During the early Sui Dynasty, Wei Prefecture's Sima Jingsu was a man of exceptional talent, sharp and capable, with a straightforward and incorruptible character that earned him deep respect from his colleagues.
In 604 AD, Emperor Yang of the Sui Dynasty seized the throne by killing his father, Emperor Wen. He ordered his imperial censor, Xue Daoheng, to evaluate all officials across the empire. After his investigation, Xue reported to the throne, saying of Sima Jingsu, "His heart is as firm as iron and stone, growing even more steadfast with age—meaning he is resolute, seasoned, and utterly loyal." The emperor nodded in approval.
However, Emperor Yang soon indulged in wine and women. This allowed the military officer in charge of guarding the capital, General Yuwen Shu of the Left Guard, to seize control of the court. Civil and military officials feared his power, daring to be angry but not to speak out.
Jing Su, the governor of Weizhou, was in charge of the very region that was the hometown of the powerful official Yuwen Shu. Yuwen Shu frequently sent messengers with personal letters to Jing Su, ostensibly to build goodwill but in truth to pressure him into protecting Yuwen Shu's relatives and friends there. Each time, Jing Su returned the letters unopened to the messenger, who dared not report this truth back. Thus, Yuwen Shu remained under the illusion that Jing Su had accepted his favor.
Relatives and friends of the powerful official Yuwen Shu, relying on his influence, bullied the common people and ran amok in the countryside. When the victims brought their grievances to the magistrate Jing Su, he showed no favor to Yuwen Shu. Instead, he immediately sent officers to arrest the offenders and punished them according to the law.
When Yuwen Shu learned of this, he harbored a deep grudge and repeatedly spoke ill of Jing Su before Emperor Yang. The emperor had originally intended to promote Jing Su to the position of Grand Administrator, but seeing Yuwen Shu's opposition, he was forced to abandon the plan.
Later, Emperor Yang indulged in pleasure-seeking, neglecting state affairs until the dynasty teetered on the brink of collapse. Seeing that the Sui Dynasty's fate was sealed, Sima Jingsu retired to his hometown.
Later, people used the idiom "iron and stone heart" to describe someone with a very hard heart, unmoved by emotion.
Source: *Book of Sui*, Biography of Jing Su
Meaning of the Idiom: Later, the Chinese idiom "铁石心肠" came to describe how someone with a very hard heart is unmoved by emotion.