Undoubtedly the soul is present in the heart of the living entity, and it is the source of all the energies for maintaining the body. The energy of the soul is spread all over the body and this is known as consciousness. On account of this consciousness spreading of energy of the soul all over the body, you can feel pains and pleasures in any part of the body. The soul is individual and he is transmigrating from one body to another, just as a person transmigrates from babyhood to childhood, from childhood to boyhood, from boyhood to youthhood and then to advanced old age. Then the change called death takes place when we change to a new body just as we change our old dress to a new dress. This is called transmigration of the soul.
When a soul wants to enjoy this material world, forgetting his real home in the spiritual world, he takes this life of hard struggle for existence. This unnatural life of repeated birth, death, disease and old age can be stopped when his consciousness is dovetailed with the Supreme consciousness of God. That is the basic principle of our Krishna Consciousness Movement.
So far as heart transplant is concerned, there is no question of success unless the soul is there in the heart. So it has to be accepted. Just like in sexual intercourse, if there is no soul there is no conception, no pregnancy. Contraception means to make the womb deteriorated so that it no longer is a good place for the soul. That is against the order of God. By the order of God a soul is sent to a particular womb and by this contraceptive, he is denied that womb and has to be placed in another. That is disobedience of the Supreme. Just as a man who is supposed to live in a particular apartment. If the situation there is so disturbed that he cannot enter the apartment then he is put at a great disadvantage. That is illegal interference and is punishable.
The undertaking of "soul research" would certainly mark the advancement of science. But advancement of science will not be able to find out the soul. It can simply be accepted on circumstantial understanding. You will find in the Vedic literature that the dimension of the soul is one ten-thousandth times smaller than the point. The material scientist cannot measure the length and breadth of a point. Therefore it is not possible for the material scientist to capture the soul. You can simply accept the soul's existence by taking it from authority. What the greatest scientists are finding we've explained long ago. As soon as one understands the existence of the soul, he can immediately understand the existence of God. The difference between God and the soul is that God is a very great soul and the living entity is a very small soul, but qualitatively they are equal. Therefore God is all-pervading and the living entity is localized. The nature and quality is the same.