What are ghosts? Well, everyone knows the answer to that: they are the dead having a good look at us. But what happens if they're not?
Maybe they're the ones that aren't dead but we are. Living in both our own little paradise and punishment. This world you're living in could be the afterlife. Who knows? Though if you think about it, it is possible. We could be dead and in the spirit world enjoying a sort of life, while we wait for Heaven or Hell or whatever.
Yes, that must be it; this is Limbo. A waiting room for the dead. A time to do everything you didn't do when you were alive.
Wait, this wouldn't make sense, you would cry. How could we possibly not remember dying? I mean, it is the last important moment in your life. And it is also devastating for us, leaving your loved ones like that and not being able to do all the things you wanted to do in your short eighty or so years of life. So if it's devastating, we may just forget about it - quickest way of coming to terms with it - and keep our mental health intact. Seriously, you don't want a crazy guy in the afterlife messing everything up. That wouldn't be cool.
Then again, if this is the afterlife why are there so many bad people? Well, they must have been bad in the living world and are re-enacting the crimes they committed. Don't worry; they'll get their just desserts when we finally move on from this life/death, whatever you want to call it.
So, the next time you see a ghost floating by or hear whispers in an empty room, just think, they might not be the dead you are seeing or hearing, but the living that have accidentally wandered in for a brief moment.