Every one knows what it is to start a piece of work, either intellectual or muscular, feeling stale--or _cold_, as an Adirondack guide once put it to me. And everybody knows what it is to "warm up to his job." The process of warming up gets particularly striking in the phenomenon known as the "second wind." On usual occasions we make a practice of stopping an occupation as soon as we meet the first effective layer (so to call it) of fatigue. We have then walked, played or worked "enough," so we desist. That amount of fatigue is an efficacious obstruction on this side of which our usual life is cast. But if an unusual necessity forces us to press onward, a surprising thing occurs. The fatigue gets worse up to a certain critical point, when gradually or suddenly it passes away, and we are fresher than before. We have evidently tapped a level of new energy, masked until then by the fatigue-obstacle usually obeyed. There may be layer after layer of this experience. A third and fourth "wind" may supervene. Mental activity shows the phenomenon as well as physical, and in exceptional cases we may find, beyond the very extremity of fatigue-distress, amounts of ease and power that we never dreamed ourselves to own, sources of strength habitually not taxed at all, because habitually we never push through the obstruction, never pass those early critical points. Again Professor James says: 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"?