I’ve never been comfortable with the way we usually regard someone’s story of having seen (or sensed) a ghost, which is either (1) they’re mistaken, they actually saw something else; or (2) they imagined it. These explanations don’t give our natural world the credit it is due.
In a previous life — by which I mean way back in 2014 — I reported on a story from National Geographic in which a young woman with epilepsy being evaluated for surgery actually had the sensation of a ghost induced when doctors stimulated a specific part of her brain. It was a shadow person, standing directly behind her, and mimicking her movements. To her, it was as real as could be. If she leaned forward, the shadow person would embrace her from behind. When undergoing certain tests, the shadow person actively tried to interfere, e.g., tried to take cards away when she was taking a flash card test.
Last year I did a Skeptoid episode where listeners wrote in with their ghost stories and I tried to explain them. During my research I learned about bereavement hallucinations, in which we actually have a real visual hallucination of a recently deceased loved one. It turns out these are extremely common. In fact, an astonishing 80% of elderly people report a bereavement hallucination after losing their spouse. 80%. That means it’s the rule, not the exception. No wonder we (in the collective) believe in ghosts — and there are many more examples.