Why Does Robert Caro Reduce a Book to One Sentence?

Robert Caro reduces a new book to one sentence, or at most a paragraph, before he starts writing, so he still knows what he is trying to do years later. He is never sure a book is on the right track. The sentence is the device he uses to try to stay there.

He learned the habit the hard way with The Power Broker, because he skipped it on that book. Before he starts, he thinks the whole book through to the end. He can spend a long time just asking what the book is about. He tells himself that he had better fit the answer into one sentence, or three years from now he will be writing and asking what he is trying to do.

Before the writing begins, he tries to get the book down to a sentence or at most a paragraph that says what the book is really about.