作為當代新儒家的主要創建者，牟宗三也因提出宋明儒學「三系說」而著名於世。依牟宗三，在朱熹所代表的「理學」及陸象山與王陽明所代表的「心學」之外，胡五峰與劉蕺山形成了「第三系」。但是，雖然牟宗三的「三系說」已經改變我們對中國哲學的瞭解，至今卻似仍沒有學者試圖找出此一學說的可能來源。為了填補此一空白，本文旨在探索「三系說」某些隱藏的根源。我們將指出：首先，「三系說」可能受到早期海德格在其《存有與時間》第二卷手稿中「三種超越性（或差異）」之劃分所影響。其次，就某種程度而言，海德格的三種倫理學區分也對牟宗三的「三系說」有所影響。這揭示海德格對於牟宗三的可能影響遠超其對康德圖式論之解釋，特別是牟宗三藉深化其「本質倫理學」一概念對於前後期朱熹之分別作出一嶄新的釐清。此外，通過劉蕺山對於朱熹早年與胡五峰學派之關連的評述作為證據，本文將指出：雖然劉蕺山事實上沒有引述胡五峰，但這不足以否定他對後者理論的熟稔。總而言之，於面對現今不少學者提出質疑的情況下，本文將發展出一對「三系說」的嶄新辯護。

As a major founder of Contemporary Neo-Confucianism, Mou Zongsan (1909-1995) is also well-known for introducing the threefold typology of Song-Ming Neo-Confucianism. According to Mou, besides the School of Principle (li) as represented by Zhu Xi (1130-1200) and the School of Mind (xin) as represented by Lu Xiangshan (1139-1193) and Wang Yangming (1472-1529), a third lineage was formed by Hu Wufeng (1105-1162) and Liu Jishan (1578-1645). However, despite the fact that Mou's typology has revolutionized our understanding of Chinese philosophy, thus far no one has tried to identify its possible sources. In fulfilling this gap, this paper will argue that there are some hidden sources of this provocative typology. First of all, it can be traced to the influence from the early Heidegger's idea of the threefold transcendence (or difference) in his manuscripts for the second volume of Being and Time. Secondly, to some extent, Heidegger's distinction of three types of ethics also contributes to Mou's classifying of Song-Ming Neo-Confucianism. All this will not only uncover Heidegger's decisive influence upon Mou other than his appreciation of Heidegger's interpretation of Kant's schematism, but also give rise to a deeper understanding of the distinction between the early and the later Zhu. Besides, in terms of Liu's appreciation of Zhu's early affinity to Hu Wufeng's school, one can counterbalance the thesis that the de facto lack of Liu's reference to Hu implies the elimination of any de jere connection between their doctrines. Given the recent challenges raised by different scholars, in terms of this clarification of its origins one can develop a new defense of Mou's typology.