When peace was declared in 1763, he was not yet thirty. With his eminent gifts of mind and person, a brilliant career in society lay open to him, but he aimed to be something more than a mere man of fashion. His first literary productions were biographical sketches of two of his brother officers, MM. de Closen and de. Belsunce, which appeared in the _Mercure_, in 1765. He wrote a lively and graceful little essay on the "Union of Poetry and Music,"--the same subject which Marmontel afterward treated in his poem of _Polymnie_. The great quarrel between the schools of Gluck and Piccini did not break out until ten years later; but mutterings of the coming tempest were heard already. Italian music had its enthusiastic admirers and its implacable foes, and in the midst of their disputes Monsigny and Grétry had just given to France a lyric school of her own by creating the comic opera. M. de Chastellux, like everybody else in those days, was passionately fond of the theatre, and he espoused the cause of Italian music with the ardor that characterized everything he did. About the same time he fell into the society of the Encyclopoedists, and allied himself with Helvétius, d'Alembert, Turgot, and the rest of the philosophical party, who received the illustrious recruit with open arms. About the same time that M. de Chastellux left the army, and made his debut in civil life, the Scottish historian and philosopher, David Hume, arrived in Paris, with the British ambassador, Lord Hertford. He became the lion of the day. Courtiers and philosophers fell down and worshipped him; his skeptical opinions were eagerly imbibed, and the three years that he spent in the French capital became, owing to his extraordinary influence, one of the most important epochs in the literary history of the eighteenth century. M. de Chastellux shared in the general enthusiasm; and the "Essays" and "Political Discourses" of Hume, together with the _Essai sur les moeurs et l'esprit des nations_ of Voltaire, which had appeared a few years before, wrought upon his mind a deep and lasting impression. The united influence of these two authors led him to a course of study which resulted in a work upon which his reputation was finally established. This was his celebrated treatise, "On Public Felicity; or, Considerations on the Condition of Man at different Periods of his History," in two volumes. It bears a resemblance to both its parents. It is historical, like the _Essai sur les moeurs_, and dogmatic, like the "Essays" and "Discourses." And that is one of its defects. The "Considerations" on the condition of man at various periods serve by way of introduction to the author's theory of public felicity; but the second part is inferior to the first. The body of the book is sacrificed to the introduction.