Nyāya Sūtras 8

8.2

atha śabdaḥ ---

8.3

āptopadeśaḥ śabdaḥ ||

Verbal testimony is the instruction of an authoritative person.

8.4

āptaḥ khalu sākṣātkṛtadharmā yathādṛṣṭasyārthasya cikhyāpayiṣayā prayukta upadeṣṭā/

An authoritative person is indeed an instructor who has directly realized the truth and is impelled by the desire to communicate an object just as it was observed.

8.5

sākṣātkaraṇam arthasyāptiḥ, tayā pravartata ity āptaḥ/ ṛṣyāryamlecchānāṃ samānaṃ lakṣaṇam/

Direct realization of an object is authority; one who acts by means of that is an authoritative person. The definition is common to seers, noble persons, and foreigners;

8.6

tathā ca sarveṣāṃ vyavahārāḥ pravarttanta iti/ evam ebhiḥ pramāṇair devamanuṣyatiraścāṃ vyavahārāḥ prakalpante, nāto 'nyatheti//7//

and thus the everyday practices of all proceed. In this manner, by means of these means of valid cognition, the everyday practices of gods, humans, and animals are made possible, not otherwise. ||7