From the beginning I always know what something is going to be; every impulse to write is born of an idea of form, for me. To begin I have to have the shape, the architecture. I can’t say it better than Nabokov did: “The pattern of the thing precedes the thing.”
This runs contrary to the belief that we don't know what to think until we scribble it down first. For Sontag, she wrote the book in her head so all she needed to do was solidify it on paper.