Though the mathematical sciences remained the objects of his special study, D'Alembert was as free as the other great men of the encyclopaedic school from the narrowness of the pure specialist. He naturally reminds us of the remarkable saying imputed to Leibnitz, that he only attributed importance to science, because it enabled him to speak with authority in philosophy and religion. His correspondence with Voltaire, extending over the third quarter of the century, is the most instructive record that we possess of the many-sided doings of that busy time. His series of _eloges_ on the academicians who died between 1700 and 1772 is one of the most interesting works in the department of literary history. He paid the keenest attention to the great and difficult art of writing. Translations from Tacitus, Bacon, and Addison, show his industry in a useful practice. A long collection of synonyms bears witness to his fine discrimination in the use of words. And the clearness, precision, and reserved energy of his own prose mark the success of the pains that he took with style. He knew the secret. Have lofty sentiments, he said, and your manner of writing will be firm and noble.[100] Yet he did not ignore the other side and half of the truth, which is expressed in the saying of another important writer of that day--By taking trouble to speak with precision, one gains the habit of thinking rightly (_Condillac_). Like so many others to whom literature owes much, D'Alembert was all his life fighting against bad health. Like Voltaire and Rousseau, he was born dying, and he remained delicate and valetudinarian to the end. He had the mental infirmities belonging to his temperament. He was restless, impatient, mobile, susceptible of irritation. When the young Mademoiselle Phlipon, in after years famous as wife of the virtuous Roland, was taken to a sitting of the Academy, she was curious to see the author of the Preliminary Discourse to the Encyclopaedia, but his small face and sharp thin voice made her reflect with some disappointment, that the writings of a philosopher are better to know than his mask.[101] In everything except zeal for light and emancipation, D'Alembert was the opposite of Diderot. Where Diderot was exuberant, prodigal, and disordered, D'Alembert was a precisian. Difference of temperament, however, did not prevent their friendship from being for many years cordial and intimate. When the Encyclopaedia was planned, it was to D'Alembert, as we have said, that Diderot turned for aid in the mathematical sciences, where his own knowledge was not sufficiently full nor well grounded. They were in strong and singular agreement in their idea of the proper place and function of the man of letters. One of the most striking facts about their alliance, and one of the most important facts in the history of the Encyclopaedia, is that henceforth the profession of letters became at once definite and independent.