46 THE GRAMMAR OF SCIENCE 



received at one time or another, and hence we may say- 

 that exertion, whether immediate or deferred, is to a large 

 extent the product, directly or indirectly, of sense- 

 impressions. 



^ 4. — The Nature of TJiougJit 



There are still one or two points to be noted here. In 

 the first place, the immediate sense -impression is to be 

 looked upon as the spark which kindles thought, which 

 brings into play the still remaining impresses of past 

 sense-impressions. But the complexity of the human 

 brain is such, its stored sense -impressions are linked 

 together in so many and diverse ways — partly by continual 

 thinking, partly by immediate sense-impressions occurring 

 in proximity and so linking together apparently discordant 

 groups of past impressions — that we are not always able 

 to recognise the relation between an immediate sense- 

 impression and the resulting train of thought. Nor, on 

 the other hand, can \ye always trace back a train of 

 thought to the immediate sense-impression from which it 

 started. Yet we may take it for certain that elements of 

 thought are ultimately the permanent impresses of past 

 sense -impressions, and that thought itself is started by 

 immediate sense-impressions.^ 



This statement must not be in any way supposed to 

 narrow the material of thought to those combinations of 

 " external objects " which we associate with immediate 

 sense-impressions. Thought once excited, the mind passes 

 with wonderful activity from one stored impression to 

 another, it classifies these impressions, analyses or simplifies 

 their characteristics, and forms general notions of properties 

 and modes. It proceeds from the direct — what might 

 perhaps be termed the physical — association of memory, 

 to the indirect or mental association ; it passes from 



1 The exact train of thought which follows an immediate sense-impression 

 depends largely on the physical condition of the brain at the time of its 

 receipt, and is further largely conditioned by the mode in which stored sense- 

 impressions have been previously excited, i.e. the extent to which memory 

 has been exercised in the past. 



