Duke Ai asked which wood should be used for the altar of the spirits of the earth. Zai Yu replied: “The men of Xia used pine; the men of Yin used cypress; the men of Zhou used chestnut, for they said the people should fear.” When Confucius heard of this, he said: “You do not explain away what had already been done; you do not argue about affairs that have already been concluded; you do not condemn what has already happened.”
Confucius is not amused by his disciple Zai Yu’s (untranslatable) pun on the character 栗 (lì) which means both “chestnut” and “tremble”. He probably saw it as unwarranted criticism of the Zhou Dynasty with its implication that it ruled by terror.
There is some speculation that Duke Ai asked this question as an indirect way of persuading Zai Yu to support his plans to launch a crackdown against opponents of his rule since the altar was used for executions as well as ritual ceremonies.
Also known as Zai Wo (宰我) as Ziwo (子我), Zai Yu had a very fractious relationship with Confucius and receives some harsh criticism from him in the Analects. In Chapter 10 of Book 5, the sage famously castigates him when catching him asleep: “Rotten wood cannot be carved; dung walls cannot be troweled. What’s the point of scolding him anymore?”