George Santayana, Marianne S. Wokeck, Martin A. Coleman, James Gouinlock (2015). “The Life of Reason Or The Phases of Human Progress: Reason in Art, Volume VII, Book Four”, p.136, MIT Press
George Santayana, James Seaton (2014). “Genteel Tradition in American Philosophy and Character and Opinion in the United States: And Character and Opinion in the United States”, p.143, Yale University Press
George Santayana (1937). “The Works of George Santayana: Dialogues in limbo. Platonism and the spiritual life. A long way round to Nirvana. The prestige of the infinite. Ultimate religion”