न जायते म्रियते वा कदाचि न्नायं भूत्वा भविता वा न भूयः।अजो नित्यः शाश्वतोऽयं पुराणो न हन्यते हन्यमाने शरीरे॥
na jāyate mriyate vā kadācinnāyaṃ bhūtvā bhavitā vā na bhūyaḥajo nityaḥ śāśvato ’yaṃ purāṇona hanyate hanyamāne śarīre
Word by word
The verse says
It is not born and it does not die. It did not come into being and will not stop being. Unborn, constant, always there, ancient — it is not killed when the body is killed.
In plain words
The most famous description of the self in the book, and it works entirely by negation.
Not born. Does not die. Did not begin. Will not end. Krishna is not telling you what it is. He is clearing away everything it is not.
The last line is the one that answers Arjuna: when the body is killed, this is not killed.
Where you've felt this · in creative work
Trying to describe a piece of music to someone who has not heard it. You end up saying what it is not like. That is not a failure of vocabulary. Some things are only reachable by clearing space around them.
Going deeper
The method here has a name in Indian philosophy: neti neti — not this, not this. You approach what cannot be described by removing what it is not.
There is a reason for that, and it is not evasion. Every positive description would use terms drawn from the world of changing things — and that is precisely the category being excluded. To say the self is like anything is already to have mis-stated it.
The word purāṇa — ancient — is interesting company for nitya, eternal. Not just old. Prior. Whatever this is, it was there before the terms you are using to ask about it.
And it is worth stepping back to see what Krishna has done over verses 11 to 20. Arjuna's grief rested on a single assumption: that he is about to end these people. Krishna has not disputed the deaths. He has not said the war is good. He has gone after one assumption and taken it apart from every direction.
What remains is the question Arjuna actually has to answer, and the rest of the chapter turns toward it: if you are not the author of the outcome, what exactly is yours to do?