A.— MATHEMATICAL AND PHYSICAL SCIENCES 37 



remember Clerk Maxwell's immortal account of the proceedings of our 

 Section at the Belfast Meeting sixty-two years ago, when Mr. Herbert 

 Spencer regretted ' that so many members of the Section were in the 

 habit of employing the word Force in a sense too limited and definite 

 to be of any use in a complete theory of evolution. He had himself 

 always been careful to preserve that largeness of meaning which was too 

 often lost sight of in elementary works. This was best done by using 

 the word sometimes in one sense and sometimes in another, and in this 

 way he trusted he had made the word occupy a sufficiently large field 

 of thought.' 



Is it heresy to suggest that some of us who have sung Canticles in 

 praise of indeterminism and the disappearance of causality have given 

 a similar generousness of meaning to these words ? 



Similar considerations apply to the term observable, which has suffered a 

 sea-change in transference from its ordinary usage in the realms of per- 

 ception. There is quite as much complicated physical theory lying between 

 the perceptually observable marks on a photographic plate and the 

 inferred frequencies, as there is between similar preceptual observables 

 and the non-observable electron orbit or state which was inferred in order 

 to subsume the perceptual facts. A similar generosity of treatment is 

 accorded to the term observe when it is applied to the conceptual experi- 

 ment for the determination of the position of a particle such as an electron. 



Which brings us round to the starting-point of this discourse. Many 

 of us who desire to proceed with our measurements untrammelled by 

 these philosophic doubts have asked if there is not some canon by which 

 the plain man could test his everyday beliefs. I suggest that a starting- 

 point at least to this end is provided by a study of Karl Pearson's work, 

 and that, with certain reservations and additions to the method discussed 

 in the Grammar of Science, we may develop a canon which will serve 

 as a guide through the jungle of additional perceptual facts which 

 the physical science of the twentieth century has added to that of its 

 predecessors. 3 



Those who discuss the doctrine of causality do so with little reference 

 to the attitude taken by the philosophers, and it may not be without 

 interest — it certainly has some bearing on present-day thought — to con- 

 sider the development of the notion of cause since the time of Newton. 

 The views of Locke, Newton's elder contemporary, are clear and simple. 

 He remarks : ' Thus, finding that in that substance which we call wax, 

 fluidity, which is a simple idea that was not in it before, is constantly 

 produced by the application of a certain degree of heat, we call the simple 

 idea of heat in relation to fluidity in wax the cause of it, and fluidity the 

 effect. ... So that whatever is considered by us to conduce or operate 

 to the producing any particular simple idea, whether substance or mode, 

 which did not before exist, hath thereby in our minds the relation of a 

 cause and so is denominated by us.' 



Newton, dominated as he was by the principle of causality and ever 



3 In what follows I have drawn on the material of an article which I wrote 

 some four years ago (Nature, vol. 45, 1932, p. 130). See also Broad, Perception, 

 Physics and Reality. 



