From Bai Juyi's (
白居易) Mid-Tang letter
《与元九书》 (Letter to Yuan Zhen):
文章合为时而著,歌诗合为事而作, 'Essays should be written for the times, songs and poems should be composed for real events.' This manifesto underpinned his New Yuefu (
新乐府) movement, which used poetry to expose social problems.