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.103, 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”