George Santayana, Marianne S. Wokeck, Martin A. Coleman, James Gouinlock (2014). “The Life of Reason Or The Phases of Human Progress: Reason in Religion, Volume VII, Book Three”, p.114, MIT Press
Henry David Thoreau, Barry Andrews (2005). “True Harvest: Readings from Henry David Thoreau for Every Day of the Year”, p.29, Unitarian Universalist Association of Congregations