Depends on what you mean by ghost. I think that spirits from the otherside/heaven can interact with us. Mediums can perceive them and communicate with them. They are actually all around us, just in a different reality/dimension than us. A different frequency. Then there are those spirits that have not gone back to the otherside. They are in the void. These spirits are stuck there because they dont wish to give up their material obsessions. They also try to interact with humans for whatever reasons. Some of these spirits are malevolent. Seeking the life force of living beings. Attracted to negative vibrations. They even attempt, at times, to occupy a human body...possesion. But this is rare. Then their are the very highly spiritual entities such as angels and the like that help protect and guide us while we're here on earth. I think the typical "ghost" may be just an imprint or recording of a persons energy that is somehow trapped in our reality.