Katha-2-3-12-naiva vācā

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

If the Brahman could be perceived by the operation of the intellect, etc., it can be specifically apprehended as this or that. When the intellect, etc., cease to act, Brahman which is not cognised, in the absence of the cause of cognition, does certainly not exist; for, it is only when anything is perceived by the instruments of cognition, it is reputed in the world to exist; and the contrary is said to be non-existing; and, therefore, yôga is useless; or, the Brahman not being known should be known as non-existing. If it is thus urged, it is thus replied ‘true not by speech, not by mind, not by the eye, not by other senses, could he be known; still, though devoid of any attributes, being known as the origin of the universe, he certainly exists; for, that into which effects are absorbed must certainly exist; for, here also, this effect traced back in the ascending series of subtlety leads only to the conviction of something as existent (in the last resort).’ The intellect, even in the ultimate analysis of all the objects of perception, is still pregnant with a belief in the existence of something; for, the intellect is our authority in the comprehending of the real nature of existence and non-existence. If the universe had no existing cause, then the effect (the world) being inseparably connected with non-existence would be apprehended as nonexisting. But this is not so; it is perceived as existing only as pot, etc., made of earth, is perceived in combination with earth. Therefore, the cause of the world, the âtman, must be known as existing. Wherefore? In any other than him who believes in existence, follows the drift of the agâmâs and is possessed of faith, i.e., one who argues that there is no âtman, the source of the universe, and that all this effect not connected with any cause is absorbed into nonexistence and who thus sees perversely, how can that Brahman be truly known? The meaning is it cannot at all be known.

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