William Penn (1807). “Fruits of solitude in reflections and maxims relating to the conduct of human life: Also, his advice to his children relating to their civil and religious conduct, to which are added extracts from the writings of William Law & Judge Hale; with a "Description of the happy man & true gentleman."”, p.28
Albert Schweitzer, William Larimer Mellon (1996). “Brothers in Spirit: The Correspondence of Albert Schweitzer and William Larimer Mellon, Jr”, p.18, Syracuse University Press