It is an essay entitled "While the World Sleeps," from the anthology A Way of Being Free by Nigerian writer Ben Okri. The piece is written specifically for poets, but it is a rallying cry for writers of all stripes.
This is my favourite passage:
"Poets, be cunning. Learn some of the miracles. Survive. Weave your transformations in your life as well as in your work. Live. Stay alive. Don't go under, don't go mad, don't let them define you, or confine you, or buy your silence. If they do confine you, burst out of their prisons with wilder fatidical songs. Be a counter-antagonist, break their anti-myths. Where the enemies breed destruction, sow seeds of startling light. Keep sowing. Time will reap. Weave your songs by whatever means you can. 'What doesn't kill us makes us stronger,' wrote Nietsche. There is no reason why the poet, if possessed of practical intelligence, cannot survive as well as the politician, or the banker. Don't become a dying breed. Dare to stick around for the hard and beautiful harvest. We need you even as we antagonise you. Remember: it is from the strength of your antagonists that you derive your greater authority. They make it absolutely necessary for you to be more than yourself. Follow Melville's precept, which he had nailed to his writing desk: 'Be true to the dreams of thy youth.' After your untimely and much lamented death, we would shout about how much we miss the uniqueness of your voice, your demanding presence, your duende. Don't wait till you're dead to know that in reality the whole of life is on your side."