i believe beyond the "medical science" defining everything we go through.

For seeing ghosts, demons etc.. looking at it truly scientificly, i take into account frequency ranges in our perception, which can evidently be influenced by drugs, medications, awareness.

So if your seeing a demon, or a creature appear then disappear, would to me, indicate there is an influencing force on my perception, which works by processing information flow at a rate, frequency, other levels. If these demons are showing themselves at will, would indicate they have the ability to do so, or its somehow being induced.

inducement to me would be a magnetized flux allowing the occular/auditory centers to perceive those frequencies.