I'm in the process of writing a story which prominently features ghosts. I suppose that ghosts are a relatively common occurrence in fantasy tales. However, I would like to develop a coherent and consistent context for ghosts in my fantasy world. For example, I'm interested in classifying ghosts into categories, as well as offering explanations for what ghosts are and where they come from. Have any of you tried this? What approach did you take? Also, are there any resources that may provide a starting point? I recall the Ghostbuster's talking about Tobin's Spirit Guide as a useful reference. Does anything similar to this exist in reality?