Verse 1.2.8

न नरेणावरेण प्रोक्त एष सुविज्ञेयो बहुधा चिन्त्यमानः ।
अनन्यप्रोक्ते गतिरत्र नास्त्यणीयान् ह्यतर्क्यमणुप्रमाणात् ॥ ८ ॥

na nareṇāvareṇa prokta eṣa suvijñeyo bahudhā cintyamānaḥ |
ananyaprokte gatiratra nāstyaṇīyān hyatarkyamaṇupramāṇāt || 8 ||

8. This atman now explained cannot easily be known, of taught by a person of inferior intellect being variously regarded. When it is taught by a preceptor, one with the Brahman, there is no further travel, his being subtler than the subtle and not arguable.

Shankara’s Commentary:

Com.—Why so? Because, taught by a man of worldly understanding, the atman which you ask me about, is not easily knowable, because he is variously discussed by disputants, whether he exists or not, whether he is a doer or not, whether he is pure or not and so forth. How then can he be well-known is explained. If the atman is taught by a preceptor who is free from the notion of duality and who has become one with the Brahman, none of the various doubts exists, such as whether he is or not, etc., because the nature of the atman absorbs all such doubtful alternatives; or, the text may be thus construed: when the atman, which is none other than his own Self, is taught, there is no knowing any other thing; for, there is no other knowable; for, the knowledge of the oneness of the atman is the highest state of knowledge. Therefore, there being nothing else to be known, knowledge stops there; or, Gatiratra nasti, may mean there is no travelling into Samsara when the atman, not distinct from the Self has been taught; because, emancipation, the fruit of such knowledge, is its necessary concomitant. Or, it may mean that when the atman is explained by a preceptor who is become one with the Brahman to be taught, there is no failing to understand it. The meaning is that as in the case of the preceptor, the hearer’s knowledge of the Brahman will take the form, ‘I am not other than that.’ Thus the atman can easily be known when explained by the preceptor versed in the agamas, to be no other than one’s self; otherwise, the atman will be subtler than even the subtle and cannot be known by dint of one’s mere intelligent reasoning When the atman is established by argument to be something subtle by one man, another argues it to be subtler than that and another infers it to be something yet subtler; for, there is no finality reached by mere argumentation.