本文以多瑪斯大著《神學大全》第二部的第一部分的第18問的11個論題為主要素材，探討關於人性行為的善與惡之一般問題。首先，解析「人性行為」的意涵，並確定「人性行為」即是「倫理行為」；其次，分別從人性行為的對象、環境和目的等三方面論述人性行為的善與惡；接著，探討是否有中立性行為的存在，環境對人性／倫理行為善惡之影響，以及道德內在化的特點；最後結論出人性行為的善必須出於整體完全的原因，也就是包含各個因素―對象、環境和目的－的正確、正當與正直。

This paper is focused on Summa Theologia, Ⅰ-Ⅱ, q. 18, art. 11 by Thomas Aquinas, inquiring the general question about the good and evil of human acts. First of all, we analyze the immanence of ”human acts,” identifying ”human acts” with ”moral acts.” Secondly, the good and evil of human acts is addressed in terms of the objects, circumstances and ends of these acts. Then we will discuss the existence of indifferent acts, the impact of the circumstances on the good and evil of human/moral acts, as well as the characteristics of the interiorization of morality. Finally, it is concluded that the good of human acts must result from the correctness, righteousness and justness of all of the factors, that is, objects, circumstances and ends.