“On the Method of Theoretical Physics" The Herbert Spencer Lecture, delivered at Oxford (10 June 1933); also published in Philosophy of Science, Vol. 1, No. 2 (April 1934), pp. 163-169., p. 165. [thanks to Dr. Techie @ www.wordorigins.org and JSTOR] The Philosophy of Science print version is available online here.”

Source: Wikiquote