初唐時期佛性論爭所關涉諸議題中，以「一乘」最備受注目。「一乘」爲印度大乘佛典的通用觀念，其中以《法華經》的論說，影響中國佛性思想曆展最爲深遠。《法華經》盛唱佛陀爲高下根器不同眾生所開講的三乘教法，同樣是導向一種佛果，因此約其根本旨趣看，都是一佛乘法。初唐以前的中國佛性論者，都接受《涅桀經》的一切眾生皆具佛性主張爲正義，由是均視《法華經》這三乘同歸一種佛乘的說法，爲這主張的濫觴。及至初唐時期，法相宗興起，倡說五種種性，聲言有三類種性眾生，是決定沒有佛種性，因而永遠不能成佛。從這觀點出發，法相宗人援引《解深密經》、《攝大乘論》的一乘釋義，提出密意一乘的觀念；並以此觀念爲本，對《法華經》三乘同歸一種佛乘的說法，作出會通；宣稱它是基於度化五種種性中的不定種性者之密意，而方便設立，並非意味《法華經》否認種性差別的存在。法相宗之部分種性眾生無佛種性的教說，爲向來相信全分眾生有佛性的傳統佛性論者，帶來重大衛擊，引曆激烈爭辯；當中以法寶和慧沼的對論最值得留意。在一乘問題方面，法寶站在傳統佛性論者的立場，界別「究竟」和「密意」兩種一乘，約九方面力辯《法華經》的一乘教法爲究極，不可以跟見於《解深密經》那種權設的一乘混同。慧沼則堅持法相宗的做法，對法寶分立兩種一乘所提出的九方面理由，逐一加以駁斥。在論辯中，雙方各自援引有利自方觀點的經論說話作爲支持；又依自身立場，給予對方舉證的經論說話另一義的解釋。初唐時期發生的佛性論爭，也便是在這各自表述，爭持不下的情況下，逐漸淡出中國佛教理論界。

The debates in Early Tang (late seventh and early eighth centuries) over whether all sentient beings were capable of attaining Buddhahood formed an important chapter in the history of the development of the Buddha-nature teaching in China. The debates involved a variety of interrelated topics, among which the "one vehicle" was the most central. All of the pre-Tang Buddha-nature theorists, heavily influenced by the teaching of universal Buddhahood of the Mahāyāna Mahāparinirvāna-sūtra, believed that all sentient beings could become full-fledged Buddhas; and they saw in the teaching of "one vehicle" of the Lotus Sūtra, which proclaimed that there was in truth only one path and one goal of Buddhahood despite the appearance of three different "vehicles," the key scriptural confirmation of their belief. Thus, when members of the Faxiang School challenged this belief in the Early Tang by maintaining the differential of existence of beings of the śrāvaka and pratyekabuddha "vehicles" of determinate lineage, they found it necessary to give the concept of "one vehicle" a new interpretation. This paper examines the concept of the "one vehicle" as it appeared in the Lotus Sūtra and a number of authoritative Indian Buddhist texts, describes its pre-Tang Chinese interpretations, presents its new interpretation given by the Faxiang School, and analyses the subsequent debates between critics and supporters of the new interpretation over its meaning and import.