He left France shortly after and travelled in several countries of Europe, studying men and nations, storing his mind with information; he was comparatively free from prejudice, and believed that useful examples and precepts might be obtained even from "the great Turk." "As surely," did he write some years later to his brother Robert, "in the great Turk, though we have nothing to do with him, yet his discipline in war matters is ... worthy to be known and learned. Nay even the kingdom of China which is almost as far as the Antipodes from us, their good laws and customs are to be learned."[170] In such a disposition of mind he visited successively Germany, Austria, Hungary, and Italy. The most interesting incident of his journey was the acquaintance he made with a Frenchman, the political thinker Hubert Languet, from whom Milton, a long time before Rousseau, probably derived his ideas of the social contract "foedus," says Languet, "inter [principem] and populum," and his theories on the right of insurrection.[171] A most tender friendship was formed between the revolutionary writer and the aristocratic Sidney. They began a correspondence which did not cease till the former's death in 1581. Languet had great influence over his young friend, and was constantly giving him most manly advice and that best suited to strengthen his character, warning him especially in very wise fashion against a melancholy unsuitable to his age, which in the grave Huguenot's opinion was only a useless impedimentum in life. "I readily allow," wrote Sidney, in answer to his friend's remonstrances, "that I am often more serious than either my age or my pursuits demand."[172] That this tendency to pensiveness left its trace on his features may be seen in most of his portraits, among others in that by Isaac Oliver, of which we give a reproduction.