In plain words
When anything is placed in the fire with mantras, it too takes on the very form of the fire.
As the Food, So the Understanding
As the flame, so the smoke. As the smoke, so the cloud; as the cloud, so the rain. As the rain, so the crop; as the crop, so the cooking; as the cooking, so the head. That is why understanding depends on the cooking. With sāttvic food, sāttvic qualities grow.
The Sword of Jñāna
Next, the sword of Jñāna. The six inner foes grow into a mighty tree and drive humanness out of mind. With the sword of Jñāna we must cut down those trees and lay a path to a life of peace. So in the fourth chapter, the Jñāna Yoga, explaining the sacred varṇas and the yajñas, Kṛṣṇa said, “What you must perform is Jñāna-yajña, nothing else.”
When women, men, boys and the aged perform this sacred yajña in the spirit of offering it to Īśvara, that very doing takes shape as the karma of Jñāna. For this yajña, virtue is the chief thing. The ground is the heart. The feelings themselves are the offerings, Ānanda the fruit. These alone are the karmas performed for Brahmānanda, for Paramānanda.
“Who am I? Where have I come from?” we must ask ourselves. No one belongs to anyone. These have come only from attachment and possessiveness. To look at this bond and fancy, “My friends, gurus,” is a mistake, it is madness. You are not the one who kills, nor the one who dies.
So What If an Old Garment Is Thrown Away?
Who grieves when an old garment is thrown away? These bodies are old garments. Then a doubt arose in Arjuna. “Swami, for the old, eighty or ninety years of age, one may speak of an old body; but how can even a young body be called an old garment?” he asked.
Kṛṣṇa smiled and answered thus.
Even in the Young, the Body Is Old!
“Arjuna! You have no authority to decide what is new and what is old,” said Kṛṣṇa.
Some time ago you brought home a piece of cloth. You put it away in the cupboard. In a while you forgot all about it; then one day, when you opened the cupboard again, there it was. You gave it to the tailor and had a new coat stitched. As you bent down over some task, the coat tore. Here the coat is certainly new. But the cloth is old stock. Yet to you it looks like a new coat. A body that has come through many births is old, whoever's it may be. The body is no more than an old garment that crumbles and falls away.
For a body that is born, dying is natural. To grieve over this is the mark of the ignorant. To root out Ajñāna, Jñāna is needed. Having taught Arjuna this kind of Jñāna-yajña, Kṛṣṇa leads Arjuna into the battle. Think of it not as fighting your kinsmen but as fighting the evil qualities within you. These evil qualities came in as thieves and now behave as masters. “Think of Me as you fight, to destroy the thieves,” Kṛṣṇa taught.