Weakness of nerves is not only the cause of physical diseases, but it leads to insanity. There is one principal cause of physical diseases as well as of mental diseases: overstrained nerves, exhausted nerves; and that person whose nerves are exhausted, in spite of all virtue and goodness, goodwill and desire to do right, will prove to be doing wrong, to his own surprise, because he has lost self-discipline. His high ideals are of no use to him, for he has not got himself in hand. His qualifications, his knowledge, his attitude, his morals will all prove to be futile in the absence of that nervous force which keeps man fit and capable of doing all that it is proper for him to do in the world.