This body of literature includes the third of the “three baskets” (Skt., ), its commentaries, and later exegetical texts.
Indeed, the names of the “eighteen schools” are indicative of their origins in characteristic doctrines, geographical locations, or the legacy of particular founders: for instance, Sarvāstivāda (“advocates of the doctrine that all things exist”), Sautrāntikas (“those who rely on the and Pudgalavāda (“those who affirm the existence of the person”); Haimavatas (“those of the snowy mountains”); or Vātsīputrīyas (“those affiliated with Vātsīputra”) respectively.
As noted by Gethin (1998, 52), rather than sects or denominations as in Christianity, “at least some of the schools mentioned by later Buddhist tradition are likely to have been informal schools of thought in the manner of ‘Cartesians,’ ‘British Empiricists,’ or ‘Kantians’ for the history of modern philosophy.” It is customarily assumed that the multiple ancient Buddhist schools transmitted their own versions of Abhidharma collections, but only two complete canonical collections are preserved, representing two schools: the Sarvāstivāda, who emerged as an independent school from within the Sthaviras around the second or first century BCE, became dominant in north, especially northwest India, and spread to central Asia; and the Sinhalese Theravāda, a branch of the Sthaviras that spread out in south India and parts of southeast Asia.
A generally accepted tradition has it that some time around the beginning of the third century BCE, the primitive Buddhist community divided into two parties or fraternities: the Sthaviras (Pali, Theriyas) and the Mahāsāṅghikas, each of which thenceforth had its own ordination traditions.
Throughout the subsequent two centuries or so, doctrinal disputes arose between these two parties, resulting in the formation of various schools of thought ( V 12–13.
The number eighteen, though, became conventional in Buddhist historiography for symbolic and mnemonic reasons (Obeyesekere 1991) and, in fact, different Buddhist sources preserve divergent lists of schools which sum up to more than eighteen.