According to the teachings of the old physiology, our stomachs were fire-boxes of the human engine; food was fuel, and the stomach was supposed to transform this fuel into work or energy by a process not entirely clear. Just as it is impossible for the lifeless iron and steel, within itself to transform coal and water into dynamic power, and to apply that power to its own locomotion, so it is impossible and entirely incompatible with reason for mere muscular tissue of the body to extract enough energy from the food we eat to perform the work necessary for that transformation itself, besides enough more to carry on all the functional activities of the system, and at the same time to do hundreds of foot-tons of physical labor. In this fact lies the key to some understanding of the phenomenon of rest and sleep. The old physiology was really never able to explain how it was possible for the digestive apparatus to extract, from the amount of food consumed, the enormous amount of energy which the average person expends each day. REST AND RE-CREATION [Sidenote: Change of occupation not re-creation] These terms are often confused. When one is engaged in some occupation or activity other than his regular vocation, it is commonly called "re-creation." This is a misconception, because it is merely a change in activity and must also be more or less destructive to other sets of nerves or muscular tissue. It is not in reality re-creation--it simply throws the life-power into a new channel, which is more responsive, and calls for less action from those parts of the mechanism which have been employed in the work from which one is seeking relief. It is for this reason that we find some pleasure in a new and different activity, though it, too, may be destructive to the human cell. [Sidenote: Specialization in business not conducive to health] One may alternate from one kind of activity to another indefinitely, which would be better than _no_ change, but the human mechanism would finally give way under such violation of fundamental law. The mental worker may change, however, to any manual labor requiring little thought, and the physical worker to some form of mental labor, with far better effect. But, in our present civilization, specialization has become so far advanced that the physical laborer is seldom qualified for mental work, and the mental worker has almost neglected manual training. _True rest and re-creation is found in mental tranquility and sleep._ SLEEP From observation and study of the state we call sleep, we notice that as night approaches and the activities of the day wear upon us, both the nervous and the muscular organisms relax, so that it becomes more and more difficult to maintain a positive and an active attitude of mind. There is a tendency toward cessation and rest, which gradually brings upon us that passive condition called sleep. [Sidenote: Evidence of acquired energy during sleep]