Über Wahrheit und Lüge im außermoralischen Sinn (in English: "On Truth and Lies in a Nonmoral Sense", also called "On Truth and Lie in an Extra-Moral Sense"Walter Kaufmann's translation, appearing in The Portable Nietzsche, 1976 edition. Viking Press.) is an (initially) unpublished work of Friedrich Nietzsche written in 1873, one year after The Birth of Tragedy. It deals largely with epistemological questions of truth and language, including the formation of concepts.
According to Paul F. Glenn, Nietzsche is arguing that "concepts are metaphors which do not correspond to reality." Although all concepts are human inventions (created by common agreement to facilitate ease of communication), human beings forget this fact after inventing them, and come to believe that they are "true" and do correspond to reality. Thus Nietzsche argues that "truth" is actually:
These ideas about truth and its relation to human language have been particularly influential among postmodern theorists, and "On Truth and Lies in a Nonmoral Sense" is one of the works most responsible for Nietzsche's reputation (albeit a contentious one) as "the godfather of postmodernism."