or filling in words which have been omitted from a certain text, we cannot easily make two tasks sufficiently alike to be able to compare the results obtained from them. But none of these methods solve the chief problem, namely, the determination of the point at which fatigue begins to be permanently harmful. There is no doubt that in moderate degrees fatigue is a perfectly normal phenomenon, involving no detriment to our future efficiency. Otherwise most people would be wrecked before they are fully grown. The experience of athletes and soldiers shows that even rather high degrees of fatigue are compatible with the normal growth of bodily strength. The same may be true for mental life. The assertions of great damage done to children by school work are--so far as normal children are concerned--certainly greatly exaggerated. QUESTIONS 106. What are the effects of fatigue? 107. Into what complication does fatigue enter with practice? 108. What attempts have been made at measuring fatigue? 109. What is the chief problem in connection with fatigue? 110. Is the fatigue of school work harmful? _C. THE EXPRESSIONS OF MENTAL LIFE_ § 12. PERCEPTION AND MOVEMENT The impression upon the mind is not the ultimate end of the nervous processes originating in the sense organs. The end is rather activity of the motor organs of the body, which we may here, accepting the naïve conception of matter and mind, regard as effects or expressions of mind. The complications of the mental life of a grown person tend to make this connection between mind and motor activity often obscure and doubtful. It seems that often we receive impressions quite passively. Nevertheless the connection exists. Every impression made upon the mind by the external world is in some way responded to by movement. The movement may occur in the stimulated sense organ itself, in the arms, the hands, the fingers, the legs, the feet, the head, the vocal organs, also in the internal organs, the heart, the blood vessels, the alimentary canal, the lungs. The significance of many of these movements is but insufficiently understood, for example, laughing, weeping, blushing, trembling. But those movements which directly affect the organism’s surroundings are easily understood. They may be classed under two headings, self-preservation and play. Another way of classifying them is to distinguish movement toward the object perceived and movement away from the object, without taking these terms in too literal a sense.