§ 4. Retentiveness.—Retentiveness in some form is an indispensable condition of development or progress of anykind. Advance would be impossible unless the results of prior process persisted as the basis and startingpoint of subsequent process. In marching, each step has its point of departure from the new position secured by the previous step. In marking time there is continual reversion to the same position and no advance. No house could be built if each brick vanished as it was laid and had to be replaced anew. A rope cannot be formed of dry sand, which crumbles away as it is put together. Similarly, mental development would be impossible unless previous experience left behind it persistent aftereffects to determine the nature and course of subsequent experience. These aftereffects are called, in psychology, traces or dispositions, and the psychological law of retentiveness may bo stated as follows: when and so far as mental development takes place through mental conditions, it does so because specific modes of consciousness leave behind them specific traces or dispositions, which determine the nature and course of subsequent process, so that when they are modified it is modified.

The persistence of dispositions is not absolute; they tend to decay, and may perhaps disappear altogether if they are not maintained by renewal of the corresponding mental processes, or of mental processes connected with these. In this respect there is a great difference between different individuals. Some are more retentive than others. But even in the most retentive minds, traces tend to fade away: "so that if they be not sometimes renewed by repeated exercise of the senses, or reflection on those kinds of objects which at first occasioned them, the print wears out, and at last there remains nothing to be seen." Thus the experiences, "as well as children, of our youth, often die before us ; and our minds represent to us those tombs to which we are fast approaching, where, though the brass and marble remain, yet the inscriptions are effaced by time, and the imagery moulders away."* The differences in the retentive power of individuals are, in part at least, differences in original endowment, and cannot be explained on psychological grounds. As Locke remarks, some minds retain the characters drawn on them "like marble," others "like freestone," and others ''little better than sand." The ultimate explanation of this difference in original endowment must take a physiological form.