Nyāya Sūtras 2.5

2.5

tatra yasyepsājihāsāprayuktasya pravṛttiḥ sa pramātā, sa yenārthaṃ pramiṇoti tat pramāṇam, yo 'rthaḥ pramīyate tat prameyam, yad arthavijñānaṃ sā pramitiḥ, catasṛṣu caivaṃvidhāsv arthatattvaṃ parisamāpyate/
kiṃ punas tattvam? sataś ca sadbhāvo 'sataśüEcāsadbhāvaḥ/ sat sad iti gṛhyamāṇaṃ yathābhūtam aviparītaṃ tattvaṃ bhavati/

Among these, he who is prompted to activity by the desire to obtain or avoid is the cognizer; that by which he cognizes an object is the means of valid cognition; the object which is cognized is the object of valid cognition; that knowledge of the object which arises is the valid cognition; and the reality of things is fully contained within these four. What, then, is reality? It is the existence of the existent and the non-existence of the non-existent. The existent, being grasped as existent, is reality—accurate and non-erroneous.