Charles Sanders Peirce, Nathan Houser, Christian J.W. J. W. Kloesel (1992). “The Essential Peirce, Volume 1: Selected Philosophical Writings? (1867–1893)”, p.56, Indiana University Press
George Berkeley (1752). “Alciphron: Or, The Minute Philosopher. In Seven Dialogues Containing an Apology for the Christian Religion Against Those who are Called Free-thinkers”, p.112
To an Oxford historian assisting him during the 1930s on a biography of the first Duke of Marlborough; quoted in Stafford, Churchill and Secret Service (1998).