What is the difference between the case where the will to learn is necessary, and the case where it is unnecessary? The difference is that in the one case we observe facts for the purpose of committing them to memory, and in the other case we observe the facts without any such intention. In both cases we remember what we have definitely observed, {348} and fail to remember what we have not observed. Sometimes, to be sure, it is not so much observation as doing that is operative. We may make a certain reaction with the object of learning it so as to make it later, or we may make the reaction for some other reason; but in either case we learn it. What is essential, then, is not the will to learn, but the doing and observing. The will to learn is sometimes important, as a directive tendency, to steer doing and observing into channels relevant to the particular memory task that we need to perform. But committing to memory seems not to be any special form of activity; rather, it consists of reactions that also occur without any view to future remembering. Not only do we learn _by_ doing and observing, but doing and observing _are_ learning. Retention We come now to the second of our four main problems, and ask how we retain, or carry around inside of us, what we have learned. The answer is, not by any process or activity. Retention is a resting state, in which a learned reaction remains until the stimulus arrives that can arouse it again. We carry around with us, not the reaction, but the machinery for making the reaction. Consider, for example, the retention of motor skill. A boy who has learned to turn a handspring does not have to keep doing it all the time in order to retain it. He may keep himself in better form by reviewing the performance occasionally, but he retains the skill even while eating and sleeping. The same can be said of the retention of the multiplication table, or of a poem, or of knowledge of any kind. The machinery that is retained consists very largely in brain connections. Connections formed in the process of {349} learning remain behind in a resting condition till again aroused to activity by some appropriate stimulus. But the machinery developed in the process of learning is subject to the wasting effects of time. It is subject to the law of "atrophy through disuse". Just as a muscle, brought by exercise into the pink of condition, and then left long inactive, grows weak and small, so it is with the brain connections formed in learning. With prolongation of the condition of rest, the machinery is less and less able to function, till finally all retention of a once-learned reaction may be lost.