Youl-gok's educational philosophy is fundamentally based on the principle that man's original humanity (Ursprünglishe Menschlichkeit) is good. So his aim of education is at restoring this good humanity and being a Sungin(聖人). But humanity is not always good because of his ki(氣). If we modify this ki(氣) and restore the original human nature, all men will become good. Man's original humanity is Sung(誠, sincerity). So sincerity is principle of educational aim. To build up a man of sincerity, sincerity is not only the principle of educational aim but also the principle of educational method, for sincerity is realized by being sincere. If sincerity is realized by being sincere. Sincerity is realized by practice and doing. Practice and doing emphasize the practical things, utility, and experience. "Learning by doing" is his principle of education itself. Sincerity is an ideal of personal building. However, his individual self-realization is connected directly with the social self-realization. Therefore, he puts emphasis on the social participation and social service. Man of Sung(誠, sincerity) is not only good as an individual being but also as a social being.