Darwin had a habit. When he finished reading a book, he would go back through it and underline the passages he had marked and then, in the margins, write what he actually thought — not what the author said, but his own reaction, his disagreement, the question the passage raised that the author had not considered. By the time he was done with a book, it was covered in a second book: his.
This was not annotation in the academic sense. He was not glossing the text for a student. He was thinking in the margins because the margins were where thinking happened — adjacent to someone else's argument, in conversation with it, pushed forward by it without being constrained by it.
Nabokov worked differently but arrived at the same place. He composed on index cards, each card containing a scene or a passage, the cards shuffled and reshuffled as the novel took shape. The physical separateness of each thought made rearrangement possible. A sentence written in a notebook is buried under other sentences; a sentence written on a card can be moved. Ada and Pale Fire were assembled from something that looked, in the early stages, like a loose pack of notes.
"A writer's notebook is the only place where he is absolutely free."
— Vladimir Nabokov
Newton borrowed books from the Cambridge library in his student years and returned them full of marginal notes that had nothing to do with the original text. He was using them as props. The page of Descartes or Wallis or Barrow gave his eye something to anchor on while his mind went somewhere else entirely, and the margin was the closest surface. The calculus — or what he called the method of fluxions — was developed in those margins. The borrowed books were returned. The notes remained in his own notebooks.
There is a pattern here worth attending to. The great notebooks are not repositories of finished thoughts. They are the places where the thinking happened — sideways, adjacent, in reaction to something else. The Origin of Species started in a notebook Darwin kept in 1837, two years before he had even articulated the theory of natural selection. He was writing around it, approaching it from different directions, feeling the shape of it before he had the shape. The notebook made that possible because a notebook will accept a thought that is not ready, that does not know what it is yet, that needs to sit next to three other half-formed things before it starts to cohere.
The margin is the part of the page that has no official function. That is why it works. The centre of the page is where you put what you mean to say. The margin is where you put what you actually think.
In the A6 grid, every line is potential margin. The grid does not distinguish between the main thought and the aside — it treats them the same, gives them the same space, the same neutral surface. A diagram sits next to a fragment of prose sits next to a single word you couldn't place but couldn't discard. The notebook doesn't know which one matters. Neither do you, yet.
That is the point. The margin is where the thing arrives before you know it has arrived. Keep the margin open. Write in it freely. The ideas that started revolutions looked, in the first notebook, a lot like stray marks in the space beside someone else's argument.