WHEN someone’s body can no longer perform its functions in the natural world in response to the thoughts and affections of its spirit (which it derives from the spiritual world), then we say that the individual has died. This happens when the lungs’ breathing and the heart’s systolic motion have ceased. The person, though, has not died at all. We are only separated from the physical nature that was useful to us in the world. The essential person is actually still alive. I say that the essential person is still alive because we are not people because of our bodies but because of our spirits. After all, it is the spirit within us that thinks, and thought and affection together make us the people we are.
We can see, then, that when we die we simply move from one world into another. This is why in the inner meaning of the Word, “death” means resurrection and a continuation of life.
The deepest communication of our spirit is with our breathing and our heartbeat; thought connects with our breathing, and affection, an attribute of love, with our heart. Consequently, when these two motions in the body cease, there is an immediate separation. It is these two motions, the respiratory motion of the lungs and the systolic motion of the heart, that are essential ties. Once they are severed, the spirit is left to itself; and the body, being now without the life of its spirit, cools and decays.
The reason the deepest communication of our spirit is with our breathing and our heart is that all our vital processes depend on these, not only in a general way, but in every specific.
After this separation, our spirit stays in the body briefly, but not after the complete stoppage of the heart, which varies depending on the cause of death. In some cases the motion of the heart continues for quite a while, and in others it does not. The moment it does stop, we are awakened, but this is done by the Lord alone. “Being awakened” means having our spirit led out of our body and into the spiritual world, which is commonly called “resurrection.”
The reason our spirit is not separated from our body until the motion of the heart has stopped is that the heart answers to affection, an attribute of love, which is our essential life, since all of us derive our vital warmth from love. Consequently, as long as this union lasts there is a responsiveness, and therefore the life of the spirit is [still] in the body.
from Heaven and Hell, Sections 445-447