What is the story of the first year of Baoyuan, Emperor Wu of the Han Dynasty?

In Zi Tong Zhi Jian (volume 17) in the first year of Baoyuan, Emperor of the Han and Yuan Dynasties, it was said: "We should keep what we have learned, take the rule of the world as the principle, and save the world as the principle, but we should change it." To sum up, if we inherit a well-managed country, the monarch only needs to digest and learn from the previous way of governing the country. If we inherit and rule a country with poor people, the monarch must change his course. Here, the author's point of view is that the monarch must govern the country according to time and situation, and treat it simply dialectically.