“The less he sees of other writers the more of a writer he will ultimately become. When he sees scarcely anyone except other writers, he is ready for New York.”

"The Secret Miracle"; Variant: Like all writers, he measured the achievements of others by what they had accomplished, asking of them that they measure him by what he envisaged or planned.
Source: Ficciones (1944)

Source: The Word and the Bomb

Source: On a writer’s responsibility in “The Literature of Uprootedness: An Interview with Reinaldo Arenas” https://www.newyorker.com/books/page-turner/the-literature-of-uprootedness-an-interview-with-reinaldo-arenas in The New Yorker (2013 Dec 5)

L’écrivain original n’est pas celui qui n’imite personne, mais celui que personne ne peut imiter.
The Oxford Dictionary of Quotations (1979) 3rd edition
Variant translations:
The original style is not the style which never borrows of any one, but that which no other person is capable of reproducing.
As translated by Charles I. White (1856) Part 2, Book 1, Chapter 3
An original writer is not one who imitates nobody, but one whom nobody can imitate.
Bartlett's Familiar Quotations (1980) 15th edition.
Le génie du Christianisme (1802)

Source: Flaubert's Parrot

The Expanding Universe (1963)
Context: I heard a famous author say once that the hardest part of writing a book was making yourself sit down at the typewriter. I know what he meant. Unless a writer works constantly to improve and refine the tools of his trade they will be useless instruments if and when the moment of inspiration, of revelation, does come. This is the moment when a writer is spoken through, the moment that a writer must accept with gratitude and humility, and then attempt, as best he can, to communicate to others.
A writer of fantasy, fairly tale, or myth must inevitably discover that he is not writing out of his own knowledge or experience, but out of something both deeper and wider. I think that fantasy must possess the author and simply use him. I know that this is true of A Wrinkle in Time. I can’t possibly tell you how I came to write it. It was simply a book I had to write. I had no choice. And it was only after it was written that I realized what some of it meant.
Very few children have any problem with the world of the imagination; it’s their own world, the world of their daily life, and it’s our loss that so many of us grow out of it.

"Goodbye school" in Boy: Tales of Childhood (1984)

Source: 1920s, Sceptical Essays (1928), Ch. 8: Eastern and Western Ideals of Happiness