康德在《道德底形上學之基礎》一書的導論中主張：「每一個人都得承認：一項法則若要在道德上有效，亦即作為一項責任底根據，就得具有絕對必然性」，而根據康德這種絕對必然性的基礎只能來自純粹理性，因此道德法則有效性的基礎只能建立在「純粹實踐理性」上。但叔本華認為實踐理性只能是「精審的」（prudential），不可能是「純粹的」，因此他認為道德的基礎與其說是「純粹實踐理性」，不如說是形而上的意志。對牟宗三來說康德把人的「純粹實踐理性」當作一種有限的實踐能力無法作為道德真正的基礎。牟先生認為康德的「純粹實踐理性」應該轉化為一種更強意義，即無限的實踐理性。本文首先嘗試分別分析叔本華和牟宗三對康德倫理學的批判，並且提出康德的可能回應（第二、三節），之後並對兩人的倫理學基礎的學說做一比較（第四節）。

In the Preface of Groundwork of the Metaphysics of Morals Kant holds that “everyone must grant that a law, if it is to hold morally, that is, as a ground of an obligation, must carry with it absolute necessity.” According to Kant, this absolute necessity can only be derived from pure reason, and therefore the validity of moral law can only be grounded on pure (practical) reason. But for Schopenhauer, practical reason can only be prudential, never pure. Therefore he holds that the foundation of morals is a metaphysical will rather than pure practical reason. For Mou Zongsan, the validity of moral law cannot be grounded on pure practical reason in Kantian finite sense, but instead on a stronger sense of an infinite practical reason. This article attempts firstly to analyze Schopenhauer’s and Mou Zongsan’s criticism of Kant’s ethics (Sections 2 and 3), and secondly to compare their teachings of the foundation of morals (Section 4).