Among the Eight Great Prose Masters of the Tang and Song Dynasties, the Six Song Prosaists frequently employed the term "human nature" in their writings. This particular phenomenon has attracted many researchers' attention and discussion. However, there hasn't been any profound study which reveals the causes behind. Therefore, this article explores their motives by probing into the historical context of the Northern Song Dynasty and further organizes the origins of relevant thought. In addition, it suggests that the historical circumstances of the Northern Song Dynasty not only led to the prevalent application of "human nature" in the works of the six Song Prosaists, but also indirectly affected their writing style.