24 SECTIONAL ADDRESSES 



true not merely of the scientist ; the layman holds the same belief. I 

 may exemplify this by a quotation from that epitome of the reasoned 

 thought of the ordinary man, the detective story. After Watson has 

 expressed admiration at one of the most brilliant guesses of Sherlock 

 Holmes he is met with the reply : ' No, no ; I never guess. It is a 

 shocking habit, destructive of the logical faculty.' The reader is en- 

 couraged to revere the great detective by being told that all his arguments 

 are Aristotelian syllogisms. The scientist forms his opinions in much 

 the same way as Holmes really did, but he is apt to feel that this is a 

 fault in himself and that he ought to be forming them by the severe 

 principles of formal logic in the manner that appealed to Watson. Of 

 course there are branches of knowledge for which this can be done, but 

 somehow they are not the interesting ones ; indeed, outside pure mathe- 

 matics any subject is apt to become dead and uninteresting as soon as 

 it is brought down to this form. The really live branches of physics 

 call for a very different kind of thought, for a review of a system of 

 interconnected facts and for a perception of conjectured analogies, and 

 so on. This is vaguer, but it is more important, and our system ought 

 to give importance to the important things, so that the actual habit of 

 thought which the intelligent man finds the most useful is acknowledged 

 as the right one. 



In general literature there is a particular kind of writing which we all 

 admire on account of its direct simplicity ; it is to be found in much 

 English of the seventeenth century, but specially in the work of many 

 French authors, both early and late. It is a delight to read and often 

 admirably achieves its aim of clarifying the subject-matter — but not 

 always. There are two ways of writing simply about a subject. One 

 is to understand and make clear the simplicity of it ; the other is to leave 

 out all the difficult parts. When we entertain the idea that everything 

 can be brought down to the Aristotelian syllogism, are we not doing this 

 last ? Is it not possible that when a subject is brought down to these 

 terms, it is merely that we have picked out from it the easy parts and con- 

 cealed all the rest ? If we turn our attention to the question of why 

 we believe in our various theories, we can see that there is often a quite 

 illusory simplicity in their presentation. 



Why do we believe in the various theories that we are all agreed to 

 accept .'' Once a theory has become well established someone usually 

 gets to work to find a system of axioms, postulates, indefinables and so 

 on from which it may be derived. For example, classical mechanics is 

 based on Newton's Laws, or whatever system has been substituted for 

 them by later criticism. The direct derivation of everything from an 

 axiomatic basis has an attractive simplicity, but it tends to make us 

 think we believe the theory because of the axioms, whereas the 

 axioms are only a convenient shorthand summarising a wide field of 

 information, and they are believed in merely because we believe in • 

 the theory. This may be seen by an occurrence of a few years 

 ago. There was a letter to Nature pointing out a rather fundamental 

 contradiction in the quantum theory — I do not think the author meant 

 it as strongly as the accident of his wording implied. One's immediate 

 feeling was that the idea must be wrong (as indeed it proved to be), but 



