There are two ways, I think, to approach nonfiction writing. You can set out on your journey armed with a thesis and collect supporting facts along the way--a perfectly legitimate approach. From this prosecutorial style, we get our best polemics, satires, and exposes. Partisan, one-sided and tending to justify a preconceived viewpoint, this is the art of the legal brief and indictment. This literature more often sounds like a trumpet blast, a call to arms, than an invitation to sober analysis and reflection.
But there's another type of nonfiction writing in which the writer surrenders all preconceived belief and submits to the material. That's not to say that the latter writers are mental eunuchs without firm opinions or airheads mindlessly soaking up facts. They, too, begin their journey carrying the bulky baggage of prejudice, although they may not know it or admit to it. The difference is that they zealously search for facts that contradict their working hypotheses. They like to stub their toes on hard, uncomfortable facts strewn in their paths. They want information that will explode, like a prankster's cigar, in their faces.
Ron Chernow in The Writing Life, 1995