It is one of the most difficult things to measure that I know. Take it in your own case: what tires you one day does not tire you another day. The individuality of it, the disturbing factors when we try to measure it, are perfectly extraordinary. Such a disturbing factor in our calculations is "second wind"--mental or physical. A number of men marching along will grow less tired as time goes on by the acquisition of what we call "second wind." We do not know what it is. We have tried to connect it with the condition of the heart, to say that the heart finally gets to deal with the volume of blood that is running through it so that there is no overplus of blood stored in any one chamber at any moment. But we do not really know anything about that. We do not know what second wind is; but it is important to know that it exists. Moreover, as Professor William James pointed out in that essay called "The Energies of Men," there are "mental second winds." Just when a man is worn out he often finds new strength. He often cannot get his best strength until he pushes himself even to despair. In the spiritual experiences of the world's saints and heroes we find that it was just when it seemed as if they were about to go under that this second wind, or third wind, for it sometimes comes again and again, this mitigation of fatigue without rest, comes to them. This is a most disturbing fact. If we were like a pitcher which is emptied out and filled up, we should know all about fatigue very soon. We are like a pitcher to a certain extent, but the similarity is disturbed by such factors as second wind, and disturbed, moreover, by mental and emotional intruders like music. A military band coming upon a body of marching men will give them strength when they had no strength. That is not a sentimental but a practical fact which army men have to take advantage of. Then the fact that many people can rest by change of work without stopping, is also disconcerting. We say to a person, "You have been working hard all day; you must stop, lie down, go to bed." That person disobeys, keeps going on something different, is altogether fresh next morning, and we have to confess that we were wrong. It is a very familiar experience that one may be almost dead from one point of view, but quite fresh from another, as one wants no more meat, but has plenty of room for dessert. Some people can rest by change of work and some cannot. It is very important for us to keep finding out in a great number of ways which of the classes into which people's bodies are divided we each belong to. Do we belong in the class of the people who must get their rest by giving up, by the abolition of all function, or in the class who rest by the change of function, by doing something different from the day's work? It is a question of fact and must be found out by each individual for himself.