Human beings' vision is very short-sighted, and they cannot see another type of "life" that naturally forms in this world, that is, ghosts.
However, there are always people who are gifted or given certain opportunities. They can see these "lives" that are different from their own, but this naturally comes with a price. At least, they have to protect the order of this "world".
If birds, animals, flowers, grass, insects, and fish have spirituality, they are demons; if vessels have intelligence, they are spirits; and dead lives that stay in the world for various reasons are ghosts. ——This is the exorcist's definition of "ghost". The evil ghosts commonly referred to by humans are mostly spirits and ghosts with unresolved grievances and vengeance towards humans. They are usually combined with demons with grievances and are collectively referred to as "demons". The exorcist's duty is to protect humans from demons and at the same time ensure that demons, spirits and ghosts do not interfere with the normal life of humans.