नैषा तर्केण मतिरापनेया प्रोक्तान्येनैव सुज्ञानाय प्रेष्ठ ।
यान्त्वमापः सत्यधृतिर्बतासि त्वादृङ्नो भूयान्नचिकेतः प्रष्टा ॥ ९ ॥

naiṣā tarkeṇa matirāpaneyā proktānyenaiva sujñānāya preṣṭha |
yāntvamāpaḥ satyadhṛtirbatāsi tvādṛṅno bhūyānnaciketaḥ praṣṭā || 9 ||

9. This idea cannot be reached by mere reasoning. This idea, Oh dearest, leads to sound knowledge, only if taught by another; thou hast reached it; Oh, thou art fixed in truth. May we find, Oh Nachiketas, a questioner like thee!

Shankara’s Commentary:

This idea of the atman knowable by the agamas and taught by a preceptor who is one with the atman is not to be attained by dint of one’s intelligent reasoning; or, it may mean, cannot be dispelled by mere intelligent reasoning; for, a logician not versed in the agamas will postulate something created by his own intelligence. Therefore alone, this idea arising from the agamas helps one, Oh dearest, to sound knowledge, only when taught by a preceptor who is not a logician and who is conversant with the agamas. What is that idea which can not be attained by reasoning is explained. That idea which you have now attained by my granting of the boon. Oh thou art fixed in truth. Death says this of Nachiketas with sympathetic favour, for the purpose of enlogising the knowledge which he was going to inculcate. May we find, Oh Nachiketas, a questioner like thee, a son or a disciple.