My belief is that all humans have two minds sharing the one body. A book I read claims that some neurologists back in the 1960's discovered that each hemisphere of the brain contains a separate and unique consciousness, they believe we exist in the left dominant hemisphere while this second mind exists in the right hemisphere.
It's like we're the driver of the body and this other mind is a passenger along for the ride, from my own experience this passenger mind is capable of putting thoughts, ideas, impulses and images into my mind (which is how they usually get us to do what they want) as well as talking to me directly which I hear like I would my own thoughts.
Most of these passengers seem to be happy to just influence our lives on a passive level but at times they want more than that so they speak to us, mine was bored and wanted to have fun at my expense so he messed with my mind and tried to get me to believe all sorts of rubbish in the beginning, things like he was real people in the world that could speak to me telepathically and wanted to do me harm but eventually I got wise to his deceptions and figured out he was internal and had been a part of me all my life, so much so I call him my 'twin within' because he's like my twin brother that was born with me and grew up along side me but I just never knew he was there in the background, not until I was 36 when he made his presence known to me.
So if you ask me if I believe we're mentally ill I'd say no, I believe everyone on the planet is like this, the only difference is we have a bad one that wants to mess with us instead of staying hidden from us or sometimes we have one that wants to be friends with us, and they could be our greatest friends, they know everything about us and they even know what we dream about at night.
The book I mentioned earlier that taught me so much about them was recommended to me by another member and is called 'The Daemon - A guide to your extraordinary secret self' by Anthony Peake. It explains in more detail about how they were discovered, what they're capable of and how they do the things they do.