天台哲學的一心說主要源於《大乘起信論》「一心二門」的命題，該論認為「一心」既有清靜無漏的性質，又具染污有漏的性質。天台諸師慧思（515－577）、智顗（538－597）、湛然（711－782）、知禮（960－1028）均承襲了《大乘起信論》的觀點，以開展他們的「一心」說。本文在真妄問題方面，描述四人的「一心」說均為真妄和合論。在色心問題方面，舖陳四人皆主張「心具」說，且知禮更倡「色具」說。在本體問題方面，說明四人均將「一心」視為宇宙萬法的最高本體。在修行問題方面，指出四人皆將「一心」用於禪觀實踐。有關「一心」的真妄、圓具、本體、觀修等問題，都是天台哲學的重要課題。上述諸師透過對這些課題的回應，從而建構了天台哲學的理論體系，而天台哲學的精華即在「一念三千」說與「色心雙具」論。

The main source of the concept of "one mind" as taught in Tiantai philosophy is the "Treatise on the Awakening of Faith in the Mahayana". This scripture teaches the concept of "the two gates of the one mind": the "one mind" comprises the two qualities of purity and impurity. The Tiantai-masters Huisi (515-577), Zhiyi (538-597), Zhanran (711-782), and Zhili (960-1028) all accepted and further developed this concept of the "Awakening of Faith in the Mahayana". Concerning the problem of the relationship true/false, this paper explains that these four masters taught that true and false actually are united. As to the problem of form/mind, they all held that "all dharmas are completely inherent to the mind," but Zhili even further proposed a concept of the "complete inherence to the dharmas." With respect to ontological positions, the paper explains that the four masters considered the "one mind" to be the highest essence of all dharmas. Concerning religious practice, they all took the "one mind" as an object of meditation and practice. All of these problems are central issues of Tiantai philosophy. The Tiantai-masters constructed the system of Tiantai philosophy in response to these issues. The essence of Tiantai philosophy is the teaching of "the whole universe in one thought" and "the comlete inherence in both form and mind."