Focusing on Yu Hyong-Won`s concept of Silli(實理), this dissertaticiri aims to examine his philosophical thoughts generated, justified at least, his study of statecraft which has been characterized by searching for ancient ritual institution(古禮). The Silli was a concept which well-reflected neo-confucians` idea about the principle(理). Neo-confucians, induding Thu Xl, claimed that human could realize the principle given in the human nature through moral hehavior, They criticized buddhists view on the principle as Gongli(空理, empty principle), and called their own view on the principle as Silli(true principle). Because of its trueness, the principle can be transcendentally immanent in human nature, The Silli was also connected to the sincerity(誠). Thu Xl attached importance to the practice based on the ritual(禮), to he ckxser to the sincerity of the nature. This concept of Silli was maintained by philosophers of Chosun dynasty like Yi Hwang and Yi Vi, To highlight moral self-cultivation, they used the concept of Silli in connection with the sincerity as common, despite of fundamental difference between their view on the principle. Yu strongly emphasized the Silk in his philosophical thoughts. Through this concept, He could insist that because the principle was true. it could control the material force(氣), although it couldn`t exist without the material force, He claimed that the standard for human behavior must he drawn from the Silli and moral nature of human by Silli was the standard for behavior, According to Yu`s opinion, the human behavior included not only the personal activity hut also the state undertaking. He insisted that the national system must be based on the way(道) of great kings. For Vu, the way was the standard for the state undertaking In addition, Yu tried to find out a proper standard for the human behavior, and paid attention to the mind(心).He made the moral mind (道心) the standard which the human mind(人心) must follow, because he thought that a wrong behavior would be done when the moral mind wasn`t established properly and consequently the human mind flowed into the desire, If the moral mind should be established properly, the Silli in the human nature would appear as a certain foam of the ritual, according to Yu, Therefore, Yu expressed the ritual as an appeared foam of the Still. To establish the moral mind, a roan should keep the attitude of respect by following the ritual, Yu sought for the standard for the state undertaking as well as personal activities, He thought that the state undertaking had to be based upon the way of great kings, and paid attention to the national institution as the method to cariy out the way. Like other confucian scholars in Chosun dynasty, Yu also regarded Three Dynasties(三代) as a great reign that the way was frilly carried out, But he thought that the way and the method were inseparable, and of course, so the great reign of Three Dynasties and its institutions were, Therefore, he designed large-scale reformations for the Chosun dynasty, standing on the hasis of the ancient ritual institution, and winte 《Ban-gye Su-rok(磻溪隧緣). Radical ideas in 《Ban-gye Su-rok》solidified Yu`s intellectual position as a pioneer of the Silhak(實學).