The Sophists introduced a profusion of knowledge among the people. They made investigations in language, logic, and the theory of cognition. They taught literature, history, grammar, the principles of the dialectic, the eristic, and rhetoric――all subjects concerned with the art of human expression. They studied and taught the special subjects concerned with human relations, like ethics, the theory of knowledge, psychology, and politics. Anything that had a place in Greek culture was systematically and skillfully presented by such men as Protagoras, Gorgias, Hippias, and Prodicus, who were men of encyclopædic erudition. The Sophist took the education of the Greek child at the age of sixteen, after he had received his elementary training, first at home and then at the hands of the teacher at school. The Greek boy’s education was naturally divided into two parts: gymnastics for the body and music for the soul. Under music was included geometry, performance on the lyre, pronunciation, the chorus and poetry, astronomy, physics, and geography. At the age of sixteen he got his instruction by meeting public men, such as the Sophists, in the street, in the Agora, and other public places. It was at this period of his life that the Sophist took his education into those higher branches which were necessary for his success in politics, society, and law. Thus the instruction of the Sophist was usually for a specific purpose, and thus rhetoric, dialectic, and the mental sciences were in great demand.