Naomi E. Maurer, Vincent van Gogh, Paul Gauguin (1998). “The Pursuit of Spiritual Wisdom: The Thought and Art of Vincent Van Gogh and Paul Gauguin”, p.112, Fairleigh Dickinson Univ Press
Martin Farquhar Tupper (1848). “Gems from Tupper: Consisting of Extracts from the Following Works: Proverbial Philosophy; Thousand Lines; Twins; Hactenus; An Authr's Mind; Geraldine, and Other Poems; Heart; Crock of Gold; and Probabilities, an Aid to Faith”, p.60