THE FACTS OF SCIENCE 47 



perceiving to conceiving. The mental association or 

 recognition of relation between the impresses of past 

 sense-impressions has probably, if we could follow it, as 

 definite a physical side as the physical association of im- 

 mediate sense-impressions with past impresses. But the 

 physical side of the impress is only a reasonable inference 

 from the physical nature of the immediate sense-impression, 

 and we must therefore content ourselves at present by 

 considering it highly probable that every process of 

 thought has a physical aspect, even if we are very far as 

 yet from being able to trace it out. 



This process of mental association we can only 

 recognise as certainly occurring in our individual selves. 

 The reason why we infer it in others we shall consider 

 later. The amount of it, however, in our individual selves 

 must largely depend on the variety and extent of our 

 store of impresses, and further on the individual capacity 

 for thinking, or on the form and development of the 

 physical organ wherein the process of thinking takes 

 place, i.e. on the brain. The brain in the individual man is 

 probably considerably influenced by heredity, by health, 

 by exercise, and by other factors, but speaking generally 

 the physical instruments of thought in two normal human 

 beings are machines of the same type, varying indeed in 

 efficiency, but not in kind or function. For the same two 

 normal human beings the organs of sense are also machines 

 of the same type and thus within limits only capable of 

 conveying the same sense - impressions to the brain. 

 Herein consists the similarity of the universe for all 

 normal human beings. The same type of physical organ 

 receives the same sense-impressions and forms the same 

 " constructs." Two normal perceptive faculties construct 

 practically the same universe. Were this not true, the 

 results of thinking in one mind would have no validity 

 for a second mind. The universal validity of science 

 depends upon the similarity of the perceptive and reasoning 

 faculties in normal civilised men. 



The above discussion of the nature of thought is of 

 course incomplete ; it offers no real explanation of the 



