Hmm... if you read the right material and personal accounts, it is hard to deny the reality of invisible & intelligent spirit creatures and indeed physical manifestations of the above. I am just about finished with a book called "The Dark Gods / Anthony Roberts" which is superb. It details many accounts which are both curious and worrying, but which ultimately enhance ones own faith in the cloaking influence of malevolant forces.
I also have a number of friends who have had very "real" & sometimes sinister encounters, which are not easy to explain away. Several being witnessed by 2 or 3 individuals at the moment of manifestation.
I have no doubts in my mind, but sometimes choose to doubt in an effort to get a better nights sleep