Alfred Marshall (1842–1924) British economist
Letter to A.L. Bowley, 27 February 1906, cited in: David L. Sills, Robert King Merton, Social Science Quotations: Who Said What, When, and Where http://books.google.com/books?id=WIKQbew5YKcC&pg=PA151 Transaction Publishers, 2000. p. 151.