T`oegye [Yi Hwang, 1501-1570) was a great Korean Neo-Confucianist representative of the 16th century. Although his Neo-Confucianism belongs to the School of Chu Hsi, his metaphysics is rated as more profound in theory and snore systematic than that of the Chinese philosopher. T`oegye`s ethical theory has its basis on his profound metaphysics. I will examine his ethical theory and his metaphysical theory as the foundation of the former in this article. T`oegye recognized as proper the traditional Confucian ethical system which is based on the five moral rules governing the human relations between father and son, king and subject, husband and wife, brothers young and older, and friends. An ethical act is one that observes the five cardinal principles of morality within their framework. The foremost aim of his ethics was to pursue a rightful order among the five human relations. For that purpose he also recognized the five norms or virtues-Love(親), Righteousness(義), Distinction(別), Order(序) and Fidelity(信). The virtues of Benevolence(仁), Righteousness(義), Civility(體), Wisdom(智), and Fidelity(信), are characteristics of human nature, being innate in it. They constitute the ability or function of human nature which enables man to display the norms such as Love, Righteousness, Distinction, Order, and Fidelity. The reason why these characteristics of human nature can be regarded as capable of transforming themselves into norms of conduct can be founded in the view that the rule of $quot;ought to$quot; coincides with the f principle of necessity, in other words, that there is a coincidence between Sollen and Sein. As T`oegye believed that Sollen coincides with Sein, he not only proposed such a theory of norms but went so far as to say that man`s ethical conduct is in coincidence with the order of nature and furthermore helps, the creation of nature. According to T`oegye, the grounds for seeing the rule of $quot;ought to$quot; and the principle of necessity as coinciding with each other are provided by the notion that ethical conduct is a realization of human nature. In other words, he believed that such ethical conducts as conforming to Love, Righteousness, and Distinction are spontaneous manifestations of human nature corresponding to Benevolence. Righteousness, and Civility respectively. This belie is the basis of seeing the coincidence. Mencius` $quot;Theory of Four Clues (Principles)$quot; can also be regarded as showing, an example of the spontaneous manifestation of human nature. For instance, Mencius observed that $quot;seeing an indiscreet child crawling toward a well. anybody automatically feels pity for it and grabs it.$quot; This kind of $quot;Theory of Spontaneous Outflow of Compassion$quot; furnishes another ground for T`oegye`s thought of coincidence between Sollen and Sein. T`oegye never rejected, however, that there is evil in human conduct. Impulse or desire which leads one to bad conduct has its origin in the body or temperament, according to him. He regarded this as being not original to human nature. He asserted that such characteristics as can be decribed as $quot;Benevolence, Righteousness, Civility, and Wisdom$quot; are original to human nature, or, in other words, they are man`s inborn nature. On this basis, he believed that this original human nature is capable of over-coming impulses generated by the non-original human nature. This assertion means that ethical conduct is inevitably possible, again conforming to his view of coincidence between the rule of $quot;ought to$quot; and the principle of necessity. T`oegye`s ethical theory, as we have seen above, is basically related to his cosmology. Now we shall examine concretely the relationship between his consmology and his theory of coincidence between Sollen and Sein which he applied to all matters All things, according to T`oegye, have their own original nature, which is the same as their essence. It is a proper rule that things act in accord