Kṛṣṇa is giving a practical way of understanding the presence of soul. He says, yena sarvam idaṁ tatam: "One thing, which is spread all over the body, that is avināśi." That means immortal. So what is that which is spread all over the body? It is not the skin, not the . . . This is also spread over the body. The skin, the bone, the marrow, the blood—they are also spread over the body. But Kṛṣṇa is not meaning these material things. The actual active principle within this body is the spirit soul. Now, in medical science, bones are being replaced; flesh also being replaced; blood is also being replaced by injection. So everything can be replaced materially by scientific improvement. But when that thing which is immortal, that is gone, it cannot be replaced. Even nowadays the heart is being replaced, but no medical science or any science has ever been able to replace the life.
So he's giving the example that that presence of the soul is perceived by consciousness. So the flesh is there, the bone is there, the blood is also there, but when the consciousness is not there, that means it is finished. Because the consciousness is there, therefore any part of the body, if we pinch, we can feel pains and pleasure. And when the consciousness is not there, when the man is dead, the flesh is there, the blood is there and other ingredients are there, but there is no more life because it is useless now. Therefore Kṛṣṇa says that avināśi tu tad viddhi yena sarvam idaṁ tatam. The tatam means "spread." The consciousness is spread. If we pinch in this part, by consciousness you'll feel pain, or this part or any part of the body, because the consciousness is there, you feel pains and pleasure.
So Kṛṣṇa points out that this consciousness is immortal, avyaya. Avyaya means which is never annihilated, avyaya. Vyaya means there is whole thing, and you can take out part of it, but avyaya means you cannot take a part of it also; it is always wholesome. So this consciousness is avyaya, means never annihilated, and avināśi means immortal. So when this consciousness is transferred from this body to another body, that is called transmigration of the soul. But one thing we must know it, that as there is consciousness in my body, as there is consciousness in your body, similarly, there is consciousness in the whole universe. So long the consciousness is there in my body and the consciousness is there in your body or everyone's body, the body is very nice, very fresh, everything is going nicely. So as soon as the consciousness is gone, immediately this lump of matter begins to decompose. Similarly, this material cosmic manifestation is working very nicely—the sun is rising; the moon is rising; the seasons are changes; the air is blowing; the light is there; heat is there—why? Because there is big consciousness. As that small consciousness in your body and my body, keeping this body fit, similarly, there is another, greater consciousness, which is keeping this material world fit and working order.