In September of the year he turned 40 years of age, he authored "The Essentials of Confucianism," which is rated as a most valuable book showing examples for a good Confucian life. The Yul Gok Chônjip (The Complete Works of Yul Gok) was compiled after his death on the basis of the writings he bequeathed.
Yul-Gok was well known for his development of a school of thought concerning the philosophy of the 12th century Confucian scholar Chu-Hsi. Chu-Hsi established the concepts of "li" (reason or abstract form) and "chi" (matter or vital force). He proposed that these two concepts were responsible for all human characteristics and the operation of the universe. As he defined the concepts, they are very similar to the concepts of body and soul in found in Western philosophy and religion. The "li," however, is not totally synonymous with the idea of an individual represents groups or models for each form of existence. Yul-Gok's school of thought supported the concept that the "chi" was the controlling agent in the universe and that the "li" was a supporting component. Experience, education, and practical intellectual activities were stressed in this school of thought. The other major school of thought, stemming from the philosophy of Chu Hsi ,was fostered by Yi Hwang (Yi ToiGye), who proposed that the "li" controlled the "chi" and stressed the importance of moral character building
Yul-Gok's school of thought was carried over into his personal life. In fact, he took sincerity very seriously. "A sincere man," he felt, "was a man that knew the realism of heaven." He once wrote that a house could not sustain harmony unless every family member was sincere. He felt that, when confronted with misfortune, a man must carry out a deep self-reflection to find and correct his own mistakes. In addition to his commitment to society, Yul-Gok emphasized the value of practical application. The reason for study, he asserted, was to apply the knowledge one gained. As an example of his dedication to this belief, he is said to have manufactured his own hoes and worked at the bellows, which was not usually done by a person of his stature.