Address at the Sorbonne, Paris, 23 Apr. 1910. Richard M. Nixon quoted this passage in his address to the nation announcing his decision to resign the presidency, 8 Aug. 1974. See Theodore Roosevelt 1; Theodore Roosevelt 2; Theodore Roosevelt 5
Kurt Vonnegut (2016). “If This Isn't Nice What Is? (Much) Expanded Second Edition: The Graduation Speeches and Other Words to Live By”, p.131, RosettaBooks