24. Bull. Soc. Math, de France, 15, 1887, 203-16 = Œuvres, 11, 79-91. He expressed this view again in an article “Les Geometries non-euclidiennes” in the Revue Générale des Sciences, 2, 1891, #23. An English translation is in Nature, 45, 1892, 404-7. See also his Science and Hypothesis, Chapter 3, in The Foundations of Science, The Science Press, 1946.