“Write only if you cannot live without writing. Write only what you alone can write.”

Source: A Room of One's Own

English note by the hand of the poet in the same paper sheet: Your poems are of interest to mankind; your liver isn't. Drink till you write well and feel sick. Bless your poems and be damned to you.
Ibid., p. 229
The Book of Disquiet
Original: Se um homem escreve bem só quando está bêbado dir-lhe-ei: embebede-se- E se ele me disser que o seu fígado sofre com isso, respondo: o que é o seu fígado? É uma coisa morta que vive enquanto você vive, e os poemas que escrever vivem sem enquanto.

Penguins and Golden Calves (2003)
Context: I have advice for people who want to write. I don't care whether they're 5 or 500. There are three things that are important: First, if you want to write, you need to keep an honest, unpublishable journal that nobody reads, nobody but you. Where you just put down what you think about life, what you think about things, what you think is fair and what you think is unfair. And second, you need to read. You can't be a writer if you're not a reader. It's the great writers who teach us how to write. The third thing is to write. Just write a little bit every day. Even if it's for only half an hour — write, write, write.

Interview with Herbert Mitgang, "Mrs. Lessing Addresses Some of Life's Puzzles," The New York Times (22 April 1984)
Context: You can only learn to be a better writer by actually writing. I don't know much about creative writing programs. But they're not telling the truth if they don't teach, one, that writing is hard work and, two, that you have to give up a great deal of life, your personal life, to be a writer.

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