How Much Do We Know About Ghosts?

On a scale of one to ten, how much do we know about ghosts? I think that people who are open to it, so you have amazing spiritualists around the world, gurus, prophets, spiritual people would be right up there on a ten. But from a science point of view, and isn't that disappointing? That the science side of it is very low, but yet the spiritual side of it was so high. I wish we could sort of meet in the middle a little bit.