopédie,' dates from the middle of the century. It was his own vast enlargement of Ephraim Chambers's Cyclopædia of 1727, of which a bookseller had demanded a revision in French. D'Alembert was secured as his colleague, and in 1751 the first volume appeared. The list of contributors includes most of the great contemporary names in French literature. From these, Diderot and D'Alembert gathered the inner group known as the French Encyclopædists, to whose writings has been ascribed a general tendency to destroy religion and to reconstitute society. The authorities interfered repeatedly, with threats and prohibitions of the publication; but the science of government included the science of connivance for an adequate consideration, and the great work went forward. Its danger lurked in its principles; for Diderot dealt but little in the cheap flattery which the modern demagogue addresses to the populace. D'Alembert, wearied by ten years of persecution, retired in 1759, leaving the indefatigable Diderot to struggle alone through seven years, composing and revising hundreds of articles, correcting proofs, supervising the unrivaled illustrations of the mechanic arts, while quieting the opposition of the authorities.