The fact is that each and every living entity is covered by two kinds of bodies, one subtle and one gross. So the subtle body causes the living entity to take another gross body. We have got experience that in dreaming the gross body is inactive while the subtle body is continuing activities; and again in deep sleep the subtle body is continuing activities. The living entity after being carried by the subtle body to another gross body, it sometimes remains in deep sleep in the womb of the mother. Then gradually the subtle body begins to act within another gross body. When this gross body is fully developed befitting to come out of the mother's body, that is called another birth. Again he changes to another gross body imperceptibly. Just like in the film thousands of pictures are moved imperceptibly. It looks that the body in the picture is moving, but the audience cannot understand that there are so many pictures in the spool. They think that the picture is moving. Similarly every second every moment we are changing the body.
Letter to Svarupa Damodara dasa, 23 June, 1975