Nyāya Sūtras 2.16

2.16

tasyāḥ pṛthakprasthānāḥ saṃśayādayaḥ padārthāḥ/ teṣāṃ pṛthagvacanam antareṇādhyātmavidyāmātram iyaṃ syāt, yathopaniṣadaḥ/ tasmāt saṃśayādibhiḥ padārthaiḥ pṛthak prasthāpyate/ tatra nānupalabdhe na nirṇīte 'rthe nyāyaḥ pravartate, kiṃ tarhi? saṃśayite 'rthe/ yathoktaṃ --- ``vimṛśya pakṣapratipakṣābhyām arthāvadhāraṇaṃ nirṇayaḥ'' iti/ vimarśaḥ --- saṃśayaḥ/ pakṣapratipakṣau --- nyāyapravṛttiḥ/ arthāvadhāraṇaṃ --- nirṇayas tattvajñānam iti/ sa ca ``ayaṃ kiṃsvid'' iti vastuvimarśamātram anavadhāraṇaṃ jñānaṃ saṃśayaḥ, prameye 'ntarbhavann evam arthaṃ pṛthag ucyate/

Doubt and the other categories are its distinct methodological subjects. Without their separate mention, this would be merely a science of the soul, like the Upaniṣads. Therefore, it is established separately by means of categories such as doubt. In that regard, reasoning proceeds neither concerning an unapprehended object nor concerning an ascertained object. Concerning what, then? Concerning a doubted object. As has been stated: 'Ascertainment is the determination of an object by means of a thesis and counter-thesis after deliberation.' Deliberation is doubt; thesis and counter-thesis are the operation of reasoning; the determination of the object is ascertainment, which is knowledge of reality. And that doubt, being merely a deliberation upon an entity—a non-determinate cognition in the form 'What might this be?'—though included within the object of valid cognition, is stated separately for this purpose.