Theodore Roosevelt (2015). “Theodore Roosevelt on Bravery: Lessons from the Most Courageous Leader of the Twentieth Century”, p.13, Skyhorse Publishing, Inc.
Lord Philip Dormer Stanhope Chesterfield, Philip Dormer Stanhope Earl of Chesterfield (1855). “The Works of Lord Chesterfield: Including His Letters to His Son, Etc : to which is Prefixed, an Original Life of the Author”, p.533
Theodore Roosevelt (2015). “Theodore Roosevelt on Bravery: Lessons from the Most Courageous Leader of the Twentieth Century”, p.13, Skyhorse Publishing, Inc.