306. Amīr al-mu’minīn, peace be upon him, said: The fixed limit of life is enough to remain watchful. 
 The idea here is that lighting may flash a hundred thousand times, tempests may rise, the earth may quake and mountains may collide with each other, but as long as the fixed period of life has yet to run its course no occurrence can cause any harm, nor the typhoon of death put out the flame of life; for there is a fixed hour for death, and nothing can cut it short before that time. In this way, obviously death itself is the watchman and guardian of life.
The hemistich of a verse says : What is known as death is the watchman of life.