Katha-1-2-18-na jāyate

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

The particle ‘Om’ has been pointed out as a prop of and as a substitute for the âtman, asked about in the text beginning with ‘Anyaira dharmat,’ etc., and devoid of all attributes, for the benefit of the ignorant and the middling class of men who wish to attain the Brahman, manifested and unmanifested. Now, this text is indroduced for the purpose of directly ascertaining the real nature of the âtman, to attain whom the word ‘Om’ was mentioned as a prop; he is not born, i.e., produced; nor does he die; various modifications are incidental to a thing which is produced and not eternal. Of those, the first and the last modifications namely birth and death are at the outset denied of the âtman, with the object of denying all modifications by the expressions ‘he is not born nor does he die.’ Vipaschit, intelligent; for, he is by nature of indestructible intelligence. Again, this âtman came not from anything, i.e., from any other cause; nor did any other real thing proceed from this âtman; therefore, this âtman is unborn, eternal, everlasting, undecaying (for, whoso is not everlasting decays; but he is everlasting); therefore, ancient, i.e., new, even formerly; (for, that which undergoes a development of its parts, is then said to be new); for instance a pot, etc.; but the âtman who is of a contrary nature is ancient, i.e., incapable of development; this being so, he is not slain or affected, even though the body is slain by swords, etc. Though in it, he is in it like the âkâs.

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