566 PSYCHOLOGY. 



pies, such as those of Similarity and Contrast, before they 

 could begin to do justice to the richness of the facts. 



THE ELEMENTARY LAW^ OF ASSOCIATION". 



I shall try to show, in the pages which immediately 

 follow, that there is no other elementary causal law of asso- 

 ciation than the law of neural habit. All the materials of 

 our thought are due to the way in which one elementary 

 process of the cerebral hemis23heres tends to excite what- 

 ever other elementary process it may have excited at some 

 former time. The number of elementary processes at 

 w^ork, however, and the nature of those which at any time 

 are fully eflective in rousing the others, determine the 

 character of the total brain-action, and, as a consequence 

 of this, they determine the object thought of at the time. 

 According as this resultant object is one thing or another, 

 we call it a product of association by contiguity or of as- 

 sociation by similarity, or contrast, or whatever other sorts 

 we may have recognized as ultimate. Its production, how- 

 ever, is, in each one of these cases, to be explained by a 

 merely quantitative variation in the elementary brain-pro- 

 cesses momentarily at work under the law of habit, so that 

 'psychic contiguity, similarity, etc., are derivatives of a sin- 

 gle profounder kind of fact. 



My thesis, stated thus briefly, will soon become more 

 clear ; and at the same time certain disturbing factors, 

 which co-operate with the law of neural habit, will come to 

 view. 



Let us then assume as the basis of all our subsequent 

 reasoning this law : Wlien tico elementary brain-processes 

 have been active together or in immediate succession, one of 

 them, on reoccurring, tends to propagate its excitement into the 

 other. 



But, as a matter of fact, every elementary process has 

 found itself at different times excited in conjunction with 

 many other processes, and this by unavoidable outward 

 causes. AYhich of these others it shall awaken now be- 

 comes a problem. Shall 6 or c be aroused next by the 

 present a ? We must make a further postulate, based, how- 

 ever, on the fact of tension in nerve-tissue, and on the fact 



