My thesis, stated thus briefly, will soon become more clear; and at the same time certain disturbing factors, which coöperate with the law of neural habit, will come to view.
Let us then assume as the basis of all our subsequent reasoning this law: When two elementary brain-processes have been active together or in immediate succession, one of them, on re-occurring, tends to propagate its excitement into the other.
But, as a matter of fact, every elementary process has unavoidably found itself at different times excited in conjunction with many other processes. Which of these others it shall awaken now becomes a problem. Shall b or c be aroused next by the present a? To answer this, we must make a further postulate, based on the fact of tension in nerve-tissue, and on the fact of summation of excitements, each incomplete or latent in itself, into an open resultant (see [p. 128]). The process b, rather than c, will awake, if in addition to the vibrating tract a some other tract d is in a state of sub-excitement, and formerly was excited with b alone and not with a. In short, we may say:
The amount of activity at any given point in the brain-cortex is the sum of the tendencies of all other points to discharge into it, such tendencies being proportionate (1) to the number of times the excitement of each other point may have accompanied that of the point in question; (2) to the intensity of such excitements; and (3) to the absence of any rival point functionally disconnected with the first point, into which the discharges might be diverted.
Expressing the fundamental law in this most complicated way leads to the greatest ultimate simplification. Let us, for the present, only treat of spontaneous trains of thought and ideation, such as occur in revery or musing. The case of voluntary thinking toward a certain end shall come up later.
Spontaneous Trains of Thought.—Take, to fix our ideas, the two verses from 'Locksley Hall':
"I, the heir of all the ages in the foremost files of time,"
and—
"For I doubt not through the ages one increasing purpose runs."
Why is it that when we recite from memory one of these lines, and get as far as the ages, that portion of the other line which follows and, so to speak, sprouts out of the ages does not also sprout out of our memory and confuse the sense of our words? Simply because the word that follows the ages has its brain-process awakened not simply by the brain-process of the ages alone, but by it plus the brain-processes of all the words preceding the ages. The word ages at its moment of strongest activity would, per se, indifferently discharge into either 'in' or 'one.' So would the previous words (whose tension is momentarily much less strong than that of ages) each of them indifferently discharge into either of a large number of other words with which they have been at different times combined. But when the processes of 'I, the heir of all the ages,' simultaneously vibrate in the brain, the last one of them in a maximal, the others in a fading, phase of excitement, then the strongest line of discharge will be that which they all alike tend to take. 'In' and not 'one' or any other word will be the next to awaken, for its brain-process has previously vibrated in unison not only with that of ages, but with that of all those other words whose activity is dying away. It is a good case of the effectiveness over thought of what we called on [p. 168] a 'fringe.'