Krishna has just stated that the self neither slays nor is slain. Now he pushes deeper, unpacking what that means: the self has no birth event, no death event, and no moment of becoming something other than what it already is.
na jāyate mriyate vā kadācin nāyaṃ bhūtvā bhavitā vā na bhūyaḥ | ajo nityaḥ śāśvato 'yaṃ purāṇo na hanyate hanyamāne śarīre ||
This self is never born and never dies at any time. It did not come into being and will not come into being again. It is unborn, eternal, always-existing, and ancient. It is not killed when the body is killed.
Arjuna is grieving because he thinks killing a body kills a person. Krishna's whole argument here depends on dismantling that equation. This is not a metaphysical detour. It is the direct response to the presenting problem. The grief is real; the premise underneath the grief needs examination.
Notice Krishna does not say 'your self will survive.' He says 'the self is not born, does not die.' This is description, not reassurance. He is not offering comfort. He is pointing at something he is claiming Arjuna already is, and asking him to look at it clearly instead of past it.
The verse systematically removes every time-based predicate: beginning, ending, change over time, age. What is left is something that cannot be located on a timeline. Most of our fear about death is fear of a future event. But if the self is not in time, then 'future' and 'event' do not apply to it. The fear is structurally misaimed.
This verse is sometimes read as training in detachment from the body. That is not quite the move. The move is re-identification: stop taking yourself to be the body, and recognize what you actually are. Detachment is a practice you do with effort. Recognition is seeing something already true. Krishna is asking for the second thing.
If we read Krishna as the steadier interior of the mind addressing its own turbulent surface, then this verse is the integrating intelligence pointing inward and saying: 'You, the one watching all this panic, were never born. You do not die when the performance ends.' The chaos on the surface is real. But the witness of the chaos is untouched by it.
Person A is identified with their role, reputation, and continuity of story. A career setback feels like partial death. A relationship ending feels like a piece of themselves is gone. They live in a constant low-grade fear of the losses that are coming, because everything they call 'me' is made of things that can be taken. Person B has bumped up against something in themselves that did not change even when everything around them collapsed. They cannot quite explain it, but they noticed it: something watching the whole unraveling, unraveled. They do not have a doctrine about it. They just remember it when the fear starts, and it takes some of the power out of the fear.
Verse 2.21 follows immediately with a rhetorical question: if a person knows the self is indestructible and unborn, who exactly is being killed, and who is doing the killing? Krishna tightens the logic he has just laid down. When ready, say: "2.21"