本文嘗試以兒童哲學小說《思考舞臺》第5章第10節〈三頭巨人〉的內容為據，討論該節的主題——真實與謊言的相關問題。其中，此主題關係到倫理學的範疇，也就是一個人性∕倫理行為的善惡通常關涉到三個面向，分別是事的真假、意圖和後果（對人是否有傷害），就像一個三頭巨人般。藉由陳明宣爸爸的親身故事，分析不同情況的倫理性；也藉由多瑪斯的名言：「惡是來自於任何一個個別的缺點，而善是出於整體完全的原因。」結論出：唯有當三頭巨人——三項考量∕行為或行動的三個面向，亦即說的事是真的、說話的用意是善的（沒有惡意）及後果對人是沒有害處的，方才可確信所做的行為在倫理上是善的、是正確的。

Based on “The Three-Headed Giant,” Chapter Five, Section Ten of Lisa, this article attempts to discuss the issues about truth and lie, the theme of this very section. This theme involves ethics, that is, the three aspects implicated in the good and evil of human/ethical acts—truthfulness, intention and consequences—in the form of the three heads of the giant. Discussing the story of the father, we analyze the ethicalness of different situations. In view of Aquinas’s words, “evil results from each particular defect, but good from the whole and entire cause,” we conclude: we can be assured one’s behaviors are good and right only when the three aspects—truthfulness, intention and consequences—are fulfilled.