Friedrich Nietzsche (2016). “BEYOND GOOD AND EVIL - Prelude to a Philosophy of the Future: The Critique of the Traditional Morality and the Philosophy of the Past”, p.50, e-artnow
"Human, All Too Human: A Book for Free Spirits". Book by Friedrich Nietzsche, transl. by Helen Zimmern, Section IX, "Man Alone with Himself"/ aphorism 633, 1909-1913.