3. Again, in the study of the child's mind we have the added advantage of a corresponding simplicity on the bodily side; we are able to take account of the physiological processes at a time when they are relatively simple--that is, before the nervous system has grown to maturity. For example, psychology used to hold that we have a "speech faculty," an inborn mental endowment which is incapable of further analysis; but support for the position is wanting when we turn to the brain of the infant. Not only do we fail to find the series of centres now known to be the "speech zone," but even those of them which we do find have not yet taken up this function, either alone or together. In other words, the primary object of each of the various centres involved is not speech, but some other and simpler function; and speech arises by development from a union of these separate functions.
4. In observing young children, a more direct application of experiment is possible. By "experiment" here I mean both experiment on the senses and also experiment directly on consciousness by suggestion, social influence, etc. In experimenting on adults, great difficulties arise through the fact that reactions--such as performing a voluntary movement when a signal is heard, etc.--are complicated by deliberation, habit, custom, choice, etc. The subject hears a sound, identifies it, and presses a button--_if he choose_ and agree to do so. What goes on in this interval between the advent of the incoming nerve process and the discharge of the outgoing nerve process? Something, at any rate, which represents a brain process of great complexity. Now, anything that fixes or simplifies the brain process, in so far gives greater certainty to the results. For this reason experiments on reflex actions are valuable and decisive where similar experiments on voluntary actions are uncertain and of doubtful value. Now the child's mind is relatively simple, and so offers a field for more fruitful experiment; this is seen in the reactions of the infant to strong stimuli, such as bright colours, etc., as related further on.
With this inadequate review of the advantages of infant psychology, it is well also to point out the dangers of the abuse of it. Such dangers are real. The very simplicity which seems to characterize the life of the child is often extremely misleading, and this because the simplicity in question is sometimes ambiguous. Two actions of the child may appear equally simple; but one may be an adaptive action, learned with great pains and really very complex, while the other may be inadaptive and really simple. Children differ under the law of heredity very remarkably, even in the simplest manifestations of their conscious lives. It is never safe to say without qualification: "This child did, consequently all children must." The most we can usually say in observing single children is: "This child did, consequently another child may."