Personal experience is one thing, but scientific evidence is another matter. Part of the difficulty in investigating ghosts is that there is not one universally agreed-upon definition of what a ghost is. Some believe that they are spirits of the dead who for whatever reason get "lost" on their way to The Other Side; others claim that ghosts are instead telepathic entities projected into the world from our minds.

Still others create their own special categories for different types of ghosts, such aspoltergeists, residual hauntings, intelligent spirits and shadow people. Of course, it's all made up, like speculating on the different races of fairies or dragons: there are as many types of ghosts as you want there to be.

There are many contradictions inherent in ideas about ghosts. For example, are ghosts material or not? Either they can move through solid objects without disturbing them, or they can slam doors shut and throw objects across the room. Logically and physically, it's one or the other. If ghosts are human souls, why do they appear clothed and with (presumably soulless) inanimate objects like hats, canes, and dresses — not to mention the many reports of ghost trains, cars and carriages?

If ghosts are the spirits of those whose deaths were unavenged, why are there unsolved murders, since ghosts are said to communicate with psychic mediums, and should be able to identify their killers for the police. And so on; just about any claim about ghosts raises logical reasons to doubt it.

Ghost hunters use many creative  methods to detect the spirits' presences, often including psychics. Virtually all ghost hunters claim to be scientific, and most give that appearance because they use high-tech scientific equipment such as Geiger counters, Electromagnetic Field (EMF) detectors, ion detectors, infrared cameras and sensitive microphones. Yet none of this equipment has ever been shown to actually detect ghosts.

Other people take exactly the opposite approach, claiming that the reason that ghosts haven't been proven to exist is that we simply don't have the right technology to find or detect the spirit world. But this, too, can't be correct: Either ghosts exist and appear in our ordinary physical world (and can therefore be detected and recorded in photographs, film, video, and audio recordings), or they don't. If ghosts exist and can be scientifically detected or recorded, then we should find hard evidence of that yet we don't. If ghosts exist and cannot be scientifically detected or recorded, then all the photos, videos, and other recordings claimed to be evidence of ghosts cannot be ghosts. With so many basic contradictory theories — and so little science brought to bear on the topic — it's not surprising that despite the efforts of thousands of ghost hunters on television and elsewhere for decades, not a single piece of hard evidence of ghosts has been found.