Richard P. Feynman's letter to Koichi Mano (February 3, 1966), as quoted in "Perfectly Reasonable Deviations from the Beaten Track: The Letters of Richard P. Feynman" edited by Michelle Feynman and Timothy Ferris (pp. 198, 201), April 5, 2005; also quoted in "Wise Man" by Freeman Dyson, www.nybooks.com. October 20, 2005.