IN THESE WORDS Goethe expresses a characteristic feature belonging to the deepest foundation of human nature. Man is not a uniformly organized being. He always demands more than the world gives him of its own accord. Nature has endowed us with needs; among them are some that are left to our own initiative to satisfy. Abundant are the gifts bestowed upon us, but still more abundant are our desires. We seem born to be dissatisfied. Our thirst for knowledge is but a special instance of this dissatisfaction. If we look twice at a tree and the first time see its branches motionless, the second time in movement, we do not remain satisfied with this observation. Why does the tree appear to us now motionless, now in movement? Thus we ask. Every glance at nature evokes in us a number of questions. Every phenomenon we meet sets us a problem. Every experience contains a riddle. We see emerging from the egg a creature like the mother animal; we ask the reason for this likeness. We notice that living beings grow and develop to a certain degree of perfection and we investigate the conditions for this experience. Nowhere are we satisfied with what nature spreads before our senses. Everywhere we seek what we call explanation of the facts.
The something more which we seek in things, over and above what is given us directly in them, divides our whole being into two aspects; we become conscious of our contrast to the world. We confront the world as independent beings. The universe appears to us to have two opposite poles: I and world.
We erect this barrier between ourselves and the world as soon as consciousness first dawns in us. But we never cease to feel that, in spite of all, we belong to the world, that there is a bond of union between it and us, that we are not beings outside, but within, the universe.
This feeling makes us strive to bridge over the contrast. And in this bridging the whole spiritual striving of mankind ultimately consists. The history of man's spiritual life is an incessant search for unity between us and the world. Religion, art and science all have this same aim. In the revelation God grants him, the religious believer seeks the solution of the problems in the world which his I, dissatisfied with the world of mere phenomena, sets him. The artist seeks to imprint into matter the ideas of his I, in order to reconcile with the world outside what lives within him. He, too, feels dissatisfied with the world as it appears to him, and seeks to embody into the world of mere phenomena that something more which his I, reaching out beyond it, contains. The thinker seeks the laws of phenomena, and strives to penetrate with thinking what he experiences by observing. Only when we have made the world-content into our thought-content do we again find the unity from which we separated ourselves. We shall see later that this goal will be reached only when the task of the scientific investigator is understood at a much deeper level than is usually the case. The whole situation I have described here, presents itself to us on the stage of history in the contrast between a unified view of the world or monism,  and the theory of two worlds or dualism.  Dualism pays attention only to the separation between I and world, brought about by man's consciousness. All its efforts consist in a vain struggle to reconcile these opposites, which it calls spirit and matter, subject and object, or thinking and phenomena. The dualist feels that there must be a bridge between the two worlds, but he is unable to find it. In as far as man is aware of himself as “I,” he cannot but think of this “I” as belonging to spirit; and in contrasting this “I” with the world he cannot do otherwise than reckon the perceptions given to the senses, the realm of matter, as belonging to the world. In doing so, man places himself within the contrast of spirit and matter. He must do so all the more because his own body belongs to the material world. Thus the “I” belongs to the realm of spirit, as part of it; the material things and events which are perceived by the senses belong to the “world.” All the problems connected with spirit and matter, man finds again in the fundamental riddle of his own nature. Monism pays attention only to the unity and tries either to deny or to efface the contrasts, which are there nevertheless. Neither of these two views is satisfactory, for they do not do justice to the facts. Dualism sees spirit (I) and matter (world) as two fundamentally different entities and cannot, therefore, understand how they can interact upon each other. How should spirit know what goes on in matter, if the essential nature of matter is quite alien to spirit? And how, in these circumstances, should spirit be able to act upon matter, in order to transform its intentions into actions? The most clever and the most absurd hypotheses have been put forward to solve these problems. But, so far, monism has fared no better. Up to now it has tried to justify itself in three different ways. Either it denies spirit and becomes materialism; or it denies matter and seeks its salvation in spiritualism;  or it maintains that since even in the simplest entities in the world spirit and matter are indivisibly bound together, there is no need for surprise if these two kinds of existence are both present in the human being, for they are never found apart.
Materialism  can never arrive at a satisfactory explanation of the world. For every attempt at an explanation must of necessity begin with man's forming thoughts about the phenomena of the world. Materialism, therefore, takes its start from thoughts about matter or material processes. In doing so, it straightway confronts two different kinds of facts, namely, the material world and the thoughts about it. The materialist tries to understand thoughts by regarding them as a purely material process. He believes that thinking takes place in the brain much in the same way that digestion takes place in the animal organs. Just as he ascribes to matter mechanical and organic effects, so he also attributes to matter, in certain circumstances, the ability to think. He forgets that in doing this he has merely shifted the problem to another place. Instead of to himself, he ascribes to matter the ability to think. And thus he is back again at his starting-point. How does matter come to reflect about its own nature? Why is it not simply satisfied with itself and with its existence? The materialist has turned his attention away from the definite subject, from our own I, and has arrived at a vague, indefinite image. And here again, the same problem comes to meet him. The materialistic view is unable to solve the problem; it only transfers it to another place.
