I will be not the first one who notes that the list of padārtha ‘categories’ at the beginning of the Nyāyasūtra is somehow strange.
Let me, therefore, repeat it here (the padārthas have been emphasised):
pramāṇaprameyasaṃśayaprayojanadṛṣṭāntasiddhāntāvayavatarka-nirṇayavādajalpavitaṇḍāhetvābhāsacchalajātinigrahasthānānāṃ tattvajñānān niḥśreyasādhigamaḥ
(the translation is purely indicative:)
The summum bonum is achieved through the knowledge of the reality of means of knowledge, objects of knowledge, doubt, purpose [of one’s investigation], example, conclusive view, parts [of the syllogism], reasoning, ascertainment, discussion [aiming at the establishment of truth], agonistic debate, sophistry, pseudo logical reasons, deceit, futile answer and points of defeat.
A first problem regards the fact that the first two items seem to already include the rest of the list, so that one does not really understand why others are separately listed. My usual attitude is to give credit to a text and try to make sense of it, but in this case, let us look at it through the eyes of a thought-provoking Indian philosopher, Daya Krishna, who wrote:
We have two different types of topics which have been discussed and enumerated as pramāṇa, prameya, saṃśaya, prayojana, dṛṣṭānta, siddhānta, avayava, tarka, nirṇaya on the one hand and vāda, jalpa, vitaṇḍā, hetvābhāsa, chala, jāti and nigrahasthāna on the other. The latter obviously relate to discussion and argument between persons while the former seem to be more internatl to the nature of the argument itself. The latter therefore have psychological elements intermixed with other things while the former seem to be more logical in character. (2004, p. 61)
The ‘extensional’ enumeration of the subject-matter of the Nyāya-sūtras, thus, is an amalgamation of two different discourses, the one relating to the forms of argumentation between different persons debating a point and winning or losing in the argument, and the other relating to the theory of proof of justification. The mixing of these different discourses in the first sūtra is a sign of a basic confusion in the mind of Gautama, the supposed author of the Nyāya-sūtra. (pp. 48–49).
Now, Daya Krishna is a philosopher and his words about the Nyāyasūtra should tell us more about his own philosophical position than about Nyāya itself. Nonetheless, the point seems to me, in this case, worth further reflection.