the mind can do some weird stuff, I myself have been diagnosed with schizophrenia which is seeing things that are not there and can have whats called tactile hallucinations, now the if you wanna talk about a ghost the only one i know is the holy ghost, hate to say this because i know this can be touchy, but start reading the bible and you will see things in a different view, I myself have been almost possessed and its not a fun ride when it happens, now as far as the science goes we really have along way to go with mental health, its not easy to tell people that you see things that are not there but believe me when it starts messin with your body, you'll get down on your knees and start talking to God real fast.