How does the matter stand with the spiritualistic view? The extreme spiritualist denies to matter its independent existence and regards it merely as product of spirit. But when he tries to apply this view of the world to the solution of the riddle of his own human nature, he finds himself in a corner. Confronting the I, which can be placed on the side of spirit, there stands, without any mediation, the physical world. No spiritual approach to it seems possible; it has to be perceived and experienced by the I by means of material processes. Such material processes the “I” does not find in itself if it regards its own nature as having only spiritual validity. The physical world is never found in what it works out spiritually. It seems as if the “I” would have to admit that the world would remain closed to it if it did not establish a non-spiritual relation to the world. Similarly, when we come to be active, we have to translate our intentions into realities with the help of material substances and forces. In other words, we are dependent upon the outer world. The most extreme spiritualist — or rather, the thinker who, through absolute idealism, appears as an extreme spiritualist — is Johann Gottlieb Fichte.  He attempts to derive the whole edifice of the world from the “I.” What he has actually accomplished is a magnificent thought-picture of the world, without any content of experience. As little as it is possible for the materialist to argue the spirit away, just as little is it possible for the idealist to argue away the outer world of matter.
The first thing man perceives when he seeks to gain knowledge of his “I” is the activity of this “I” in the conceptual elaboration of the world of ideas. This is the reason why someone who follows a world-view which inclines toward spiritualism may feel tempted, when looking at his own human nature, to acknowledge nothing of spirit except his own world of ideas. In this way spiritualism becomes one-sided idealism. He does not reach the point of seeking through the world of ideas a spiritual world; in the world of his ideas he sees the spiritual world itself. As a result of this, he is driven to remain with his world-view as if chained within the activity of his “I.”
The view of Friedrich Albert Lange  is a curious variety of idealism, put forward by him in his widely read History of Materialism. He suggests that the materialists are quite right in declaring all phenomena, including our thinking, to be the product of purely material processes, only, in turn, matter and its processes are themselves the product of our thinking.
“The senses give us the effects of things, not true copies, much less the things themselves. To these mere effects belong the senses themselves, as well as the brain and the molecular vibrations which are thought to go on there.”
That is, our thinking is produced by the material processes, and these by the thinking of the “I.” Lange's philosophy, in other words, is nothing but the story — applied to concepts — of the ingenious Baron Münchhausen,  who holds himself up in the air by his own pigtail.
The third form of monism is the one which sees the two entities, matter and spirit, already united in the simplest being (the atom). But nothing is gained by this, either, for here again the question, which really originates in our consciousness, is transferred to another place. How does the simple being come to manifest itself in two different ways, if it is an indivisible unity?
To all these viewpoints it must be objected that it is first and foremost in our own consciousness that we meet the basic and original contrast. It is we who detach ourselves from the bosom of nature and contrast ourselves as “I” with the “world.” Goethe  has given classic expression to this in his essay On Nature although at first glance his manner may be considered quite unscientific: “We live in the midst of her (nature) yet are we strangers to her. Ceaselessly she speaks to us, and yet betrays not her secrets.” But Goethe knew the other side too: “All human beings are in her and she is in all human beings.”
Just as true as it is that we have estranged ourselves from nature, so is it also true that we feel: We are within nature and we belong to it. That which lives in us can only be nature's own influence.
We must find the way back to nature again. A simple consideration can show us this way. We have, it is true, detached ourselves from nature, but we must have taken something of it over with us, into our own being. This essence of nature in us we must seek out, and then we shall also find the connection with it once again. Dualism neglects this. It considers the inner being of man as a spiritual entity quite alien to nature, and seeks somehow to hitch it onto nature. No wonder it cannot find the connecting link. We can only understand nature outside us when we have first learned to recognize it within us. What within us is akin to nature must be our guide. This points out our path. We shall not speculate about the interaction of nature and spirit. But we shall penetrate the depths of our own being, there to find those elements which we took with us in our flight from nature.
Investigation of our own being must bring the solution of the riddle. We must reach a point where we can say to ourselves: Here I am no longer merely “I,” here I encounter something which is more than “I.”
I am aware that many who have read thus far will not have found my discussion “scientific” in the usual sense. To this I can only reply that so far I have not been concerned with scientific results of any kind, but with the simple description of what everyone experiences in his own consciousness. A few expressions concerning the attempts to reconcile man's consciousness and the world have been used only for the purpose of clarifying the actual facts. I have, therefore, made no attempt to use the expressions “I,” “spirit,” “world,” “nature,” in the precise way that is usual in psychology and philosophy. Ordinary consciousness is unaware of the sharp distinctions made by the sciences, and up to this point it has only been a matter of describing the facts of everyday conditions. I am concerned, not with how science, so far, has interpreted consciousness, but with how we experience it in daily life.