César Chesnau, a French writer, who distinguished himself as a philosophical grammarian, was born at Marseilles on the 7th of July 1676. His life consisted of a succession of misfortunes; and his merits, considerable as they were, seem to have been entirely overlooked and neglected by his contemporaries. His father died while he was yet an infant; and his mother, by her extravagance, dissipated his patrimony. He was educated in his native town by the Fathers of the Oratory, into whose congregation he entered, but left them at the age of twenty-five, and repaired to Paris, where he married, and was admitted an advocate in 1704. He soon, however, quitted the bar; separated from his wife, to whom he gave up the little he possessed, and went to reside with the President de Maisons, in the capacity of tutor to his son. But his prospects in this quarter were blasted by the death of his patron, by whose family he was not treated with that respect and gratitude which were due to his talents and his services. He was afterwards successively tutor to the son of Law, the famous projector, and of the Marquis de Beaufremont. It was during this last period that he published the results of his grammatical investigations, which were received with great coldness. At a subsequent period he opened an establishment for education in the suburb St Victor, which scarcely afforded him the means of subsistence; and he expired, at length, under the accumulated pressure of years, infirmities, poverty, and neglect, on the 11th of June 1756, at the age of eighty.

Dumarsais possessed no ordinary talents. His researches are distinguished alike by their accuracy, ingenuity, and depth. As a man, he combined the greatest purity of morals and simplicity of character with a rare degree of manly fortitude in the midst of his misfortunes; yet during the greater part of his life he was left to languish in obscurity, and his merits scarcely attracted any notice until nearly half a century after his death. His works on philosophy and general grammar, however, are worthy of attention. Of these, the best is his Treatise on Tropes or Figures. D'Alembert and Voltaire have both paid a just and discriminating tribute to the merits of Dumarsais. An edition of his works was collected by Duchosal and Millon, and published at Paris in 1797, in seven vols. 8vo.

In the year 1804, the French Institute proposed a prize for an Eloge on Dumarsais, which was gained by M. Degrande, whose work was published at Paris, 1805, in 8vo. A previous and well-written Eloge on the same author, by D'Alembert, is to be found in the Mélanges de Littérature, and prefixed to the above-mentioned edition of the Works of Dumarsais. See also Biographie Universelle.