The existence of causal laws perhaps deserves to rank as a postulate, or may perhaps be proved probable, on the existing evidence, if induction is assumed. Here our proviso is relevant, that a postulate need not be supposed to hold universally. We shall assume that there are causal laws, and try to discover them; but if none are found in a given region, that merely means that science cannot conquer that region. There are at present important regions of this kind. We do not know why a radio-active atom disintegrates at one moment rather than another, or why a planetary electron changes its orbit at one moment rather than another. We cannot be sure that these occurrences severally are governed by laws; but if they are not, science cannot deal with them individually, and is confined to statistical averages. Whether this will prove to be the case, we cannot yet say. FOOTNOTES: [37] Jeans, op. cit., p. 29. [38] To show the depth of Dover cliff, Shakespeare says: "The crows and choughs that wing the midway air Show scarce so gross as beetles." [39] A Treatise on Probability. By John Maynard Keynes. Macmillan, 1920. Le Problème Logique de l'Induction. Par Jean Nicod. Paris, Alcan, 1924. Review of the above by Braithwaite, Mind, 1925. The Foundations of Probability. By R. H. Nisbet. Mind, January, 1926. [40] Nature, May I, 1926, supplement. [Pg 169] CHAPTER XVII WHAT IS AN EMPIRICAL SCIENCE? IT would be generally agreed that physics is an empirical science, as contrasted with logic and pure mathematics. I want, in this chapter, to define in what this difference consists. We may observe, in the first place, that many philosophers in the past have denied the distinction. Thorough-going rationalists have believed that the facts which we regard as only discoverable by observation could really be deduced from logical and metaphysical principles; thorough-going empiricists have believed that the premisses of pure mathematics are obtained by induction from experience. Both views seem to me false, and are, I think, rarely held in the present day; nevertheless, it will be as well to examine the reasons for thinking that there is an epistemological distinction between pure mathematics and physics, before trying to discover its exact nature.