Life force
Life force is a concept that was used in the past to explain what happens when a person dies i.e., he looses his life force. A force is a popular term in science because physics is based on forces. Complex chemistry is occurring in the body of a live person. When the body is alive many processes are occurring like the absorption of oxygen by the lungs, its transport to the different cells in the body through the veins, arteries, and capillaries by pressure (force) created by the pumping of the heart. The interactions of atoms and molecules in the body are produced by the electromagnetic forces. There is no specific "life force" in the body there is just the complex functioning of the body produced by known physical forces. When the body dies chemistry still occurs but because the major organs no longer function properly there is no oxygen nor other chemical needed by the cells to keep the cell functioning properly. The complex functioning that the body was producing is now much less complex. There is no life force lost upon death, what is lost is the complex functioning of matter that is required to produce the before death animated complex behavior, consciousness and ixperiencitness. What is necessary to restore the "life force" to a body is to restore this high level of functioning that a body had before death. We can remove the term "life force" and simple state that what is necessary to restore the body to a living state again after death is to nearly to exactly restore the high level of functioning that a body had before death. To restore the same or close consciousness that the person had before death, to the after death body, requires the restoration of the right structure and then functioning of the brain. If the structure and functioning of the after death body is different enough from the before death body then a different consciousness will be produced. This different consciousness can still be one that the before death person would experience after death. But it might not be a coherent continuity from the consciousness that existed right before death. If the structure and functioning of the after death body was very different from the before death body but still functioning complex enough to be alive and well, the after death body might produce a consciousness that does not have the same ixperiencitness as the before death body.