Krishna has just stunned Arjuna by claiming he taught this yoga to Vivasvan (the sun-god) at the dawn of creation. Arjuna is confused: Krishna was born in this age, Vivasvan long before. How can that be?
aparaṃ bhavato janma paraṃ janma vivasvataḥ | katham etad vijānīyāṃ tvam ādau proktavān iti ||
Your birth is recent; the birth of Vivasvan was much earlier, in the distant past. How am I to understand this — that you taught him at the beginning?
Arjuna's question is not a failure of faith. It is what a careful, honest mind does when a claim exceeds its categories. He does not dismiss. He asks. That is exactly the right move, and Krishna will reward it with one of the Gita's most direct statements about identity across time.
The confusion here is specifically about time. Arjuna's mental model places every person inside a single, bounded timeline. Krishna's claim violates that model. What is about to be resolved is not a factual dispute but a mistaken assumption about what a 'self' is and whether it is exhausted by the body it currently occupies.
Read from inside: Arjuna is addressing the surface of Krishna, the biographical person born in Mathura. He has not yet learned to address the deeper faculty that Krishna represents. The question 'how could you have taught this then?' is the voice of surface-mind genuinely baffled by something coming from deeper than surface-mind. The answer, when it comes, will be about that depth, not about history.
This verse marks a specific stage in a conversation's arc: the moment when the questioner has been given something their current framework cannot hold, and they say so. That moment is not an obstacle to understanding. It is understanding beginning. The frame has to be acknowledged as insufficient before a better one can be offered.
Person A hears a longtime mentor say 'I have always known this about you, even before we met' and feels unsettled, maybe even a little insulted: that cannot be right, we met in 2019, you barely knew me. The continuity of the self feels threatened by a claim that exceeds it. Person B hears the same words and pauses. Not to agree or disagree, but to notice that their model of 'who I am' is tied entirely to biographical memory, and that this may be a smaller container than what is actually here. They ask, with genuine curiosity: how do you mean that? And the conversation goes somewhere real.
Krishna responds directly in verse 4.5, opening with 'Many births of mine have passed, and of yours too' and begins explaining how he knows his own past lives even though Arjuna does not know his. The teaching on eternal identity begins in earnest. When ready, say: "4.5"