Dasan is a great scholar and philosopher in the late period of Choson Dynasty (朝鮮王朝). Two invasions from foreign countries ended up with the impoverishment and incapacity of the country, which made the Choson citizens judge that Sungri-hahk (性理學), or Neo-confucianism could not relieve them from poverty nor could realize the public welfare. This drift of opinion produced Sil-hahk (實學), or Realism, which was compiled by Dasan. The current academic features of Sil-hahk were the same in goal as Kyongseh Ahnmin (經世安民), but different in its methodology. Thus it could reasonably be confined to Kyongseh Sil-hahk (經世實學). In addition, there prevailed a school of thought, asserting that the study should start from radical criticism and reflection on Sungri-hahk, and turn towards pre-Chin Kyongseh-hahk (先秦經世學) which taught about practice in life. Dasan first tried to show his idea through political approaches to realize the concept, but his idea was dampened by the governmental oppression scandal during the introduction of foreign religion, but he saw prevalence in his philosophy during his stay at home for 18 years. For this reason, Dasan devoted his whole life to illumination of pre-Chin Kyongseh-hahk, leaving lots of works. However, his interpretation of Kyongjeon (經典), or the old Confucian scriptures in ground of Sil-hahk proposed new theories. The summaries are as follows: First, Dasan understood Cheon (天), the base of his interpretation on the Confucian scriptures, as God, existing inside of a man from the time he is born. On the other hand, he took Cheon as the natural basic principle in universal changes, and applied it to the logical basis in order to retort against Ri-Gi (理氣) Theory. Second, Dasan insisted that Myeong (=命, vitality) which a man was endowed with from God was the innate original nature, and it is nothing but preference (性嗜好, likes and dislikes). That is, human nature is good- directional, which means that it is fond of good, and is able to practice good; it has both physical preference (bodily virtue, 形軀的嗜好性) and mental preference (spiritual virtue, 靈知的嗜好性) inside as the same time. Third, according to Dasan, human inborn preference, called Myeong (vitality), is a priori Seong (本性) before not being used or activated by itself. Even though a man succeeded to a priori nature, once he is born, he is granted with the right of choice in mind (heart), and becomes a host in life. This is the existential human mind, the host who employs Seong, and the operator. The capability through which the mind can independently choose good and evil supported by the subjective ability of choice is called ‘autonomy (sovereign rights)’ endowed from Cheon (heaven). Fourth, Dasan explained that the mind is sometimes likely to follow ‘big body (大體, conscience)’ or sometimes follow ‘small body (小體, desire)’. The former is called Doshim (道心), the latter is called Inshim(人心). In fact, according to Dasan, Doshim and Inshim look as if ‘their inner state hesitates to make daily decisions, causes complications, and accuses each other inside.’ But ‘God always exists inside the human mind, and keeps an eye on conscience’ to urge a man to make an independent decision on a moral practice. Fifth, Dasan did not itemize ‘Kyokmul (格物)’ and ‘Chichi (致知)’ into two of eight items in the Great Learning (大學八條目), but instead he classified two of them into Sah (事) and Mul (物), and named them Kyokchi Youkjo (格致六條, Kyokchi and six items). It is outstanding that Dasan demonstrated his thought in the center of the time passage with a unique angle of Sahmul (事物, events and matters), first by ‘the principle of the end and beginning (終始原理)’, which means ‘The end in everything is the beginning’; second by ‘the principle of the cause and effect, which means’ all things existing in space have the cause and effect. From this, Eui (意, Consciousness), Shim (心, mind or heart), Shin (身, body), Gah (家, household), Gook (國, nation), Cheonha (天下, world) can be divided into six matters, or objective items (六物); Seong (誠, devoting), Jeong (正, justifying), Soo (修, training), Jeh (齊, governing), Chi (治, ruling), Pyeong (平, administrating) can be divided into six events (六事) which should be employed by the subjective mind. This methodology was deduced from the logic of knowledge-conduct, beginning-end, and cause-effect. In addition, Dasan showed his original interpretation of the Confucian scriptures by emphasizing that Sooshin (修身, self-training) is a universal principle which is common in public, and it should be regarded as the same fundamental canon. Sixth, Dasan's theoretical relation between knowledge and practice can be called as ‘the theory of Knowledge First, Conduct Second’, but he insisted ‘Unity of Knowledge and Conduct theory’, adding that the end of knowing is action, which means that knowledge and action should be combined, resulting in union. From the reason above, his theory can be found to include the trend of Shil-hahk (realism) which valued moral practice above knowing because he saw practice as the moral conduct. Thus, he understood all of moral principals [Ihn (仁, charity), Eui (義, righteousness), Ye (禮, courtesy), Ji (智, wisdom), Hyo (孝, piety), Jeh (悌, brotherhood), Jah (慈, affection), etc] as dyadic relationship between men. Furthermore, he persisted that ‘Ihn of Confucius (孔子的仁, charity)’ is a practical proposition between men, ‘not a reason of charity’.(不是愛之理) For this reason, he is considered as a realist who valued practice above principle. Last, Dasan judged that Sungri-hahk could not relieve people from impoverishment, and severely criticized Sungri-hahk and the classical examination system in his ‘the Five Teaching Theory’; instead, he strongly advocate Shil-hahk to realize the society which Joo-gong (周公) and Confucius (孔子) took as an ideal model, and devoted his whole life to the new interpretation of the Confucian scriptures, leaving a lot of works. However, the fundamental proposition of pre-Chin Seong-hahk (先秦聖學) is not in the scholiastic interpretation of moral concepts, but in the right enlightenment of ‘the core point of Cheonmyeong (天命) thought’, as mentioned in the passage: “ ‘Be sure to catch the core reason of heaven because it is in your body[Consciousness of Time (時間意識) 〈＝consciousness of morality (道德意識)＋consciousness of history (歷史意識)〉]. (＝天之曆數在爾躬, 允執厥中)’ But, it is his limitation that he did not show the core reason, which is also the limitation of post-Han Confucianism (漢儒以來).” In summary, Dasan's Shil-hahk might be highly estimated in a sense that he introduced a new academic feature which is different from old Confucian trends, and left lots of works. On the contrary, the scholiastic interpretation of the old scriptures, which is regarded as the limitation of post-Chin Hahn Confucianism as well as Dasan's limitation, will not be able to give a solution to the core subject argument in the original Confucianism and the new age. To overcome this obstacle, it is suggested that we catch the fundamental subject of pre-Chin Confucianism (先秦儒學, Original Confucian -ism), interpret it in the original order, and enlighten the ideal Confucianism so that we might able to help the upcoming generation. This is the writer's the research topic as well as the direction of the current academic approach.