Talking about authoritative ancient historical sources - here's a great post from Dan Lusthaus:
From: "Dan Lusthaus"
List Editor: Franz Metcalf
Editor's Subject: Re: QUERY>Modern use of "Theravada" (Lusthaus)
Author's Subject: Re: QUERY>Modern use of "Theravada" (Skilling)
Date Written: Sat, 23 Dec 2006 09:23:59 -0800
Date Posted: Sun, 23 Dec 2006 12:23:59 -0500
Just to complicate things a bit, if we put aside the additional concern of using "Theravada" (or any sectarian name) as a designation for lay and/or geographic communities in contradistinction to simply naming a monastic lineage, then the Chinese tradition does seem to attest to the term Thera-school.
If we look, for instance, at the three Chinese translations of Vasumitra's "Development of the Different Schools," which, unfortunately does not survive in Sanskrit or an Indic language, we find the following:
(T.2031) uses 上座部 shangzuobu to designate the Theravada school. Shangzuo literally means "High-Seated," and is a common term for Thera/Sthavira. Bu means a "school" or "sect" or something of that sort, and can correspond to --vaada (e.g., Sarvaastivaada is commonly rendered in Chinese as yiqieyou bu 一切有部 "everything exists school"). Of course, this is a translation, not a transliteration or transcription.
(T.2032; the Taisho [mis-]attributes this to Paramartha) does give us a transliteration: 體毘履 which in modern pronunciation would be: ti pi lü. Someone better versed in early fifth century Chinese phonetics may be able to suggest how that sounded back then. However it sounded, its initial consonant was a -t-, not -sth-. Kumarajiva explains the term with another common term for Thera/sthavira, laosu 老宿 which means "elder." His gloss is (此言老宿唯老宿人同會共出律部也). So its meaning is not in doubt.
(T.2033), in his version of Vasumitra, offers two forms of the name, both being translations rather than transliterations:
大德眾 da de zhong (Great Venerables)
上座弟子部 shangzuo dizi bu (High-Seat and Disciples School)
The first name is meant to convey Thera as an honorific (which is one of the common uses, even in Chinese translations). The second is interesting, since while Xuanzang (as is typical in Chinese) only indicates the "High-seated Ones" (shangzuo), Paramartha adds dizi "disciples", i.e., the high-seated (i.e., teachers) and their disciples. So we are not yet including laypersons, but he is clearly indicating that the term is not meant to be restricted only to actual "elders."
This would suggest that by Vasumitra's day the term Thera (-vada) was already a common designation. His text, of course, is one of the classic sources for the narrative by which the first schism involved the splitting off of Mahasanghikas from the Theras, and that later Sarvastivada also split from the Theras. According to him, the Vatsiputriyas split from the Sarvastivada, and each continued to engender further schisms or sectarian splits. But Thera-vada (if we can take the Chinese bu as an equivalent of vaada) remains consistent. This, of course, is centuries before Buddhaghosa.
In addition, the Foguang Dictionary (p. 719c) lists some additional transliterations (I've added canonical citations):
1. 銅鍱部 modern pronunctiation = tong ye bu. [cf. T.54.2128.646c 一切經音義]
2. 他鞞羅部 modern pronunciation = ta bi luo bu. [cf. ibid, 784b; also Guanding's commentary on the MahaNirvana Sutra T.1767.194c, re: the initial schism:
In other words, Guanding (Tiantai Zhiyi's disciple and editor, 6th-7th c) initially gives the name BOTH in translation and transliteration (High-seated + tabiluo), and then continues with the translated version as a bu/vaada.
3. 體毘履 ti pi lü (this was what Kumarajiva used, as noted above).
4. 他毘利 ta pi li [cf. T.55.2149.262a 大唐內典錄: 他毘利律(他毘利齊言宿德見僧祐錄)]
If we take the transliterations -- tabiluo, tipilü, and tapili -- and stress the middle syllable, while also taking bi/pi as an approximation for Indic -vi- , then, while they begin with an initial T- sound rather than an S- or Sth-, we get stha-VI-ra (sthavira).
So we may consider both terms (Thera- and Sthavira- "school") as attested in China at least since the end of the fourth century, and, if we can trust the Chinese representations of Vasumitra, in use in India four to five centuries earlier than that.
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