देहिनोऽस्मिन्यथा देहे कौमारं यौवनं जरा।तथा देहान्तरप्राप्तिर्धीरस्तत्र न मुह्यति॥
dehino ’smin yathā dehe kaumāraṃ yauvanaṃ jarātathā dehāntara-prāptir dhīras tatra na muhyati
Word by word
The verse says
Just as the one who lives in this body passes through childhood, youth and old age, so it passes into another body. The steady person is not shaken by this.
In plain words
This is Krishna's first real argument, and it is cleverer than it looks. He does not begin by asking Arjuna to believe anything new. He points at something Arjuna has already lived through.
You were a child once. That body is gone. Not changed - gone. Almost nothing of it is still here. Then you were young, and that body is going too. You have already survived being replaced, several times over, without once feeling that you ended.
So, Krishna says, you already accept that the body can be swapped out while you carry on. Death is one more swap. He is not asking for a leap of faith. He is asking you to keep counting.
Where you've felt this · in growing older
Find a photograph of yourself at six years old. Not one atom in that picture is still in you. The height is wrong, the voice is wrong, the face is wrong. You would not recognise the child in a crowd. And yet you look at it and say, without hesitating, "that's me." Something has stayed. You have never been able to point at it.
Going deeper
The claim is that something in you is the same at six years old and at sixty, and that it is none of the parts that changed.
Krishna never says you have a body. He says dehin - the one who dwells in a body. A dweller is not the house.
The other word worth catching is dhīra, usually translated as wise. It actually means steady, firm, hard to shake. He is not promising that understanding this will make you clever. He is saying it will make you difficult to knock over.
And notice what he does not claim. He never says grief is foolish, or that you should feel nothing when someone dies. He says the steady person is not muhyati - not bewildered, not thrown into confusion. Sorrow and panic are different things. Arjuna on the floor of that chariot is not grieving; he has lost his footing.
That is what is being treated, and the argument is deliberately modest. It is one thought, solid enough to hold on to, handed to a man who cannot currently stand up.