Zisi's most important contribution to Confucianism is the book Zhongyong 中庸 "Doctrine of the Mean" that he wrote when he dwelled in the state of Song 宋. According to the imperial bibliography Yiwenzhi 藝文志 in the official dynastic history Hanshu 漢書, there must also have been a book Zisi 子思 with 23 chapters which is lost.
There is not much known about his life except that he was venerated by the dukes Miu (Mu) of Lu 魯繆公 and Hui of Fei 費惠公 because he instructed them in the rituals, but he was never given an official post. It was assumed that Zisi was instructed by Zeng Shen 曾參, one of Confucius' important disciples, while his own most important disciple was the philosopher Mengzi 孟子.
Zisi's authorship of the Zhongyong, which became a chapter of the Classic Liji 禮記 "Records of Rites", was never doubted by Han 漢 (206 BCE-220 CE) or Tang 唐 (618-907) period scholars. The Southern Song period 南宋 (1127-1279) Neo-Confucian Ye Shi 葉適 was the first to doubt the authenticity of this text.
Yet he was quite alone with this analysis in an environment when the short Confucian texts of the "Four Books" (sishu 四書) constituted the core texts of research. Some sentences in the Zhongyong on the standardization of road tracks, the script and social relationships and political circumstances, point at political and cultural background of the Qin 秦 (221-206 BCE) and Han periods rather than the late Spring and Autumn period 春秋 (770-5th cent. BCE).
During the Northern Song period 北宋 (960-1126), Zisi was bestowed the honorific title of Marquis of Yishui 沂水侯, and in 1330 he was given the title of Duke Shusheng 述聖公 "Transmitting the [Teachings of the] Saint (i.e. Confucius)".