Theodore Roosevelt (2015). “Theodore Roosevelt on Bravery: Lessons from the Most Courageous Leader of the Twentieth Century”, p.4, Skyhorse Publishing, Inc.
"Epistolæ Ad Lucilium" by Marcus Fabius Quintilianus as reported in "Hoyt's New Cyclopedia Of Practical Quotations" by Jehiel Keeler Hoyt, p. 878-82., 1922.