One of the great difficulties in life is the inevitable downward curve in all growth process. Life is growth, and growth is a continuum of movement that goes in a fluctuating line. Each down brings a new up; each up must bring a new down in order to go up again. There can be no upward movement unless there is first a downward one. Thus, there can be no life unless it has gone through a form of death.
The principle I explain here holds true on all levels. It is indeed ascertainable on the physical level. The physical system, like all other systems or planes, also strives toward wholeness and health. When a disturbing force pulls in an opposite direction, the pull of the two directions creates the pain. You can tell that this is what actually causes the pain because when the struggle is given up and the individual lets go and gives in to the pain, the pain stops.