《汉书·志·律历志上》
三代各占据天、地、人三统中的一统，明了天、地、人三统常重合，所以轮流为岁首。升降三统的岁首，是周旋于五行之道啊。
三代各据一统，明三统常合，而迭为首，登降三统之首，周还五行之道也。
Each of the three dynasties occupied one of the three unities of heaven, earth, and man, and understood that the three unities of heaven, earth, and man often coincided, so they took turns as the beginning of the year. The rise and fall of the three unities of the year is a cycle of the Five Elements.