Dr. Singh.  When the doctors perform a heart transplant, they take a heart from someone who has just died and exchange it for a weak heart in someone else's body. Does the soul from the dead heart change places with the soul in the weak living heart?
Śrīla Prabhupāda.  No. The soul has already left the dead heart. There is no question of bringing in another soul.
Dr. Singh.  Let me see if I understand you correctly. When the doctors remove the heart of a man who has just died, the soul has already left his heart. Then, when they transplant his dead heart into the body of the patient, the patient's soul passes into the transplanted heart.
Śrīla Prabhupāda.  Yes. The soul is destined to live in a particular body for a certain number of years. You may change whatever part of the body you like, but you cannot change the duration of the life of that body.
Dr. Singh.  So the heart is just a machine-an instrument?
Śrīla Prabhupāda . Yes. It is the sitting place of the soul.
A Mustard Seed in a Bag of Mustard Seeds
Dr. Singh.  Śrīla Prabhupāda, biologists tell us that there are many species of life that can reproduce without sex. Do the  Vedas  agree with this?
Śrīla Prabhupāda . Oh, yes..
Dr. Singh.  So we cannot curtail their reproduction?
Śrīla Prabhupāda.  No. How could we? There are so many living entities that have come into this material world to enjoy life, and therefore reproduction must continue. This material world is like a jail. You cannot put an end to jails. As soon as one man leaves the jail, another is ready to enter it. This same question was discussed by Lord Caitanya Mahāprabhu.[21] One of His devotees, Vāsudeva Datta, said, "Please take all the living entities of this entire universe and release them from material bondage. And if You think they are too sinful to be rectified, then just give all their sins to me." But Caitanya Mahāprabhu said, "Suppose I were to take this entire universe and all the living entities with it. This is only one of innumerable universes. It is just like a mustard seed in a huge bag of mustard seeds. If you removed one seed from the bag, what would be the difference?" So reproduction cannot actually be stopped. Living entities are unlimited in number, and therefore it must continue.
Dr. Singh.  You have said that this material world is like a correction house to teach one to get out of material entanglement and the cycle of repeated birth and death.
Šrīla Prabhupāda . Yes. Therefore you must practice Kṛṣṇa consciousness.