The writer follows an action line just as if he were writing a true story, one with a protagonist, a complication, and a resolution. Maybe, as was the case with one McPhee narrative, it’s an Army Corps of Engineers officer riding a boat down the Atchafalaya River, inspecting Louisiana flood-control structures as he goes. But every once in a while, the writer stops the action, just pulls the curtain on the narrative. Then he goes off on a little exploration of the subject, an abstract explanation that gives depth and meaning to what the reader’s been witnessing in the narrative. They’re known in the trade as digressions, and they’re the key to making an explanatory narrative work.