LECTURE IV 



THE WORLD OF PHYSICS AND THE 



WORLD OF SENSE 



Among the objections to the reality of objects of sense, 

 there is one which is derived from the apparent difference 

 between matter as it appears in physics and things as they 

 appear in sensation. Men of science, for the most part, 

 are willing to condemn immediate data as " merely sub- 

 jective," while yet maintaining the truth of the physics 

 inferred from those data. But such an attitude, though 

 it may be capable of justification, obviously stands in need 

 of it ; and the only justification possible must be one 

 which exhibits matter as a logical construction from sense- 

 data unless, indeed, there were some wholly a priori 

 principle by which unknown entities could be inferred 

 from such as are known. It is therefore necessary to 

 find some way of bridging the gulf between the world 

 of physics and the world of sense, and it is this problem 

 which will occupy us in the present lecture. Physicists 

 appear to be unconscious of the gulf, while psychologists, 

 who are conscious of it, have not the mathematical know- 

 ledge required for spanning it. The problem is difficult, 

 and I do not know its solution in detail. All that I can 

 hope to do is to make the problem felt, and to indicate 

 the kind of methods by which a solution is to be sought. 



Let us begin by a brief description of the two con- 

 trasted worlds. We will take first the world of physics, 



IOI 



