[Footnote 50: Sidis: P. 112 of the composite volume _Pychotherapeutics_.] =What about Being Tired?= If all these things are true, why do people need to be told? If man's equipment is so adequate and his reserves are so ample, why after all these centuries of living does the human race need to learn from science the truth about its own powers? The average man is very likely to say that it is all very well for a scientist sitting in his laboratory to tell him about hidden resources, but that he knows what it is to be tired. Is not the crux of the whole question summed up in that word "tired"? If we do not need to rest, why should fatigue exist? If the purpose of fatigue seems to be to slow down our efforts, why should we disregard it or seek to evade its warnings? The whole question resolves itself into this: What is fatigue? In view of the hampering effect of misconception on this point, it is evident that the question is not academic, but intensely practical. We shall find that fatigue is of two kinds,--true and false, or physical and moral, or physiological and nervous,--and that while the two kinds feel very much alike, their origin and behavior are quite different. PHYSIOLOGICAL FATIGUE =Fatigue, not Exhaustion.= In the first place, then, fatigue very seldom means a lack of strength or an exhaustion of energy. The average man in the course of a lifetime probably never knows what it is to be truly exhausted. If he should become so tired that he could in no circumstances run for his life, no matter how many wild beasts were after him, then it might seem that he had drained himself of all his store of energy. But even in that case, a large part of his fatigue would be the result of another cause. =A Matter of Chemistry.= True fatigue is a chemical affair. It is the result of recent effort,--physical, mental, or emotional,--and is the sum of sensations arising from the presence of waste material in the muscles and the blood. The whole picture becomes clear if we think of the body as a factory whose fires continuously burn, yielding heat and energy, together with certain waste material,--carbon dioxide and ash. Within man's body the fuel, instead of being the carbon of coal is the carbon of glycogen or animal starch, taken in as food and stored away within the cells of the muscles and the liver. The oxygen for combustion is continuously supplied by the lungs. So far the factory is well equipped to maintain its fires. Nor does it fail when it comes to carrying away waste products. Like all factories, the body has its endless chain arrangement, the blood stream, which automatically picks up the debris in its tiny buckets--the blood-cells and serum--and carries it away to the several dumping-grounds in lungs, kidneys, intestines, and skin.