Nyayasutra 69.4

69.4

yatra yatra saṃśayapūrvikā parīkṣā śāstre kathāyāṃ vā, tatra tatraivaṃ saṃśaye pareṇa pratiṣiddhe samādhir vācya iti/ ataḥ sarvaparīkṣāvyāpitvāt prathamaṃ saṃśayaḥ parīkṣita iti//

Wherever an examination is preceded by doubt, whether in a treatise or in a philosophical discussion, there the reply is to be stated in this way whenever doubt is denied by the opponent. Therefore, because it pervades all examination, doubt has been examined first.