Haesu Kim U-jeong (1551, 6th year of King Myeongjo's reign 〜1630, 8th year of King Injo's reign) lived in Dongnae, Busan, from the end of the 16th century to the first half of the 17th century. He joined Joseon's battle against the Japanese invasion, but as Dongnae Castle collapsed, he was taken as a captive to Japan at the age of 42 (25th year of King Seonjo's reign,1592), where he suffered for 8 years. At 49 (32nd year of King Seonjo, 1599), he returned home. He was a scholar. His world of Confucian classics belonged to the emergence period of Korea's Confucian classics in the first half of the 17th century. His understanding of the Doctrine of the Mean was overall based on Juhui's annotations, as well as on the expanded interpretation of the Theory of Cheyong (Substance and Function), a major principle system of Neo-Confucianism. Nonetheless, his understanding of Confucian classics did not simply follow the theory of Chu-tzu but instead involved his own
theories. For instance, of Cheyong, he emphasized Yong, namely, perception.
He also stressed the reality and routineness of Jung (中) of things, namely, Sijung (時中), Haengsa (行事), and Do (道) out of Seong (性), Do (道) and Gyeo (敎), thus differentiating himself from the Neo-Confucianism system of Juhui. Such understanding of Confucian classics did not adhere to
contemplative principle of human nature such as the key Neo-Confucianism theme, Igiron (理氣論), Sadan (四端) and Chiljeongnon (七情論), but instead were somewhat similar to the viewpoint of Hangang Jeong Gu (1543〜1620)
who emphasized realistic human nature cultivation and inner mind training, namely, practical Neo-Confucianism.
Haesu's understanding of Confucian classics in the first half of the 17th century did not yet embrace the stimulating and resisting meaning of the Confucian classic world of Pojeo Jo Ik, Seogye Park Se-dang, and Baekho
Yun Hyu who were active after the second half of the 17th century.
However, one century earlier than these scholars, Haesu's Yeongnam School Confucian classics system, which was based on reality, routineness, and practicality, was yet different from Gyeongseobyeonui (經書辨疑) of Sagye
Kim Jang-saeng (1548〜1631) who was a disciple of Yulgok in the same period, and who tried to further solidify the exclusive position of the Chu-tzu theory in the Confucian classics as a member of the early Giho School.