Official historians of the Zhou Dynasty were aware of lessons concerning political gain and loss, lamented the abandonment of codes governing human relations, bemoaned the harsh enforcing of laws and government, and sang or chanted to express their feelings and emotions, in order to admonish the monarch. They knew well the vicissitudes of worldly affairs and were nostalgic for bygone customs. “A variation in poetic style” is expressive of poets' inner feelings and, for that matter, an articulation of common folk's natural inclination, but such expression should be kept within the confines of propriety and justice, a good heritage left by the late kings.

(Introductions to Mao’s Version of The Book of Songs)

Not to express feelings and emotions naturally does not conform to propriety and righteousness, so there should be joy and sorrow in poems. Feelings and emotions expressed not necessarily conform to propriety and righteousness, and thus sorrow and joy in poems should be kept within the confines of propriety and righteousness. (Liu Xizai: Overview of Literary Theories)