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George Santayana, Marianne S. Wokeck, Martin A. Coleman, James Gouinlock (2013). “The Life of Reason or The Phases of Human Progress: Reason in Society, Volume VII, Book Two”, p.64, 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”