If you have made it this far, here it is: What people think are ghosts are actually moments of vivid dreaming when they have had too much to eat or their stomach is not well. Also combined with constipation. It can also happen during severe physical pain such as injury or depression and mostly while dreaming during that period. Since they remember their dream as a reality and not the reality they think they've seen a ghost in reality. It's actually the minds powerful way of distracting itself so that it focuses on something different to ease the physical pain.