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In 655 BCE, Jin asked the small state of Yu for passage to attack its neighbor Guo. The minister Gong Zhiqi warned that Yu and Guo were like lips and teeth: when the lips are gone, the teeth grow cold. The ruler of Yu trusted Jin's gifts and kinship, granted the road, and Jin conquered Guo and then took Yu on the way home.
Yan Ying distinguishes harmony (he) from sameness (tong) using soup: a good broth blends contrasting ingredients, each correcting the others. A minister who approves whatever the ruler approves adds water to water and gives the ruler no information. Real harmony means saying 'no' where the ruler says 'yes' so that the decision is corrected.
When urged to close the village schools where people criticized the government, Zichan of Zheng refused. He argued that suppressing criticism is like damming a river: a large breach causes disaster that can't be repaired, while small channels let pressure out and show the ruler what to fix. Public complaint is treated as free diagnostic information, not a threat.
Duke Zhuang of Zheng let his ambitious brother Duan expand his city and gather troops, telling his worried ministers that 'one who does much wrong will bring about his own ruin.' Tolerating a rival can be a deliberate way to let the rival supply the evidence that justifies destroying him.