thefanof - a person with an eidetic memory/photographic memory, despite all of the controversy (which is more over the "perfect" photographic memory - it's entirely possible to have an eidetic memory, just not a perfect one) people do actually have eidetic memories. They normally remember things visually that we cannot normally remember and usually in greater detail (such as Shawn). There are studies of test subjects having extraordinary power in remembering via eidetic memories, and so we can allude that it most likely is real. But possessions? I'm sorry, but this episode kinda spoke for itself - how do we know that the "possessed" wasn't either taking drugs or was kind of loopy? Can you prove that someone is possessed via scientific study or some other credible source? Not really.
Oh, and I loved the actual episode. I kept laughing at poor Shawn's father. I know that feeling after I stopped playing video games for a while. It kinda sucks!