The Exorcist's Bestiary

You said that some people might see demons. That just sounds like a hallucination.

Hallucinations are, of course, real. They happen all the time. We are made of science. The Christian tradition works to discern between hallucinations (where the brain gets sick) and what we call open visions (where the Holy Spirit allows our brains to work in this way).

A Christian might even suspect an open vision to be a Holy Spirit-inspired, scientific hallucination. Because we live in the physical world of science, all supernatural things we experience can generally be processed in some kind of scientific way.