One subject, three tastes - Ikkasanshi

…The Spring and Autumn Annals has three schools of thought: the Gongyang School, the Guliang School, and the Zuo School. According to He Xiu, who appeared at the end of the Eastern Han Dynasty and compiled the Han Dynasty Gongyang Studies, “Zhou is the new one, Song is the old one, and the Spring and Autumn Annals are the new king. This is one department and three purposes.” In other words, when the Mandate of Heaven is changed and a new king is appointed, the descendants of Song (Yin) and Zhou are given great powers and become the three kings together with the new king. This is called “preserving the three unities,” and it describes the principles of how to treat past dynasties in a revolution. …

*Some of the terminology explanations that refer to "one subject, three principles" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…《春秋》には《公羊伝（くようでん）》《穀梁伝》《左氏伝》の3学派があるが，このうち公羊学派のとなえた説で，後漢末に現れて漢代公羊学を集大成した何休によると，〈周を新とし，宋を故とし，春秋を以て新王に当つ。これ一科三旨なり〉すなわち天命が改まって新たに王となる場合，宋(殷)と周の子孫を大国に封じて新王とともに三王とする。これは〈三統を存す〉といわれ，革命において過去の王朝をいかに処遇するかの原則を述べたものである。…

※「一科三旨」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報