The mind can be thought of as many things. Among them being an interpretive filter.
Instead of interpreting external things we can--through strong emotional misconception--project outward and interpret our projection as having come from the outside.
If you are, for example, afraid of snakes and you go out into an area well-known for being "snakey" you might well jump out of your skin if you see a piece of coiled rope or a stick shaped a certain way.
The fear is real although the coiled rope isn't.
The same is true of belief in demons, spirits, ghosts, supernatural "powers'.
Your fear creates an emotional transmission and you'll feel the pre-echo by hallucinating something from "outside" that is really from the inside.
Once you don't believe in such things any long it will be almost impossible to fear the demons!