Sima Geng (died 481 BCE), also known as Sima Niu, was a native of the State of Song. He is mentioned three times in The Analects. Sima Geng was articulate but had a quick temper. When he asked Confucius about the principle of Ren (Benevolence), Confucius taught him: "A person of Ren is sincere in heart and speaks as if restrained, reluctant to speak rashly." This meant he should think twice before acting and be cautious in speech. Still unclear, Sima Geng asked: "Does being restrained in speech and not speaking rashly count as Ren?" Confucius explained: "Since one should not be careless or perfunctory in doing things, how can one speak without careful consideration?"
Sima Geng was born into an aristocratic family in the State of Song, the youngest of four brothers. He was the younger brother of Huan Tui, a Minister of War in Song. Because Confucius had criticized Huan Tui for extravagantly building a stone outer coffin for his tomb, wasting resources without regard for the people's strength and materials, Huan Tui held a grudge. When Confucius and his disciples traveled to Song, they often practiced Li (Propriety/Ritual) under a large locust tree near their lodging, and Huan Tui sent people to cut it down. Disciples urged Confucius to leave quickly, but Confucius said, "My mission is bestowed by Heaven; what can Huan Tui do to me?" Because of this conflict, Sima Niu (Sima Geng's courtesy name) often felt uneasy. Confucius comforted him, saying, "The Junzi (Exemplary Person) is neither worried nor fearful." Sima Geng then asked, "Can one be considered a Junzi if they are neither worried nor fearful?" Confucius replied, "If one examines themselves and finds no cause for shame, what is there to worry about or fear?"
He firmly believed in Confucian doctrine and opposed rebellion against superiors and instigating disorder. His three elder brothers, led by Sima Huantui, started a rebellion in Song, which Sima Niu resolutely opposed. After his brothers failed—some killed, others fleeing to Wei—he left Wei for Qi; when his brother fled to Qi, he left Qi for Wu, vowing never to serve the same ruler as his brother.
He said, "All men have brothers; I alone have none