“Write what should not be forgotten.”

The Art of Writing

Nobel Prize Speech (1954)
Context: Things may not be immediately discernible in what a man writes, and in this sometimes he is fortunate; but eventually they are quite clear and by these and the degree of alchemy that he possesses he will endure or be forgotten. Writing, at its best, is a lonely life. Organizations for writers palliate the writer's loneliness but I doubt if they improve his writing. He grows in public stature as he sheds his loneliness and often his work deteriorates. For he does his work alone and if he is a good enough writer he must face eternity, or the lack of it, each day. For a true writer each book should be a new beginning where he tries again for something that is beyond attainment. He should always try for something that has never been done or that others have tried and failed. Then sometimes, with great luck, he will succeed.

Nobel Prize Speech (1954)

Source: Paris Review interview (1972), p. 250

Letter to L.A. Avilova (February 26, 1899)
Letters

Variant: the problems of the human heart in conflict with itself which alone can make good writing because only that is worth writing about, worth the agony and the sweat