Confucius had a disciple named Sima Niu. One day, Sima Niu asked, "Master, how does one become a noble person?" Confucius replied, "By being free from worry and fear." Puzzled, Sima Niu pressed, "Can simply being free from worry and fear make one a noble person?" Confucius explained, "A noble person constantly reflects on themselves, so their heart is open and at ease—what is there to worry or fear about?" After leaving Confucius, Sima Niu met his fellow disciple Zixia and lamented, "Others all have brothers—how joyful—but I alone have none. How heartbreaking this is."
Upon hearing this, Zixia comforted him, saying, "I have heard that life and death are governed by fate, and wealth and honor are determined by heaven. A gentleman is diligent in his work, making no mistakes, and is respectful and proper in his dealings with others. In my view, brothers can be found everywhere under heaven—why should a gentleman worry about having no brothers?"
Later, the idiom "All Men Are Brothers Within the Four Seas" came to describe people everywhere as brothers.
Source: *The Analects*, Chapter "Yan Yuan"
Meaning of the Idiom: Later, the Chinese idiom "四海之内皆兄弟" came to describe people everywhere as brothers.