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(THE CONVERSATION) In the Halloween season, American culture briefly participates in an ancient tradition of making the world of the dead visible to the living: Children dress as skeletons, teens go to horror movies and adults play the part of ghosts in haunted houses.

But what if the dead played a more active, more participatory role in our daily lives?

It might appear to be a strange question, but as a scholar of late medieval literature and art, I have found compelling evidence...