§ 1. Twofold Aspect of Ideational Process. — In the last chapter we considered the characteristics of images as compared with percepts. We have now to deal with trains of ideas, — with the sequence and combination of images and their meanings. Trains of ideas, like trains of perceptual activity, have in general a certain unity and continuity of interest. They subserve some end, practical or theoretical. Those transitions in the flow of ideas which show a break in continuity of interest are in general transitions from one train to another. It should be noted that the interest which gives unity to a single train may be very slight and evanescent. Thus the train may be no more than a passing thought. It may appear to consist of a single idea; but if it tends to gratify any interest, however evanescent, it may none the less be regarded as a continuous train.

The course of a train of ideas is determined by two distinct groups of conditions. On the one hand it is reproductive, and on the other productive. The material for it must be derived from past experience. But this material is variously shaped and transformed by the total mental condition existing when the ideal revival takes place. Even when we are interested in reviving past events, as such, preserving as far as possible their original nature and order, yet the mode in which they appear to consciousness is determined by the circumstances of the present, and by all that has taken place since their original occurrence. It is for this reason that on their revival they come before consciousness as past events; whereas on their original occurrence they bore the character of present experiences.

Every train of ideas has both a reproductive and a productive aspect; though the relative dominance of the two aspects may vary indefinitely. We shall first consider the reproductive side of the process, under the head, Association of Ideas; and then the productive, under the head, Ideal Construction.