《大乘起信論》以「一心開二門」的思想架構說明了一切法的存在與究竟成佛的問題，並開展了佛教式的「兩層存有論」格局。本文主要是順著牟宗三先生視「一心開二門」為一重要的哲學格局，以及對於依此思想架構所開展出來的「兩層存有論」的重視，企圖對天臺宗性具圓教之核心概念「一念無明法性心」作一檢視與定位，希冀藉此檢視，能確實地探討「一心開二門」作為一哲學格局之普遍適用性；同時亦期能進一步探討天臺宗「一念無明法性心」之具體內容以及可能具有的特殊涵義。

　　　　　The expositional framework of "Two-gates-in-one-mind" in The Awakening of Faith in the Mahayana expounds both the ontic status of beings and the radicalization of becoming a Buddha. Furthermore, it constitutes the Buddhist "duplex ontology". Mou Tzun-san treated "Two-gates-in-one-mind" as an important philosophical scheme and deduced "duplex ontology" from it seriously. This paper follows Mou T'zun-san approach to examine the very concept of the one single mind embracing ignorance and dharma-nature simultaneously", the central concept of "hsing-chu-yuan-chiao" in T'ien-t'ai Buddhism, and discusses the usefulness and universality of "Two-gates-in-one-mind" as a philosophical scheme. This paper discusses also the concrete contents and the special significances of the one single mind embracing ignorance and dharma-nature simultaneously" in T'ien-t'ai Buddhist canon.