Letter to Theo van Gogh, from Arles, September 29, 1888. "An Examined Faith: Social Context and Religious Commitment" by James Luther Adams and George K. Beach, p. 259, 1991.
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.58, Fairleigh Dickinson Univ Press
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.41, Fairleigh Dickinson Univ Press