“That which pervades the entire body is indestructible. No one is able to destroy that imperishable soul.” (Bhagavad-gita 2.17)
Here Krishna is giving a practical way of understanding the presence of the soul. He says that the one thing which is spread all over the body is immortal. So what is that which is spread all over the body? It is not the skin, the bone, the marrow, or the blood. The active principle within the body is consciousness or the soul.
Nowadays, by medical science they can replace bones, flesh, blood, all of the organs. It seems that everything can be replaced by scientific advancement. But that thing which is immortal, when gone, it cannot be replaced. Nowadays they can even replace the heart, but no medical science, or any science, is able to replace life. So Krishna is giving an example that the presence of the soul is perceived by consciousness. The flesh may be there, the bone may be there, the blood may be there, but if the consciousness is not there, there is no life. When the consciousness is in the body, the sensation of pain and pleasure can be experienced. Without consciousness, the body is a pile of dead matter.
Krishna teaches here that the consciousness is the symptom of the soul and it is immortal; it does not perish when the body perishes.
From Srila Prabhupada’s lecture on Bhagavad-gita 2.17 — Mexico, February 17, 1975