avinasi tu tad viddhi
yena sarvam idam tatam
na kascit kartum arhati [Bg. 2.17]
Now Krsna is giving a practical way of understanding the presence of soul. He says, yena sarvam idam tatam: "One thing, which is spread all over the body, that is avinasi." 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 Krsna is not meaning this 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 things 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 Krsna says that avinasi tu tad viddhi yena sarvam idam 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 Krsna points out that this consciousness is immortal, avyaya.
>>> Ref. VedaBase => Bhagavad-gita 2.17 — Mexico, February 17, 1975