Verse 1.2.18

न जायते म्रियते वा विपश्चिन्नायं कुतश्चिन्न बभूव कश्चित् ।
अजो नित्यः शाश्वतोऽयं पुराणो न हन्यते हन्यमाने शरीरे ॥ १८ ॥

na jāyate mriyate vā vipaścinnāyaṃ kutaścinna babhūva kaścit |
ajo nityaḥ śāśvato'yaṃ purāṇo na hanyate hanyamāne śarīre || 18 ||

18. The intelligent atman is not born, nor does he die; he did not come from anywhere nor was he anything, unborn, eternal, everlasting, ancient; he is not slain though the body is slain.

Shankara’s Commentary:

Com.—The particle ‘Om’ has been pointed out as a prop of and as a substitute for the atman, 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 atman, 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 atman, 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 atman came not from anything, i.e., from any other cause; nor did any other real thing proceed from this atman; therefore, this atman 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 atman 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 akas.