I would like to continue my earlier thought by discussing the way the mind can produce things that are real, or at least seem as real to us as anything else. Reality is subjective. What is real is whatever our mind tells us is real. Our senses are not the primary means of detecting reality, our minds are.
For instance, have you ever walked right past someone you know and not noticed them? Maybe your friend says to you, indignantly, 'What, you can't even say hello?' And you respond in surprise, 'Oh, I'm sorry, I never saw you!' Your friend huffs and says, 'Yeah, yeah, you were looking right at me.'
Our eyes worked, but our mind turned the signal away to some dark corner because it was busy with something else at that moment. Our perception of reality comes from the mind, not our senses.
Another example: If you have a vivid nightmare, and wake up in a cold sweat, why did your body react with the fight-or-flight response? Because of something your senses were picking up? No, for nothing was actually happening. But your mind told your body something was happening, and thus your reality was formed.
Another example: Mental illness can produce symptoms that in times past would have been chalked up to demons. Hearing voices, having visions, and the like, can happen to a person with certain types of mental illnesses. Are those voices real to that person? Absolutely real. Is that reality subjective? Of course.
Another example: If you take LSD, will reality change for you? Yes. Will the world around you be changing, or is it only your mind that thinks it has changed? It is the mind. Your eyes may tell you that you are seeing a monster, but the eyes have been fooled by the mind. Your nose may smell something, but it's just the mind. Mess with the chemical balance of your mind, and your reality changes.
All of these examples show that something that is "real" to us, because I "saw" it, or "heard" it, or "felt" it, may not necessarily have an outside cause. And yet, and this is the key point, it will seem every bit as real to us as if there were an outside cause. Our senses will be telling us things that are indistinguishable from objective reality. It's real to us, and no one can tell us otherwise. For it really is real, at least to the mind.
What about more than one person perceiving this reality? Our mind is open to suggestion. In the middle ages, it became common to see the Virgin Mary in vision. Nobody saw aliens. In the twentieth century, visits by the Virgin Mary, while still around, were not as common. But alien visitations sure increased, as soon as science fiction writers began to write about them. If I write a book today about small, intergalactic woodchucks that have pink antennae on their heads, and the book becomes well known enough, pretty soon people will start to see intergalactic woodchucks at the foot of their beds late at night.
Now, and this is another important point, please do not mistake this message as saying that ALL such events can be explained away as tricks of the mind. Don't be offended and think I'm calling you crazy. I'm not. For one thing, these mind tricks happen to everyone, not just "crazy" people. For another, it's entirely possible that some of these events that have been related were caused by something real, objectively real. That brings me to my final point.
As Mommie Dark said, we don't know all there is to know about the phsyical universe. Theere may well be unexplained physical forces and phenomena in the world. Perhaps in time science will come to understand these new forces, and then we'll laugh and say, 'Oh, that's what caused those visions!'
Can you imagine a primitive human's reaction the first time he or she saw ball lightning? Try to tell that person this is a natural force, and you won't be believed. Clearly it was a demon! Or imagine not understanding weather patterns, and being confronted with your first thunderstorm. Can't tell you that's not the gods!
One day, we may feel the same about the stories in this thread. What at one time seemed like demon stories turned out to be nothing more than ___________, where that blank is yet to be filled in with a scientific explanation.
So remember, just because something was real to you (and I'm sure it was real to you), it doesn't mean your senses actually picked up on something, for it could have been a trick of the mind and there's absolutely no way for you to know the difference. Or maybe you did actually perceive something outside your mind, but what that is may be something entirely different from demons.