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, 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.65, MIT Press
George Santayana (1937). “The Works of George Santayana: Scepticism and animal faith. Some meanings of the word "is". Literal and symbolic knowledge. The unknowable”