Attributed to Horace from Maecenas in John Williams’ Augustus: 

I decide to make a poem when I am compelled by some strong feeling to do so–but I wait until the feeling hardens into a resolve; then I conceive an end, as simple as I can make it, toward which that feeling might progress, though often I cannot see how it will do so. And then I compose my poem, using whatever means are at my command. I borrow from others if I have to–no matter. I invent if I have to–no matter. I use the language that I know, and I work within its limits. But the point is this: the end that I discover at last is not the end that I conceived at first. For every solution entails new choices, and every choice made poses new problems to which new solutions must be found, and so on and on. Deep in his heart, the poet is always surprised at where his poem has gone.