SECTION 20. In the first part of this work I have insisted upon the great value of _health_ as the chief and most important element in happiness. Let me emphasize and confirm what I have there said by giving a few general rules as to its preservation. The way to harden the body is to impose a great deal of labor and effort upon it in the days of good health,--to exercise it, both as a whole and in its several parts, and to habituate it to withstand all kinds of noxious influences. But on the appearance of an illness or disorder, either in the body as a whole or in many of its parts, a contrary course should be taken, and every means used to nurse the body, or the part of it which is affected, and to spare it any effort; for what is ailing and debilitated cannot be hardened. The muscles may be strengthened by a vigorous use of them; but not so the nerves; they are weakened by it. Therefore, while exercising the muscles in every way that is suitable, care should be taken to spare the nerves as much as possible. The eyes, for instance, should be protected from too strong a light,--especially when it is reflected light,--from any straining of them in the dark, or from the long-continued examination of minute objects; and the ears from too loud sounds. Above all, the brain should never be forced, or used too much, or at the wrong time; let it have a rest during digestion; for then the same vital energy which forms thoughts in the brain has a great deal of work to do elsewhere,--I mean in the digestive organs, where it prepares chyme and chyle. For similar reasons, the brain should never be used during, or immediately after, violent muscular exercise. For the motor nerves are in this respect on a par with the sensory nerves; the pain felt when a limb is wounded has its seat in the brain; and, in the same way, it is not really our legs and arms which work and move,--it is the brain, or, more strictly, that part of it which, through the medium of the spine, excites the nerves in the limbs and sets them in motion. Accordingly, when our arms and legs feel tired, the true seat of this feeling is in the brain. This is why it is only in connection with those muscles which are set in motion consciously and voluntarily,--in other words, depend for their action upon the brain,--that any feeling of fatigue can arise; this is not the case with those muscles which work involuntarily, like the heart. It is obvious, then, that injury is done to the brain if violent muscular exercise and intellectual exertion are forced upon it at the same moment, or at very short intervals.