While it can be characterized as more of a philosophical methodology than a distinct school, Buddhist logic nevertheless developed largely within a clearly defined stream of Buddhism, that of Abhidharma/Yogâcāra, with the earliest texts of the system being attributed to Yogâcāra founders Asaṅga 無著 and Vasubandhu 世親, and later developments coming from the Yogâcāra philosopher Dignāga 陳那. Over time, Buddhists came to refer to their system as hetu-vidyā, which is generally classified into early 古因明 (pre-Dignāga) and late 新因明 (Dignāga and afterward).

The main purpose of 'logic,' systematized as a set of rules, was to serve as the structure for debate—concerning, of course, philosophical principles. To this end, it was obviously well-used in ancient India, and developed extensively in Tibet. While a number of logic texts were transmitted to and studied in East Asia, no significant 'live-debate' practice developed comparable with that seen in Tibet.

The early form of the school is defined as the system contained in the fifth fascicle of the Yogâcārabhūmi-śāstra, attributed to Maitreya. There, we find seven characteristics listed, as:

1. the debate in itself (vāda) 論體性
2. the setting of the debate (vāda-adhikaraṇa) 論處所
3. the foundation of the debate (vāda-adhiṣṭhāna) 論所依
4. the debateʼs ornament (vāda-alaṃkāra) 論莊嚴
5. the points of defeat in the debate (vāda-nigraha) 論墮負
6. the decision whether or not to undertake the debate (vāda-niḥsaraṇa) 論出離
7. the attributes of much utility in the debate (vāde bahukarā dharmāḥ) 論多所作法1

Important among the seven is the third item, the foundation, which includes two thesis topics, and eight kinds of proving (sādhana; for a lengthy discussion of this important term including all the major aspects of Dignāgaʼs logic, see 能立). The two thesis topics are the individual presence 自性 and the qualification 差別. For the eight acceptable means of proving, see 成法八種 (also called 八能立). the Mahāyānâbhidharma-samuccaya-vyākhyā gives a slightly different rendering of this set, paring it down to six components, the first five of which end up comprising the five components of a proof in the earlier school of Buddhist logic 五支作法.The formula in the five-part system consists of: proposition (pratijñā 宗), reason (hetu 因), example (dṛṣṭânta, udāharaṇa 喩), application (upanaya 合) and conclusion (nigamana 結). For example: 1. A voice is impermanent; 2. because it is produced by causes; 3. it is like ceramics; 4. ceramics are produced by causes and are impermanent—in the same manner a voice is also; 5. therefore, a voice is impermanent.

According to Dignāgaʼs disciple Śaṃkarasvāmin 商羯羅主, Buddhist logic includes the dual methods of self-edification 自悟 and edification of others 悟他. Other-edification includes the four aspects of valid proof (眞)能立 and refutation (眞)能破, and fallacious proof 似能立 and refutation 似能破, while self-edification includes the four aspects of valid direct perception (眞)現量, valid inference (眞)比量, fallacious direct perception 似現量 and fallacious inference.

The New School of Buddhist logic, based on the three-part syllogism 三支作法 starts in the works of Akṣapāda, and is consummated by Dignāga. After this new system is developed, it becomes the mainstream, and is transmitted into China and Tibet. The application of the three part system is laid out by Dignāga in his Nyāyamukha (Gateway to Logic), translated by Xuanzang as 因明正理門論本 (T 1628) and Yijing as 因明正理門論 (T 1629) (for a detailed article on the system presented in this text, mostly definitive for the East Asian logic tradition, see under 能立). A central part of the discourse of this tradition is the definition of what constitutes a valid proof, what doesn't constitute a valid proof, what constitutes a blatant contradiction, and the reason why. To this end, the logicians compiled tables of fallacies, which, as standardized, include nine fallacies in the thesis 宗九過, fourteen fallacies in the reason 因十四過 (which are analyzed in nine ways 九句因), and ten in the example 喩十過—totaling thirty-three.

Scholars and Texts: The definitive work by Dignāga mentioned above, as well as the Nyāyapraveśa 因明入正理論 (T 1630) by Śaṃkarasvāmin are the three most important source texts for Buddhist logic transmitted in East Asia, which is actually a relatively small number when compared with the amount of texts that Xuanzang 玄奘 brought back from other traditions. Nonetheless, these texts attracted a good amount of attention from members of the East Asian Yogâcāra tradition, as one can see from browsing the titles of the numerous commentaries on these works listed in Onoʼs Bussho kaisetsu daijiten. Unfortunately, most of these are no longer extant. In terms of the dissemination of Buddhist logic in East Asia, two of Xuanzangʼs students played a critical role: Kuiji 窺基 and Huizhao 慧沼, but these works introduce the opinions of a dozen or so other contemporary colleagues as well. In Korea, at least fifteen logic works were composed, mainly commentaries on the Nyāyamukha, but the only thing that remains is the fragment of a work by Wonhyo 元曉. In Japan, commentarial work was also extensive, with much earlier influence coming from Zenju 善珠. Some of these major works are listed below (further detailed bibliographical information is provided under 能立):

理門論述記T 1839 (1 fasc.) by Shentai 神泰

因明入正理論疏T 1840 (3 fasc.) by Kuiji 窺基

判比量論 HBJ 1.814-816 (fragment) by Wonhyo 元曉

因明義斷; T 1841 (1 fasc.) by Huizhao 慧沼

因明入正理論義纂要T 1842 (1 fasc.) by Huizhao 慧沼

因明論疏明燈鈔 T 2270 (12 fasc.) by Zenju 善珠

因明大疏抄T 2271 (41 fasc.) by Zōshun 藏俊

因明大疏融貫鈔T 2272 (9 fasc.) by Kiben 基辯

因明大疏導T 2273 (3 fasc.) by Myōsen 明詮

因明大疏裏書T 2274 (6 fasc.) by Myōsen 明詮

For the tradition of Buddhist logic overall, Dharmakīrti 法稱 is also important as the influential successor to Dignāga. Unfortunately, his great treatises, such as the Nyāyabindu and the Pramāṇavārttika were never translated into Chinese, and thus his influence in East Asia was minimal as compared with India, and later, Tibet. The Sanskrit Hetu-vidyā is transliterated as 醯都費陀.

Modern Works:

Chi, R.S.Y. 1969. Buddhist Formal Logic: A Study of Dignāgaʼs Hetucakra and K'uei-chiʼs Great Commentary on the Nyāyapraveśa. London:  Royal Asiatic Society.

Iida Shotaro. 1980. Reason and Emptiness: A Study in Logic and Mysticism. Tokyo:  Hokuseido.

Keith, Arthur Berriedale. 1921. Indian Logic and Atomism; An Exposition of the Nyāya and Vaicesika Systems. Oxford:  Clarendon Press.

Stcherbatsky, Theodore. 1930-32. Buddhist Logic. Leningrad:  Akademii Nauk.

Tachikawa, Musashi. 1971. “A Sixth-century Manual of Indian Logic.” Journal of Indian Philosophy 1 : 111–145.

Tillemans, Tom J.F. 1999. Scripture, Logic, and language: Essays on Dharmakīrti and his Tibetan Successors. Boston:  Wisdom Publications.

Tucci, Giusseppe. 1930. The Nyāyamukha of Dignāga. Leipzig and Heidelberg:  Otto Harrasowitz.

Vidyabhūsana, Satis Chandra. 1921. History of Indian Logic. Calcutta:  University of Calcutta.

Wayman, Alex. 1999. A Millennium of Buddhist Logic. Delhi:  Motilal Banarsidass.