THE ASSOCIATION OF IDEAS, 581 



could be called) would be shut up to the punctiform instant ; he would 

 obey, without noticing, the current which swept him on ; drift to his 

 conclusions, but never know why ; and act upon the suggestions of 

 experience with a fatality which would be inwardly all the blinder in 

 proportion as it was the more rational to outward semblance. I sim- 

 ply assume for his benefit the possession of a consciousness, I beg 

 that much from the reader's liberality ; and limit my ambition to 

 showing (the consciousness being granted) with what objects it is at 

 any given moment most likely to be filled. 



The laws of motor habit in the lower centers of the nervous sys- 

 tem are disputed by no one. A series of movements repeated in a 

 certain order tend to unroll themselves with peculiar ease in that 

 order for ever afterward. Number one awakens number two, and that 

 awakens number three, and so on, till the last is produced. A habit of 

 this kind once become inveterate, like the manipulations of certain 

 trades, the balancings of the body in standing or walking, the varying 

 pressure of the legs in response to the swayings of a horse's gait, may 

 go on automatically while the mind concerns itself with far other 

 affairs. And so it is with thoughts. Not only poems, but the multi- 

 plication-table, Greek verbs, and formulas of gibberish like " ana, mana, 

 mona, mike,'''' etc., cohere in the self -same order in which they have 

 once been learned. If we have blundered once in a certain place, we 

 are prone to repeat the mistake again. The higher and the lower 

 nerve-centers, then, are subject to one and the same law ; and the rea- 

 son of the law must be in both cases the same. The fact that there 

 are isolated tracts of conduction in all the centers, and that as we pass 

 from below upward the different centers have in the main different 

 characteristic functions, leads to the notion that each function, idea- 

 tional or motor, is dependent on a certain tract localized somewhere, 

 which tract when once excited may propagate the excitement to other 

 outlying tracts. The reason for the law of habit would, then, seem 

 to be that the propagation occurs easiest through those tracts of con- 

 duction which have been already most in use. Descartes and Locke 

 hit upon this explanation, which modern science has not yet succeeded 

 in improving. " Custom," says Locke, " settles habits of thinking in 

 the understanding as well as of determining in the will, and of motions 

 in the body ; all which seem to be but trains of motion in the animal 

 spirits (by this Locke meant identically what we understand by the 

 words neural j^rocess), which, once set agoing, continue in the same 

 steps they have been used to, which by often treading are worn into a 

 smooth path, and the motion in it becomes easy and as it were natural." 



Let us, then, assume as the basis of all our subsequent reasoning 

 the following law : Wheii two hrain tracts or processes have occurred 

 together or in immediate succession, any one of them, on reoccurring, 

 tends to propagate its excitement into the other. 



Now, as a matter of fact, things in the brain are much less simple 



