Regarding the Source of the Buddha-Nature Teachings
When it comes to the sources of the Buddhist teachings on tathāgatagarbha, sugatagarbha, or several other equivalents that we generally designate by the English term “buddha-nature,” for the most part this refers to sūtras, which are the scriptural discourses that the tradition considers canonical. Thus the notion of source tends to carry a strong sense of authority, in that it is the very words of the Buddha, as such sūtras are believed to be, that are most commonly referenced or cited to establish the veracity of such teachings. However, when it comes to the topic of buddha-nature, perhaps the most influential work on the subject is not a sūtra, but rather a śāstra, a term usually translated as “treatise” that refers to literary compositions rather than the oral discourses that more typically characterize the sūtras. The work in question, the Ratnagotravibhāga Mahāyānottaratantraśāstra, is itself canonical, as many such treatises of Indian origin are in the cultures to which Buddhism spread. For Tibetans, especially, the treatise enjoys an elevated status even among the great literary works of the Indian tradition, as it is believed to have been composed by the celestial Bodhisattva Maitreya, who is said to wait in the proverbial wings as the next in line to manifest complete buddhahood in this world once the last remnants of Buddha Śākyamuni’s wheel-turning activities have faded from the collective memory. Nevertheless, this work also references the authority of the sūtras to support its presentation of the seven topics (termed vajrapada in the text) upon which its five chapters are based. Therefore, one way of approaching the question of the sources of the buddha-nature teachings is to look at the sūtras that are cited in this treatise, which perhaps more than any other composition contributed to the acceptance and spread of the buddha-nature teachings themselves. The author cites numerous sources throughout the work, though the primary one is stated to be the Dhāraṇīśvararājasūtra, (also known by its alternative title The Sūtra Teaching the Great Compassion of the Tathāgatas, Tathāgatamahākaruṇānirdeśasūtra), as this text covers all of the seven topics of the treatise, while the remaining citations mentioned, both explicitly by name and implicitly by content, touch upon specific aspects of one or more of the seven topics. These sūtra sources have been well documented by traditional and modern scholars alike, though perhaps the earliest example of such an approach can be found in the work of the c. 13th to 14th Tibetan scholar Rinchen Yeshe and his work the Illumination of the Definitive Meaning: A Commentary on the Uttaratantra in Relation to the Sūtras (rgyud bla ma'i 'grel pa mdo dang sbyar ba nges pa'i don gyi snang ba), which comments upon the treatise through a further exploration of the sūtras referenced within its pages.
Another way that this question of the sources of the buddha-nature teachings has been traditionally approached is through the development of the category of so-called "tathāgatagarbha sūtras," which grouped the scriptures in which these teachings could be found. Though it is not clear how prevalent this categorization was, or whether it even gained any traction at all, in India, it certainly became a preoccupation of commentators in the Chinese and the Tibetan traditions where the buddha-nature teachings spread and, as some modern scholars might argue, took on a life of their own. In Tibet, in particular, this notion of a finite set of tathāgatagarbha sūtras led to the compilation of lists of such sūtras, often presented in groups of five or ten, though numerical variations were not uncommon. However, the texts which actually made up these five or ten certainly varied depending on the authors of the lists and their affiliations with specific philosophical schools of thought and monastic institutions. In fact, it seems that the only text that everybody agreed should certainly be included in these lists, is the actual Tathāgatagarbhasūtra, itself, though the Śrīmālādevīsūtra comes in a close second, as it appears in almost all of the available lists, though the majority of them also include at least the Aṅgulimālīyasūtra and Mahābherīsūtra. Therefore, only one sūtra is universally accepted as belonging to this group in the Tibetan tradition, though the above four are generally mentioned in most of the groups of ten. Furthermore, for those Tibetan schools, such as the Jonang, that asserted the teachings of buddha-nature and their description of ultimate truth (paramārthasatya) represented the definitive meaning of the Buddhist teachings (nītārtha), these lists of tathāgatagarbha sūtras were often expanded or conflated with the categorization of the "sūtras of definitive meaning." Therefore, the combined, or overlapping lists, of these definitive texts and the tathāgatagarbha sūtras tended to routinely number into double digits, with the most extensive lists capping out around twenty or just slightly more.
Karl Brunnhölzl has detailed the variations in these lists, along with an overview of all the sūtras that are cited in the Ratnagotravibhāga Mahāyānottaratantraśāstra, which can be read Karl Brunnhölzl's Translator's Introduction, ''When the Clouds Part'', pp. 3-12. here. His findings suggest that there are actually twenty-four sūtras of Indian origin associated with the teachings of buddha-nature, a number which he arrived at by combining all of the texts either referenced in the Ratnagotravibhāga or included the Tibetan lists of the tathāgatagarbha sūtras and the related sūtras of definitive meaning. Of course, the Chinese tradition would likely augment this list by adding, or extracting, texts known, or unknown, to them in translation, with an obvious addition being the Dasheng qixin lun, which is often translated into the slightly abbreviated English title of the Awakening of Faith, a text which modern scholars have long since conceded to being of Chinese origin. However, regardless of the grounds of these disagreements, the numerous variations among such list of sources of the buddha-nature teachings, actually highlights an important aspect of the teachings themselves. In one sense, the teachings on buddha-nature have been extrapolated from the corpus of the sūtras by those who found them compelling. Unlike other categorical series of Buddhist scriptural literature, such as the Prajñāpāramitāsūtras for instance, in which the associated texts are labeled as such, the tathāgatagarbha sūtras as a category remains much more fluid in comparison. The buddha-nature teachings have essentially been gleaned from the sūtras rather than being representative of an overt scriptural movement that can be easily demarcated as belonging to a specific time or place from which they emerged. The history of these teachings is a long one that straddles cultures, languages, and ever changing arenas of debate. To return to the questions of the source of these teachings, though there are clearly specific sūtras whose authority has been invoked in support of the buddha-nature teachings, it is the Ratnagotravibhāga that seems to deserve the most credit for bringing these to the fore. While it may not represent the absolute origin of these teachings or the terms utilized in its presentation, the engagement with this extraordinary work has played a unique role in the development of the buddha-nature teachings as they spread from India into China, Tibet, and beyond. Therefore, though it cannot be said to have invented the notion of buddha-nature, it may very well have had the greatest influence on the traditions that coalesced around the message of buddha-nature and championed its cause.
