This article draws upon related historical materials and research, old and new, to explicate the content and purpose of Ssu-ma Ch'ien's letter in reply to Jen An in order to elucidate ”Ssu-ma Ch'ien's mind.” The research process for this inquiry necessarily involved embarking on a review of the following issues:1)the time and purpose of Jen An's original letter to Ssu-ma Ch'ien, 2)the time(month and year)and purpose of Ssu-ma Ch'ien's letter in reply, 3)Ssu-ma Ch'ieh's age, 4)the Han system of officials, and 5)the political situation during the latter part of Han emperor Wu's reign. Finally, we are in a position to break through and interpret the six words with which Ssu-ma Ch'ien opened his letter: T'ai-shih kung niu-ma tsou(太史公牛馬走): Grand Historian master cow horse flees. This article first established the expression ”Ssu-ma Ch'ien's mind” so as to distinguish it from, say, ”Ssu-ma Chao's mind,” Consequently, ”Ssu-ma's Ch'ien's mind” lay in begging for castration and suffering patiently despite people's talk, so he could complete the Historical Records(Shih chi) for the purpose of washing away his disgrace and bringing honor to his father. Indeed, he made this exchange for his father and for history and culture. This essential meaning can be explicated from the six words: T'ai-shin kung flu-ma tsou.