What exactly is a ghost? That might seem like an easy question to answer- that is, if you believe in ghosts at all. A ghost is the spirit of a dead person, right? But if a ghost is just the spirit of a dead person, then what about ghost trains, ghost cars, ghost ships and other ghostly inanimate objects?
What about ghost armies fighting battles from long ago? What about prophetic ghosts- the apparitions of things that haven't happened yet? What about ghost doubles of people known to be still alive?
Sometimes I think the real reason hardcore rational materialists are so skeptical about the existence of something so widely and consistently reported by so many different people in so many different cultures for so many centuries is that the existence of ghosts carries certain philosophical implications that make them very uncomfortable.
What implications are those? That the basic “stuff” of reality might not be matter, as they have so confidently assumed for so long. That it might be mind, as philosophers like Berkeley have argued. Or that it might be pure information- information that can, under certain circumstances, become stuck in a pattern, playing out the same spectral scene over and over again.
It could be that some ghosts are indeed the spirits of the dead. It could be that they don't exist at all. But what if they are essentially just stuck information, trapped in a pattern until something happens to interrupt it? It's just an idea, of course, but I think it's an interesting one.