Richard P. Feynman's letter to Robert Bacher (April 6, 1950) as quoted in "Genius: The Life and Science of Richard Feynman", book by James Gleick, 1992.
James Boswell (1863). “The Life of Samuel Johnson, LL.D.: Comprehending an Account of His Studies and Numerous Works, in Chronological Order; a Series of His Epistolary Correspondence and Conversations with Many Eminent Persons; and Various Original Pieces of His Composition Never Before Published: the Whole Exhibiting a View of Literature and Literary Men in Great-Britain, for Near Half a Century During which He Flourished”, p.54