Symbiotic Theory is a theory used to explain the relationship between the Fears and humanity.
The theory goes that since The Fears have existed for countless millenia, they have made such a significant mark on the world that if they ceased to exist, the world would quickly die off. In other words, Earth's ecosystem would be severely impacted by the sudden disappearance of The Fears in the same way a species that others animals depend on for survival being removed from their environment would lead to their extinction. The only difference is that the symbiotic nature believed to exist between humanity and the Fears is one of a much more metaphysical nature. For example, if someone found a way to kill The Archangel, the effects would be catastrophic and wide reaching. In some stories, the afterlife could simply cease to exist and the souls it has collected would dissipate. In others, the souls could be set free, trapping them in the mortal realm and making stories about ghosts a reality.