Even though there is no scientific evidence that ghosts exist, you may not be crazy if you see one. 48% of Americans profess to believe in ghosts, and around a quarter say that they've actually seen a ghost before. I wanted to find out if there was any proof of their existence, so I spoke with Joe Nickell, allegedly the world's only paranormal investigator who had researched reported hauntings for almost 50 years. He says that he's never seen any evidence that would point towards the existence of ghosts. Nickell walks us through the various scientific explanations for why people think they see ghosts, including sleep paralysis, waking dreams, traumatic grief, and exposure to infrasound. So even though there may be no evidence that ghosts exist, that doesn't mean that you might not see one.