Wáng Ānshí zài 《 Tí Zhāng sīyè shī 》 lǐ shuō, Zhāng Jí de shī kànqǐlái hěn píngcháng, qíshí xiě de fēicháng xīnkǔ.
In this poem Wang Anshi says that Zhang Ji's poems look plain but were in fact very hard to write.
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culture
The whole poem: 苏州司业诗名老,乐府皆言妙入神。看似寻常最奇崛,成如容易却艰辛。 'The Suzhou director has long been famous as a poet; all say his yuefu are wonderful. What looks ordinary is the most striking; what seems easily done was hard won.'
history
张籍 (c. 766–830), a friend of 韩愈, held the post of 司业, deputy head of the Imperial Academy, 国子监, so he was known as Director Zhang. He wrote plain-spoken yuefu about the lives of ordinary people.