It is well known that Dignāga’s syllogism consists of three members, namely thesis (pakṣa), reason (hetu), and example (dṛṣṭānta). Dignāga rejects the Naiyāyika’s theory of the five-membered syllogism, and does not regard application (upanaya) and conclusion (nigamana) as members of the syllogism, as he considers these to be of no value. According to him, hetu and two types of dṛṣṭānta should be stated to show the three characteristics of a correct probans, that is, pakṣadharmatā, sapakṣasattva, and vipakṣāsattva. In the Pramāṇasamuccaya (-vṛtti), Dignāga explains that pakṣa, stated to show the object of reasoning, is not nessesary for a proof. However, he cannot completely abandon the statement of pakṣa, and states a pakṣa in his syllogisms.
Dharmakīrti’s syllogism drastically differs from Dignāga’s, as illustrated by a typical example as follows: “Whatever is produced is impermanent, like a pot, etc. And sound is produced.” Dharmakīrti’s syllogism consists of two members, vyāpti and pakṣadharmatā, and he never states a pakṣa. The statement of pakṣadharmatā may be regarded as hetu. However, unlike Dignāga, who states it with an ablative case as a reason, such as kṛtakatvāt, Dharmakīrti directly shows it, such as śabdaś ca kṛtakaḥ. The statement of vyāpti may be regarded as the dṛṣṭānta. Unlike Dignāga, who states two kinds of dṛṣṭānta, Dharmakīrti states only one kind. Moreover, the order of the two members is changed, with vyāpti stated first, and pakṣadharmatā stated second.
From an historical perspective, Dharmakīrti and his followers regard the statement of pakṣadharmatā in their syllogism as hetu, even though it seems to play the role of upanaya. By making the statement of pakṣadharmatā, the general rule established is applied to the subject of concern. Such a statement is nothing but upanaya, and some logicians of other schools point out that the statement of pakṣadharmatā stated in Dharmakīrti’s syllogism should be regarded as upanaya, not as hetu. Therefore, Dharmakīrti’s syllogism structually resembles Aristotle’s syllogism in that vyāpti and pakṣadharmatā correspond to the major premise and the minor premise, respectively.
