The “human soul” is that “all-unifying agency” that connects “the various organs and members, the parts and elements, that constitute the body of man” and “causeth them to function in perfect harmony and with absolute regularity, thus making the continuation of life possible.”

By nature is meant those inherent properties and necessary relations derived from the realities of things. And these realities of things, though in the utmost diversity, are yet intimately connected one with the other. For these diverse realities an all-unifying agency is needed that shall link them all one to the other. For instance, the various organs and members, the parts and elements, that constitute the body of man, though at variance, are yet all connected one with the other by that all-unifying agency known as the human soul, that causeth them to function in perfect harmony and with absolute regularity, thus making the continuation of life possible. The human body, however, is utterly unconscious of that all-unifying agency, and yet acteth with regularity and dischargeth its functions according to its will. 
(‘Abdu’l-Baha, ‘Tablet to August Forel)