Richard P. Feynman, Robert B. Leighton, Matthew Sands (2015). “The Feynman Lectures on Physics, Vol. I: The New Millennium Edition: Mainly Mechanics, Radiation, and Heat”, p.45, Basic Books
"The Feynman Lectures on Physics". Physics textbook by Richard P. Feynman, Robert B. Leighton, and Matthew Sands, volume I; lecture 44, "The Laws of Thermodynamics"; section 44-1, "Heat engines; the first law"; p. 44-2, 1964.
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.