The meaning of this can be understood by the study of the soul. Man's appetite is satisfied by food; but behind it is an appetite which is the appetite of the soul, and that appetite is never satisfied. That appetite is at the back of all the different forms of hunger and of thirst. And since man cannot trace that innermost appetite he strives all through his life to satisfy these outer appetites, which are satisfied and yet remain unsatisfied. If someone is making a study of objective things, things of the objective world, he may gain a great deal of knowledge about them, and yet there is never an end to it. The one who searches the secret of sound, the one who searches the mystery of light, the one who searches the mystery of science, they all search and search and search, and there is never an end to it, nor is there ever satisfaction. And a thoughtful person wonders if that satisfaction is to be found anywhere, the satisfaction which so to speak fulfills the promise of the soul.