It's a matter of perception, I'd say.
A normal person has eyes, bodily senses, and is able to perceive light and objects, but lacks the "spiritual senses" as it were to perceive in other realms.

Some persons have those "spiritual senses", either because they developed them, or through some sort of atavism, something which can randomly be there and go away again.

Put like this, is it easier to see what I mean ?