George Santayana, Marianne S. Wokeck, Martin A. Coleman, James Gouinlock (2014). “The Life of Reason Or The Phases of Human Progress: Reason in Religion, Volume VII, Book Three”, p.114, MIT Press
David Hume (1874). “A Treatise on Human Nature: Being an Attempt to Introduce the Experimental Method of Reasoning Into Moral Subjects; and Dialogues Concerning Natural Religion”, p.452