The terms Āgama and Nikāya denote the subdivisions of the Sūtrapiṭaka (Pāli, Suttapiṭaka; Basket of Discourses) within the canon. Āgama has the basic meaning of (received) tradition, canonical text, and (scriptural) authority, while Nikāya means both collection and group. Nikāya also denotes an ordination lineage that allows the joint performance of legal acts of the Buddhist order (saṄgha), a meaning that will not be explored in this entry.
It is not known when monks started to gather individual discourses of the Buddha into structured collections. According to tradition, the Buddha's discourses were already collected by the time of the first council, held shortly after the Buddha's death in order to establish and confirm the discourses as "authentic" words of the Buddha (buddhavacana). Scholars, however, see the texts as continuously growing in number and size from an unknown nucleus, thereby undergoing various changes in language and content. For at least the first century, and probably for two or three centuries, after the Buddha's death, the texts were passed down solely by word of mouth, and the preservation and intact transmission of steadily growing collections necessitated the introduction of ordering principles. The preserved collections reveal traces of an earlier structure that classified the texts into three, four, nine, or even twelve sections (aṅga), but this organizing structure was superseded by the Tripiṭaka scheme of arranging texts into the three (tri) baskets (piṭaka) of discipline (vinaya), discourses (sūtras), and systematized teachings (abhidharma). All Buddhist schools whose literature has been preserved divided the Sūtrapiṭaka further into sections called Āgama or Nikāya. Neither term is school-specific; the notion that the TheravĀda school used the term Nikāya while other schools used Āgama is justified neither by Pāli nor by Sanskrit sources.
There are either four or five Āgamas and Nikāyas considered canonical by the various mainstream Buddhist schools: the Dīrghāgama (Pāli, Dīghanikāya; Collection of Long Discourses); the Madhyamāgama (Pāli, Majjhimanikāya; Collection of Discourses of Middle Length); the Saὃyuktāgama (Pāli, Saṃyuttanikāya; Connected Discourses); the Ekottar(ik)āgama (Pāli, Aṅguttaranikāya; Discourses Increasing by One); and the Kṣudrakāgama (Pāli, Khuddhakanikāya; Collection of Small Texts). Some schools do not accept a Kṣudraka section as part of the Sūtrapiṭaka; others classify it as a separate piṭaka. The sequence of the five (or four) sections varies, but if included, the Kṣudraka always comes last. The names refer to the ordering principle of each section: the Dīrgha (long) contains the longest discourses; the Madhyama (middle) contains those of medium-length; and the Saṃyukta (connected) contains shorter sūtras connected by their themes. The Ekottarika (Growing by one) or Aṅguttara (Increasing number of items) comprise discourses arranged in ascending order according to numbered sets of terms, from sūtras treating one term up to those dealing with groups of ten or more. The contents of the Kṣudraka (small texts) vary significantly from version to version: Most of the works that seem to form its nucleus are composed in verse and apparently belong to the oldest strata of the canon. Some of them, such as the Dhammapada, rank among the best known Buddhist texts.
It is not known how many versions of the Sūtrapiṭaka were once transmitted by the various schools in India. Equally unknown is the number of languages and dialects used for this purpose. At present, only the Pāli Suttapiṭaka of the Theravāda school is completely preserved. Four Āgamas are available in Chinese translation: the Dīrgha, the Madhyama, the Saṃyukta, with three translations, two of them incomplete, and the Ekottarika. These were translated from the collections of different schools: The Dīrghāgama probably belongs to the Dharmaguptaka, the Madhyamāgama and Saṃyuktāgama to the (Mūla)Sarvāstivādins, and the Ekottarikāgama to the MahāsāṂghika school.
In the early twentieth century, numerous fragments of Sanskrit sūtra manuscripts were found in Central Asia, enabling scholars to recover at least a small part of the Sūtrapiṭaka of the (Mūla)Sarvāstivādins. Later, fragments of the Ekottarikāgama of the same school came to light among the Gilgit finds. Recent manuscript finds from Afghanistan and Pakistan also contain many sūtra fragments from the scriptures of at least two schools, the (Mūla)Sarvāstivādins and probably the Mahāsāṃghikas. Most notable among them is a manuscript of the Dīrghāgama of the (Mūla)Sarvāstivādins. Unlike colophons of vinaya texts, those of single sūtras or sūtra collections never mention schools, and this often renders a definite school ascription difficult. School affiliation of Āgama texts may have been less important than modern scholars tend to believe.
The different versions of the Sūtrapiṭaka are by no means unanimous with regard to the number and type of sūtras included in each section. To give one example: The Dīghanikāya of the Theravāda school contains thirty-four texts, while the Dīrghāgama in Chinese translation contains only thirty. In the incompletely preserved Dīrghāgama of the (Mūla)Sarvāstivādins, however, forty-seven texts are so far attested. Only twenty of them have a corresponding text in the Chinese Dīrghāgama, and only twenty-four correspond to texts in the Pāli version. For eight of them, a parallel text is found in the Majjhimanikāya of the Pāli; at least four have no parallel at all. The agreement between the different versions of a sūtra varies significantly. Versions may be close in some passages and loose in others. Often a considerable part of a sūtra consists of formulaic passages, and the wording of these formulas is version specific. Further differences may be found in the sequence of passages, in the names of places and persons, and also in doctrine. All this indicates a common origin, followed by a long period of separate transmissions with independent redactional changes.
There are many examples of text duplicates in two sections of the same Sūtrapiṭaka. For example, the Satipaṭṭhāna-sutta (Foundation of Mindfulness) of the Pāli canon is contained in both the Dīgha- and the Majjhimanikāya. This may be an indication of a separate transmission for each Āgama/Nikāya in earlier times, another indication being terms like Dīghabhāṇaka (reciter of the Dīgha section) to refer to the respective specialist during the phase of oral transmission in the Pāli tradition. At least in the case of the Mūlasarvāstivādins, many sūtras are also duplicated in their Vinaya.
When growth and redactional changes of the various collections came to an end, they began to form what can best be described as part of a canon of the respective schools. However, very little is known about the use or ritual and educational functions of the collections during early times. Because of their status as scriptural authority, quotations from the sūtras are numerous in the commentarial literature of the various schools. Certain sūtras also continued to be transmitted individually or in fixed selections designed for specific religious purposes, and it appears that such texts played a much more important role in the life of Buddhists than the complete collections. Not all the sūtras were collected as Āgamas/Nikāyas; the Mahāyāna sūtras, for instance, never came to be included in such a classification scheme.
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