Here's a sensible and informative piece by E.D. Hirsch on the pedagogical role of"background knowledge," from the Spring 2006 issue of American Educator (the magazine of the American Federation of Teachers). There's lots of interesting material here for teachers, historians, philosophers of education, philosophers of history, epistemologists, and others. Unfortunately, I don't have time to comment on it right now, but hope to come back to it later. Hirsch aside, take a look at the whole symposium on"background knowledge"; it's well worth reading (assuming some background knowledge, of course).