For his "Letter on the Blind," Diderot was imprisoned at Vincennes. The real cause of this punishment is said to have been a slight allusion in the "Letter" to the mistress of a minister of state. But this may not have been the only cause. There occurred about this time one of those temporary seasons of severity which are necessary under all governments to meet occasional outbursts of crime, but to which weak and corrupt governments are liable with capricious frequency. Diderot sturdily denied the authorship of the "Letter," lying as thoroughly as he had done in that piece of writing itself, when he invented the name of Inchlif and forged the ideas of Saunderson. This time there was more excuse for his untruth; for the disclosure of his printer's name might have sent that unfortunate man to prison or to the galleys. The imprisonment of Diderot himself, at first severe, was soon lightened at the instance of Voltaire's mistress, Madame du Châtelet. Diderot was allowed to see his friends, and even to wander about the park of Vincennes on parole. After three months of captivity he was released by the influence of the booksellers interested in the "Encyclopaedia." [Footnote: Morley, _Diderot_, i. 105.] The first volume of that great work was in preparation. Diderot, whose untiring energy was unequal to the task of editing the whole, and who was, moreover, insufficiently trained for the work in some branches, and notably in mathematics, gathered about him a band of workers which increased as time went on, until it included a great number of remarkable men. First in importance to the enterprise, acting with Diderot on equal terms, was D'Alembert, an almost typical example of the gentle scholar, who refused one brilliant position after another to devote himself to mathematics and to literature. Next, perhaps, should be mentioned the Chevalier de Jaucourt, a man of encyclopaedic learning, who helped in the preparation of the book with patient enthusiasm, reading, dictating, and working with three or four secretaries for thirteen or fourteen hours a day. Montesquieu, whose end was approaching, left behind him an unfinished article on Taste. Voltaire not only sent in contributions of his own, but constantly gave encouragement and advice, as became the recognized head of the Philosophic school. Rousseau, whose literary reputation had recently been made by his "Discourses," contributed articles on music for a time; but subsequently chose to quarrel with the Encyclopaedists, whose minds worked very differently from his. Turgot wrote several papers on economic subjects, and in the latter part of the work, Haller, the physiologist, and Condorcet were engaged.