I am open to the idea of evil spirits if conclusive proof were given, just as I'm open to the idea of God, but faith in such an idea is quickly waning.
My honest thoughts..... I believe that we attribute what we don't understand in the world to be something that they are not. For instance, it was once thought that someone who was schizophrenic had demons. We know now this to be a mental illness. For people that lived before this illness was discovered, these people were downright scary and still can be today. However, we no longer attribute this illness to demons.
I do believe that there is supernatural phenomena, but we are not classifying it correctly due to an incomplete understanding. I believe that as science progresses and the tools are made available to properly measure and understand these phenomena, we will see these for what they are, explainable facts of life. In time we will see these not as malevolent or benevolent spirits, but will see them for what they are.
Of course, these phenomena can often be exaggerated by our own imagination, sleep deprevation, mental illness, preconceived notions, etc. to the point that we believe we have seen or experienced something we can not.
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True experience.....
In an effort to prove the existence of God, I tried to prove the existence of the demons since God would never answer me. As an eight year old, I cried out to Satan that I would devote my life to him for one supernatural experience...... floating. Satan missed an easy opportunity to have a devoted follower. As an adult, I spent time in a graveyard at night firing off fireworks, the supposed lurking place of demons and spirits...... not one supernatural experience. Maybe they were scared of fireworks? Anyways, in my life, I've made it easy to have an experience that others have "supposedly" had, but to no avail from either side of the forces of good or evil.