592 THE POPULAR SCIENCE MONTHLY. 



Why a single portion of the passing thought should break out from 

 its concert with the rest and act, as we say, on its own hook, why the 

 other parts should become inert, are mysteries which we can ascertain 

 but not explain. Possibly a minuter insight into the laws of neural 

 action will some day clear the matter up ; possibly neural laws will 

 not suffice, and we shall need to invoke a dynamic reaction of the form 

 of consciousness upon its content. But into this we can not enter now. 



Thus the difference between the three kinds of association reduces 

 itself to a simple difference in the Amount of that portion of the nerve- 

 tract supporting the going thought ichich is operative in calling up 

 the thought which comes, but the modus operandi of this active part 

 is the same, be it large or be it small. 



The items constituting B waken in every instance because their 

 nerve-tracts once were excited continuously with those of A or its 

 operative part. This ultimate physiological law {supra, p. 583) is what 

 runs the train. The direction of its course and the form of its transi- 

 tions, whether contiguous or similar, are due to unknown regulative or 

 determinative conditions which accomplish their effect by opening this 

 switch and closing that, setting the engine sometimes at half speed, 

 and coupling or uncoupling cars. 



This last figure of speech affords itself an excellent instance of 

 association by similarity. I was thinking of the deflections of the 

 course of ideas. Now, from Hobbes's time downward English writers 

 have been fond of speaking of the train of our representations. This 

 word happened to stand out in the midst of my complex thought with 

 peculiarly sharp accentuation, and to surround itself with numerous de- 

 tails of railroad imagery. Only such details became clear, however, 

 as had their nerve-tracts besieged by a double set of influences — those 

 from train on the one hand, and those from the movement of thought 

 on the .other. It may possibly be that the prepotency of the sugges- 

 tions of the word traiyi at this moment were due to the recent excita- 

 tion of the railroad brain-tract by the instance chosen a few pages 

 back of a railroad king playing foot-ball with the stock-market. 



It is apparent fi'om such an example how inextricably complex are 

 all the contributory factors whose resultant is the line of our reverie. 

 It would be folly in most cases to attempt to trace them out. From 

 an instance like the above, where the pivot of the Similar Association 

 was formed by a definite concrete word, trai^i, to those where it is so 

 subtile as utterly to elude our analysis, the passage is unbroken. We 

 can form a series of examples. When Mr. Bagehot says that the mind 

 of the savage, so far from being in a state of nature, is tattooed all over 

 with monstrous superstitions, the case is very like the one we have just 

 been considering. When Sir James Stephen compares our belief in 

 the uniformity of nature, the congruity of the future with the past, to 

 a man rowing one way and looking another, and steering his boat by 

 keeping her stern in a line with an object behind him, the operative 



