A man who keeps these muscles in good shape will be much readier for every sort of activity than if he allowed them to yield, and one of the secrets of the army officers' power as the years go on so that, quite contrary to the usual rule in life it is men well beyond sixty who make some of the greatest successes as leaders, is because their {325} regular training and discipline keep them from letting their muscles lose tone and their powers deteriorate. The setting-up exercises of the army or navy indulged in for fifteen minutes a day--and this could be divided into two periods--would keep men in condition and prove at the same time a very salutary mortification and above all an exercise in self-control and persistent application to a good purpose that would constitute a magnificent factor for that training of the will that is so important for religion. Nerve irritation is oftener a function of insufficient exercise and air with overfeeding than of any other factor. This same thing is true for suspicion and jealousy and envy and other of the supposed inner emotions of the soul. They will disappear very often before the fresh air, while they will be fostered by life indoors and by the coddling of ills by rest and high living generally. The passions, by which a great many people mean mainly the sexual feelings, though of course they also include the tendency to overeat and especially to stimulate one's self to eating in various ways, are all fostered by being much indoors and not getting enough fresh, outdoor air and particularly cold air. There are a great many people who seem to forget that air is absolutely the most important requisite for life, and that when it is cold it takes heat away from the animal body and sets all the cell functions working at their best. Human beings are practically heat engines, and we keep on manufacturing heat all the time. As our temperature never rises except when we suffer from fever, some outlet must be found for this heat, and unless there is exposure to the air and especially air in motion that will carry heat away from us, the heat is consumed in {326} various large organs and almost invariably succeeds in making us quite miserable. It is under these circumstances of sluggish indoor living particularly that irritability of all kinds is heightened and that the tendency to lack of self-control is most manifest. It is a form of intoxication actually that comes over people and would remind us not a little of the intoxication that follows from smaller amounts of alcohol with the resultant lack of inhibition.