I totally agree with Regan's statement that spirits exist...but why and from where these spirits manifest is an important query.
Some of the evidence I have gathered over the years suggests that there is a measurable connection between human spirit energy and other phenomena that we consider paranormal. Any investigation that I have conducted, be it in the field or behind the desk, involved more than just physical evidence, witness statements and deductive intuition. Psychic and metagnomy examination, with qualified use of remote viewing, clairvoyance, afterlife communication, astral projection, hypnosis, etc., is paramount in my opinion.
The use of technical equipment is great for initial probing as well as entertainment value if it's conducted in front of television audience. But tangent spirit connection depends on developing a background of the supposed being and finding the reason for the haunting. This requires a psychic bond, an understanding of the circumstances that caused this energy to roam this plane or the initiative taken that caused the energy to come through the existence divide.
Spirit energy is not only human...there are countless spirits of different beings from times before and, possibly, after ours that pervade our conscience. The living with the ability to see these spirits will confide that many of these spectres are not of the energy similar to ours. Over the centuries and in present time, these spirits have been referred to as demons, angels, fairies, etc.
It's time that we looked beyond our three dimensional world with only a three dimensional mindset. Depth perception goes much further than the visual cortex and a stereoscopic logic.