"It is a notebook kept for the purposes of his calling by a professional writer who passionately sought truth, purity of style and perfection of language. As a writer no one could have been more conscientious. Jules Renard jotted down neat retorts and clever phrases, epigrams, things seen, the sayings of people and the look of them, descriptions of scenery, effects of sunshine and shadow, everything, in short, that could be of use to him when he sat down to write for publication; and in several cases, as we know, when he had collected sufficient data he strung them together into a more or less connected narrative and made a book of them."
Maugham goes on to say that he never cared for any of those book strung together by Renard. And in fact, reiterates the notion that a writer should keep a notebook as long as he never refers to it. He feels that a writer gets practice in noticing, in separating sensations from the herd, by writing notes. By referring to them in actual work, the writer gives these details too much power to sway the narrative of a story.
I don't know how I feel about this. But then I never traveled as extensively as Maugham has, nor do I intend to write about foreign cultures. I'd be very surprised if he never referred to his notes while writing The Painted Veil, but surely his memory is better than mine. The reason I should never refer to an old notebook because I'll see in its pages all the stories I planned to write but never did.