In these words Goethe expresses a trait which is deeply ingrained in human nature. Man is not a self-contained unity. He demands ever more than the world, of itself, offers him. Nature has endowed us with needs; among them are some the satisfaction of which she leaves to our own activity. However abundant the gifts which we have received, still more abundant are our desires. We seem born to dissatisfaction. And our desire for knowledge is but a special instance of this unsatisfied striving. Suppose we look twice at a tree. The first time we see its branches at rest, the second time in motion. We are not satisfied with this observation. Why, we ask, does the tree appear to us now at rest, then in motion? Every glance at nature evokes in us a multitude of questions. Every phenomenon we meet presents a new problem to be solved. Every experience is to us a riddle. We observe that from the egg there emerges a creature like the mother animal, and we ask for the reason of the likeness. We observe a living being grow and develop to a determinate degree of perfection, and we seek the conditions of this experience. Nowhere are we satisfied with the facts which nature spreads out before our senses. Everywhere we seek what we call the explanation of these facts. The something more which we seek in things, over and above what is immediately given to us in them, splits our whole being into two parts. We become conscious of our opposition to the world. We oppose ourselves to the world as independent beings. The universe has for us two opposite poles: Self and World. We erect this barrier between ourselves and the world as soon as consciousness is first kindled in us. But we never cease to feel that, in spite of all, we belong to the world, that there is a connecting link between it and us, and that we are beings within, and not without, the universe.