Sharpe, Alfred. "Doubt". The Catholic Encyclopedia. 5. New York: Robert Appleton. Dicapai pada 2008-10-21. A state in which the mind is suspended between two contradictory propositions and unable to assent to either of them.
Schulte, Karl Joseph (1910). "George Hermes". The Catholic Encyclopedia. 7. New York: Robert Appleton. Dicapai pada 2008-10-21.