本文的目的是比較徐復觀與牟宗三這兩位思想家對孟子性善論之詮釋。徐復觀與牟宗三是當代新儒家的兩位核心人物，他們都對孟子的性善論提出一套精彩的詮釋，然而採取不同的取向：徐復觀從思想史發展的取向，追溯性善論在中國思想發展中的源流；牟宗三則採取哲學論證的方法，透過孟子與告子的論辯，顯示孟子透過理性論證的方式理所必然地確立性善論。由於取向不同，使得他們在細節的詮釋上小有出入。本文先從周初到《中庸》天命觀的演變、孟子對性善的貞定、孟子對性與命觀念的深化、孟子與告子的論辯這四點來重述徐復觀如何論述儒家性善論的思想發展；其次以杞柳之喻與湍水之喻、生之謂性之辯、仁義內在或外在、性善之確立這四點來重述牟宗三如何論證孟子性善論的確立；最後比較徐、牟對孟子性善論詮釋之同異。

The purpose of this article is to compare Xu Fuguan and Mou Zongsan's interpretations of Mencius’theory of “xing-shan”（性善）. Xu and Mou are the two core figures of contemporary Neo-Confucianism. They both put forward a set of wonderful interpretations of Mencius’theory of xing-shan, but they adopt different approaches: Xu adopts the approach of the history of thoughts and investigates the historical development of theory of xing-shan in the Chinese humanity thoughts; Mou adopts philosophical argumentation and shows that Mencius rationally and necessarily establishes the theory of xing-shan through the debate between Mencius and Gaozi. Due to their different approaches, Xu and Mou present slightly different interpretations. This article begins with Xu’s discussion of the Confucian thought of humanity goodness by tracing the evolution of the concept of destiny from the early Zhou Dynasty to the doctrine of “Zhong-Yong”（《中庸》）. Specifically, I consider how Xu interprets Mencius’ recognition of xing-shan, the deepening of Mencius’concept of humanity and concept of destiny, and the debate between Mencius and Gaozi. Second, I introduce how Mou demonstrates the establishment of Mencius’theory of xingshan by using the metaphor of qiliu（杞柳）and turbulent water（湍水）. Then, I consider how Mou argues for the Mencius’theory of xing-shan by using the debate over humanity is human nature, the debate over ren-nei-yi-wai（仁內義 外）, and the justification of Mencius’ theory of xing-shan. Finally, I compare Xu’s and Mou’s interpretations by showing the similarities and differences in their interpretations of Mencius’theory of xing-shan.