This paper examines how two modes of understanding Heaven (Tian 天) coexisted in ancient Japan. The notion of Tian was formed in ancient China, therefore the latter part of this paper refers to Chinese thought. People in ancient China elaborated the theory of Tian at least in two ways. Confucianists of the Han dynasty conceptualized it as the agency imposing moral behaviours on people and giving rewards or punishments according to the virtue of governors. In the context of astrology and divination (shushu 術数), Tian is a kind of natural law which can be observed in everything in the world. It is not the question of ethics nor art of government, but rather the mathematical discourse. Yin-yang and Wu-Xing 陰陽五行 is the part of expressions of its...