本文旨在說明康德堅持說謊禁令之理據。在說謊禁令與人類之愛的衝突例子，康德堅持前者比後者重要，但令人難以接受。雖然很多康德學者提出不同的策略解決義務衝突問題，但並不成功，因為它們都違反了定然律令，而且很多學者都忽視了康德提出的全然義務與不全然義務區分。本文也主張，此區分於實踐理性的模態範疇能發揮評估功能，由此能幫助我們理解說謊禁令的優先意義。

The aim of this paper is to show the ground of Kant's insistence on the Lügenverbot. In the example of the conflict between the Lügenverbot and philanthropy Kant argues that the former is more important than the latter. However, it seems that this suggestion is difficult for us to accept. Although many Kantian scholars suggest different approaches to solve this problem, they are not successful, since these approaches violate the categorical imperative. Inasmuch as Kant's distinction between perfect and imperfect duty is easily neglected, this paper argues that with the help of the categories of modality of practical reason that this very distinction serves an evaluative function, which helps us to understand Kant's insistence on the superiority of the Lügenverbot.