Of course there are limits; the trees don’t grow into the sky.  But the plain fact remains that men the world over possess amounts of resource which only very exceptional individuals push to their extremes of use.  But the very same individual, pushing his energies to their extreme, may in a vast number of cases keep the pace up day after day, and find no “reaction” of a bad sort, so long as decent hygienic conditions are preserved.  His more active rate of energizing does not wreck him; for the organism adapts itself, and as the rate of waste augments, augments correspondingly the rate of repair.[49]

[Footnote 49:  Ibid., pp. 6-7.]

Another psychologist, Boris Sidis, writes:  “But a very small fraction of the total amount of energy possessed by the organism is used in its relation with the ordinary stimuli of its environment."[50] These men—­Professor James and Dr. Sidis—­represent not young enthusiasts who ignorantly fancy that every one shares their own abundant strength, but careful men of science who have repeatedly been able to unearth unsuspected supplies of energy in “worn out” men and women, supposed to be at the end of their resources.  Every successful physician and every leader of men knows the truth of these statements.  What would have happened in the great war if Marshal Foch had not known that his men possessed powers far beyond their ken, and had not had sublime faith in the “second wind”?

[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.