Shortly before this, the minister of agriculture Yan Yi had been put to death. Yan Yi had originally been a village head in Ji'nan. Because of his honesty and integrity, he was gradually promoted in office until he became one of the highest ministers. [...]
... Yan Yi was on bad terms with Zhang Tang. When someone bought an accusation against Yan Yi for some other affair, the matter was referred to Zhang Tang for investigation. Earlier Yan Yi had been talking with one of his guests, when the guest remarked that some new law that had just been put into affect did not seem very practical. Yan Yi had made no answer, but given only a subtle wry twist of his lips. Zhang Tang sent a memorial to the throne relating this affair and stating "Yang Yi, one of the nine highest ministers, having seen that the law was impractical, did not state his opinion to the emperor, but nevertheless disapproved of it in his heart. For this offence he is deserving of death!". From this time on, the crime known as "disapproval of heart" became a part of the law, and most of the high ministers and officials resorted to gross flattery in order to stay in the good graces of the emperor.
-- Sima Qian, Records of the Grand Historian; Han Dynasty II (revised edition). pp 75-76 Translated by Burton Watson. Columbia University Press 1993