"Philosophy of Science", Volume 37 (p. 157), 1934; later quoted in Roger Gerhard Newton "The Truth of Science: Physical Theories and Reality" (p. 176), 1997.
"The philosophy of Niels Bohr" by Aage Petersen in "Bulletin of the Atomic Scientists", Volume 19, No. 7 (September 1963), as quoted in Abraham Pais "The Genius of Science: A Portrait Gallery" (p. 24), 2000, and in Paul McEvoy "Niels Bohr: Reflections on Subject and Object" (p. 291), October 2000.