Nyāya Sūtras 149.4

149.4

yady anupalambhād āvaraṇaṃ nāsti, āvaraṇānupalabdhir api tarhy anupalambhāt nāstīti tasyā abhāvād apratiṣiddham āvaraṇam iti/ kathaṃ punar jānīte bhavān nāvaraṇānupalabdhir upalabhyata iti?

[Opponent:] If obstruction does not exist because it is not apprehended, then the non-apprehension of obstruction also does not exist because it is not apprehended; hence, in the absence of that [non-apprehension of obstruction], obstruction is not refuted. [Proponent:] But how do you know that the non-apprehension of obstruction is not apprehended?