lè ér bù yín, āi ér bù shāng 乐而不淫，哀而不伤
Express Enjoyment Without Indulgence and Express Grief Without Excessive Distress
This is what Confucius said of the description of love between young men and women in the poem entitled “Guan Ju” in “Ballads of Zhounan,” The Book of Songs. Later Confucian scholars regarded this as a basic requirement for poems and other literary works to advocate impartiality, peace of mind, and harmony between emotion and reason, making it a criterion for evaluating literary works. Its connotation is in accord with zhongyong (the golden mean) of Confucianism. In the more recent history, the connotation of the term has been continuously renewed to keep pace with the times.
The poem “Guan Ju” expresses enjoyment without indulgence and grief without excessive distress. (The Analects)
Ballads from the states express passion of love without indulgence. Minor court hymns make complaints and criticisms without inciting trouble. (Sima Qian: Records of the Historian)