本文將焦點集中在美國哲學家David S. Nivison（倪德衛）對孟子哲學的研究，尤其在道德心理學方面，探討倪德衛對孟子的動機、義務與道德行動的理論重建，分析其內容，檢討其論證，並提出適當的反省與批評。首先，本文觀察「意志薄弱」的問題在古代中國哲學的呈現。依倪德衛，此一問題在古代中國哲學並非少見。本文比照西方道德哲學，分別論述孔子、墨子與孟子對此一問題的處理方式，尤以《孟子》為探討焦點。其次，本文探究「一本」與「二本」的問題，討論歷來解說及其疑義，從而引入倪德衛獨特的理解——一個道德心理學式的詮釋，即動機與義務究竟是起自一個根源，還是兩個根源？結論部分，作者提出對一本二本問題自己的見解，並批評倪德衛的論點。

The paper is a close and critical examination on David S. Nivison’s research into Mencius’s philosophy, emphasizes on his moral psychological approach to the reconstruction of Mencius’s theory of human motivation, moral obligation and action. At First, the problem of “akrasia” or “weakness of will”, according to Nivison, enters ancient Chinese philosophy in general. Three theories (Confucius’s, Mencius’s, and Mozi’s) are discussed in reference to Western moral thinking and, particularly, this paper centers around Mencius. Secondly, the paper investigates into Nivison’s reconstruction of Mencius’s theory of motivation, obligation, and moral action. Related to the “akrasia” problem, Nivison’s ingenious inquiry — “Do moral obligation and motivation stem from two sources (roots) or one?” — is critically examined and the author provides his own interpretation.