system
Given the following post, respond as if you're commenting on Reddit:
human
[Mario Universe] Are Boos the spiritual remnants of humans, or are they simply an intelligent species that resemble what we think of as 'ghosts?'

Are they actual, back-from-the-dead ghosts? Luigi's Mansion has both Boos and humanoid ghosts, which suggests they are two different things. However, King Boo is revealed to be (SPOLIER) Bowser's ghost.
gpt
Pretty sure it's all based on Shinto, which is animistic, so *everything* has a ghost/spirit (which is why most things, even inanimate things, have some sort of "eyes"). 

It's theoretically possible that some of these spirits have never been anything but spirits, and that those might be boos.