Frederick also was very interested in philosophy. He wrote philosophical works, publishing some of his writings under the title of "The Works of a Sans-Souci Philosopher" Frederick corresponded with key French Enlightenment figures, including Voltaire, who at one point declared Frederick to be a philosopher-king, and the Marquis d'Argens, who he appointed as Royal Chamberlain in 1742 and later as the Director of the Prussian Academy of Arts and Berlin State Opera. Frederick even used the opera to make philosophical points critiquing superstition and the Pietism that still held sway in Prussia. However, his openness to philosophy had its limits. He did not admire the "encyclopédistes' or the French intellectual avant-garde of his time, though he did shelter Rousseau from persecution for a number of years. Once he ascended the Prussian throne however, he found it increasingly difficult to apply the philosophical ideas of his youth to his role as king.