Katha Upanishad Verse 1.2.8

Yama explains to Nachiketas that the knowledge of the Atman is extremely subtle and cannot be easily grasped if taught by someone of “inferior intellect” or through mere intellectual reasoning. However, when revealed by a preceptor who is themselves established in Brahman, all doubts and “further travel” (the cycle of birth and death) cease, for the Self is subtler than the subtlest and beyond logic.

Sanskrit Verse

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

Transliteration

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

Translation

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.