Ghosts
I don't believe in the supernatural. Please don't think this means I go through life as a nerdy killjoy who snobbily discards anything that science can't explain, deaf to the romance of life, but I think my position makes sense. I do set a lot of store by science, but "science" is just Latin for knowledge, or to put it another way, stuff we have collectively figured out so far. It's not a prescription of how or what to think.
(To quote Dara Ó Briain, "Science knows it doesn't know everything. Otherwise, it would stop.")
Imagine a world where ghosts were commonplace and their existence was uncontroversial. They wouldn't be considered supernatural, just natural. There would be some other phenomenon that some people would believe in.
Think about it: the one thing you can be sure of when you sit down to watch an episode of Ghost Adventures, Ghost Hunters, or Ghost Files is that you won't be seeing any ghosts. If they had found any, you wouldn't need to watch to find out. You would already know. Life would be like, well, Ghosts.
In this universe, there haven't been any cats in my apartment for the past six months.
Still, when I hear a little creaking noise of the floorboards settling, or a shadow moves in the edge of my vision there is a moment, before I catch myself, that I still see them.
When I was away last month, there was a cat on my bed every night, and though this is a cat I had never met before, when my foot brushed against her, I was back in my own bed on the other side of the world for a second, and it was once again a cold dark day in January.
Clearly, when people and animals leave us, they leave something behind in our brains, and if you want to call that a ghost, you won't hear any objections from me.
When I shuffle off this mortal coil, there are one or two places in this world where I wouldn't mind if, every once in a while, someone caught a glimpse of me out of the corner of their eye.