The Yuan-chueh ching (圓覺經 Perfect Enlightenment Sutra, hereafter YCC) is one of the most important texts in studies of East Asian Buddhism from the 8th century onwards. This paper examines the age, background, and intellectual characteristics of this Sutra by analyzing from a historical perspective some concepts and theories which make up its intellectual framework. The main findings are as follows: 1. The subject of the Sutra is "Yuan-chueh" (圓覺). This concept has already being seen in the Jen-wang pan-jo po-lo-mi ching (仁王般若波羅密經), Ta-sheng chih-kuan fa-men (大乘止觀法門), and Shou leng-yen ching (首楞嚴經 Heroic Valour Sutra, hereafter SLC). However, this concept is presented for the first time in the YCC as "the Root and Absolute Truth" and "Ultimate Stage" at which the training person should aim. Therefore, it is clear that the "Yuan-chueh" of this Sutra developed from the examples in those earlier texts, and especially from the thought of "Yuan" (圓) of the SLC. In addition, We can guess that ideas such as "Harmony and Non-Obstruction" (圓融無礙) and "Non-Obstruction of Totality in Complete Freedom" (圓通無礙) of Hua-yen and Tien-t'ai Buddhism lie behind this "Yuan". 2. The theory that "Sentient beings are originally Buddha" (衆生本來成佛) which is explained in this Sutra tries to eliminate the stage between the sentient beings and Buddha. This does not mean that enlightenment is possible and potential but rather that enlightenment has been achieved originally in the present reality of the sentient being. This concept that "Sentient beings are originally Buddha" does not appear before the YCC. We can therefore say that this original theory concerning the Attainment of Buddhahood developed from the thought of Buddhahood (佛性) and Tath?gatagarbha explained in a series of past Sutra and Abhidharma. Furthermore, this theory is promoted aggressively from the Attainment of Buddhahood of Hua-yen and Ch'an Buddhism, where it is expressed in the idea that "Evil passions are themselves enlightenment and birth and death is itself Nirvana" (煩惱即菩提, 生死即涅槃). 3. The Tath?gata-garbha and Last Dharma (末法) theories of this sutra are inherited from the ideas which flourished in China around the 7th and 8th centuries. There is thus an obvious and direct relationship to the Heroic Valour Sutra. 4. An original theory of training is explained in the YCC. This Sutra stands basically on the concept of sudden enlightenment (頓悟), but on the other hand the theory of training is also preached. This coexistence of the ideas, that sudden enlightenment is united with gradual practice (漸修), and that the method of the training such as ?amatha, sam?patti and dhy?na, show a clear relationship between the YCC and the SLC. 5. This Sutra has the SLC as its intellectual foundation. On top of this foundation it builds up an original theory, keeping thoughts such as the "Awakening of Faith, Hua-yen, Tien-t'ai, Ch'an, Tath?gata-garbha, and Last Dharma" in view. 6. As for the purpose of the YCC, it can be said that this Sutra is made directly for general monks, and is arranged from a basic standpoint of Mahayana Buddhism. It aims to establish a doctrinal dependence place by arranging from an overall viewpoint the influential ideas in Chinese Buddhism at that time. 7. Concerning the age of the YCC, it is certain that it was composed after the SLC. Kiyotaka Kimura estimates that the latter was composed around 713 years at the very earliest. And he says that it does not go back to the age of writing of K'ai-yuan-shih-chiao-lu (開元釋教録). By extension, we can guess that the YCC was composed in the 720s, only a few years before it appeared in K'ai-yuan-shih-chiao-lu in 730.
