Sanskrit Buddhist literature
Sanskrit Buddhist literature refers to Buddhist texts composed either in classical Sanskrit, in a register that has been called "Buddhist Hybrid Sanskrit" (BHS), or a mixture of the two. Several non-Mahāyāna Nikāyas appear to have kept their canons in Sanskrit, most prominent among which was the Sarvāstivāda. The Mahāyāna Sūtras are also in Sanskrit, with less classical registers prevalent in the gāthā portions. Buddhist Tantras too are written in Sanskrit, sometimes interspersed with Apabhramśa, and often containing notable irregularities in grammar and meter (traditionally ascribed to the esoteric nature of the texts).
Besides texts considered "Word of the Buddha" (Buddhavacana) by the traditions that transmitted them, Buddhist authors have composed treatises and literary works in Sanskrit dealing with Buddhist philosophy, logic, etc., but also with more worldly topics such as gemology, erotics, literary aesthetics, etc.
Traditional accounts vary vastly in identifying the language in which the Buddha taught, as well as in respect to the history of the non-Mahāyāna Nikāyas. While the Theravāda tradition usually upholds that the Buddha taught exclusively in the language of Magadha, other accounts offer a very different perspective on the languages of the early non-Mahāyāna schools, and in these accounts Sanskrit plays a central role.
Contemporary reconstructions are hypotheses, which differ from traditional accounts due to significantly divergent presuppositions about the nature of a Buddha, the nature of language, and the nature of history itself. Furthermore, both contemporary reconstructions and traditional accounts contain a plurality of voices and perspectives on history, relying on different and usually very fragmentary data, especially when it comes to such factual details as which language or languages the Buddha may have taught in. While we offer here a tentative and small sample of views and hypotheses, this is by no means comprehensive, nor conclusive, and should be taken as a starting point for further reflection rather than as a definitive account of the distant Buddhist past.
As for the Mahāyāna tradition, Sanskrit is a key language invested with esoteric and symbolic significance in both Sūtras and Tantras. Sanskrit grammarians and commentators justify some irregular usages in the Buddhavacana either as "Buddhist usage" (bauddhaprayoga) or by resorting to the concept of ārṣa, i.e. not unlike their non-Buddhist contemporaries. It should be understood that different types of Sanskrit texts employ different sociolinguistic registers, with different degrees of conformity to standardized rules of grammar: in this, Buddhist texts are in no way unique, and thus terms like "Buddhist Hybrid Sanskrit" should perhaps be used with some caution and contextual care.
While some hypotheses say that Buddhism was originally written in Prakrits, Sanskrit gradually became the main language of Buddhist scriptures and scholasticism in India mirroring its rise as political and literary lingua franca of the Indian subcontinent, perhaps reflecting an increased need for elite patronage. This process, it is proposed, began with the north-western Indian Buddhists of the Kushan empire (CE 30-375). The Sarvāstivādin Piṭakas were mostly transmitted in Sanskrit and many Mahāyāna sūtras such as the Prajñāpāramitā sūtra were composed in different registers of Sanskrit. The Buddhist use of classical Sanskrit for literary purposes possibly began with Asvaghoṣa (c. 100 CE), author of the Buddhacarita and one of the earliest Sanskrit dramatists. Buddhist thinkers like Nāgārjuna, Āryadeva, Asaṅga, Vasubandhu, Dignāga, Sthiramati, Dharmakīrti, Bhāviveka, Candrakīrti, etc., also wrote in Sanskrit.
The decline of Buddhism in India saw the loss of a large number of Sanskrit Buddhist texts. The use of Sanskrit as a sacred language survives in the Newar Buddhism of Nepal, and arguably the vast majority of Sanskrit Buddhist manuscripts have been preserved by this tradition.
Most Buddhist traditions have a relation to Sanskrit, and it could be argued that Sanskrit is in some sense the only pan-Buddhist language. The Theravāda tradition has relied heavily on Sanskrit grammar and lexicography; Pali commentaries and treatises often quote from Sanskrit grammars, and occasionally reproduce Sanskrit verses; while the influence of Sanskrit on South-East Asian languages and literature is omnipresent and easily perceptible. Within East-Asian Buddhism, mantras and dhāra.nīs are still recited in Sanskrit; and the same is of course true of Himālayan Buddhism, where the Tibetan tradition retained Sanskrit scholarship well into the 20th and 21st century. The Newar tradition most prominently employs Sanskrit for all ritual purposes and has produced a number of respected Sanskritists.