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, Marianne S. Wokeck, Martin A. Coleman, James Gouinlock (2011). “The Life of Reason or The Phases of Human Progress: Introduction and Reason in Common Sense, Volume VII, Book One”, p.75, MIT Press