"Suppose the earth were inhabited by two kinds of life. We're one and ghosts are the other....They may be just as real and material to them as we are to us...The earth may be just as real to them as to us but neither of us could appreciate the other's realness, or the quality of realness in the earth...The ghost pirates are not our ideas of flesh and blood yet they exist." (The Ghost Pirates, by William Hope Hodgson)