mediation

When we read books about history, we want an objective recounting of the relevant facts and developments. That's kind of what we expect - something bloodless and impartial, but interesting at the same time. History is interesting because of the facts and developments that make it up. 

The problem is that that idea - that a text can be absolutely transparent, a lens through which we can view the past or whatever is the subject of the text - is an illusion. To be effective - like, for example, to tell us about history - a text has to have a thesis. It has to be animated by some kind of argument or point - something the composer is trying to express.