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 which 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 transitions, 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 details 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 suggestions of the word train at this moment were due to the recent excitation 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 from 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, train, 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