Mankind always sets itself only such problems as it can solve; since, looking at the matter more closely, it will always be found that the task itself arises only when the material conditions for its solution already exist or are at least in the process of formation.

Marx, Karl and Friedrich Engels. The Communist Manifesto, translated by Samuel Moore and edited by Gareth Stedman Jones. Penguin Classics, 2015, pt. 2.

Flaubert, Gustave. Madame Bovary, edited and translated by Lydia Davis. Penguin Classics, 2011, pt. 3, ch. 6.

Lincoln, Abraham. "Address to the Wisconsin State Agricultural Society, Milwaukee, Wisconsin, September 30, 1859." Abraham Lincoln: Speeches and Writings 1859-1865, edited by Don E. Fehrenbacher. Library of America, 1989.

Nietzsche, Friedrich. Human, All Too Human: A Book for Free Spirits, translated by Marion Faber and Stephen Lehmann. University of Nebraska Press, 1984, preface.