Katha-1-2-9-naiṣā tarkeṇa

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

This idea of the âtman knowable by the âgamâs and taught by a preceptor who is one with the âtman 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 âgamâs will postulate something created by his own intelligence. Therefore alone, this idea arising from the âgamâs helps one, Oh dearest, to sound knowledge, only when taught by a preceptor who is not a logician and who is conversant with the âgamâs. 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 Nachikêtas with sympathetic favour, for the purpose of enlogising the knowledge which he was going to inculcate. May we find, Oh Nachikêtas, a questioner like thee, a son or a disciple.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda