Most natural things on Earth have been assigned into categories such as mineral, animal, vegetable and sub categorized further such as mamal, fish, amphibian and again and so on. Hence everything has a place in the order of things.
What about ghosts? We've mentioned Battlefield Ghosts, Phantom Hitchhikers, Road Ghosts and Poltergeists in other topics, but can ghosts be categorised, any families identified that share common traits.
If they are hallucinations this would be impossible, but there are distinct differances of appearance between black shadow ghosts, those that pass for living people and the grey mist form types. Have a think about and whether it may or may not be possible and how it put together.