In the Name of Allah, The Most Gracious, Most Merciful
The soul is man's driving force?
Praise be to Allah, The Lord (The One and Only) of the Worlds, and blessings and peace be upon our Master Muhammad, the Truthful and the Faithful.
The soul likens electricity. When is the light turned off? This happens in two cases: either by turning off the switch, or by breaking the lamp. Similarly, killing likens breaking the lamp, while natural death likens turning off the Divine power given to man by Allah.
Natural death happens when the power supply, Divinely given to the body, is turned off. Had we weighed the body of a dead man by milligram units after his soul is taken, his weight would have never lost not even one milligram. This means that the soul is the driving power due to which the brain thinks, the liver functions and the kidneys work. Thus, every system in the body has very complicated functions, and it works due to the power of the soul. However, when man reaches his term, the Divine supply (i.e. the soul) comes to an end.