WE HAVE ESTABLISHED that the elements for explaining reality are to be taken from the two spheres: perceiving and thinking. As we have seen, it is our organization that determines the fact that the full, complete reality of things, our own subject included, appears at first as a duality. Cognition overcomes this duality by combining the two elements of reality: the perception and the concept gained by thinking, into the complete thing. If we call the world as it confronts us before it has attained its true aspect by means of cognition, “the world of appearance,” in contrast to the unified whole composed of perception and concept, then we can say: The world is given us as a duality (dualistic), and cognition transforms it into a unity (monistic). A philosophy which starts from this basic principle may be called a monistic philosophy, or monism, in contrast to the theory of two worlds, or dualism. The latter does not assume that there are two sides of a single reality, which are kept apart merely by our organization, but, rather, that there are two worlds, completely different from each other. Then in the one world it tries to find the principles that can explain the other.
Dualism rests on a misunderstanding of what we call knowledge. It divides the whole of existence into two spheres, each of which has its own laws, and it lets these spheres stand opposite to and outside of each other.
It is from a dualism such as this that there arises the distinction between the perceived object and the thing-in-itself which Kant  introduced into science and which so far has not been expelled. From our discussion can be seen that it is due to the nature of our intellectual organization that a particular thing can be given us only as perception. Thinking then overcomes this separateness by referring each perception to its rightful place in the world whole. As long as the separated parts of the world whole are defined as perceptions, in this elimination we are simply following a law of our subjectivity. If, however, we consider the sum-total of all perceptions as constituting one part, and confront it with the “thing-in-itself” as a second part, then our philosophizing loses all foundation. It then becomes a mere playing with concepts. An artificial opposition is constructed, but it is not possible to attain a content for the second part of this opposition, since such content for a particular thing can be drawn only from perception.
Every kind of existence which is assumed outside the realm of perception and concept belongs to the sphere of unjustified hypotheses. The “thing-in-itself” belongs in this category. It is quite natural that a dualistic thinker should be unable to find the connection between a universal principle which he hypothetically assumes, and the given, known by experience. One can obtain a content for the hypothetical universal principle only by borrowing a content from the sphere of experience and then shutting one's eyes to the fact of the borrowing. Otherwise it remains an empty concept, a non-concept, which is nothing but a shell of a concept. Then the dualistic thinker usually maintains that the content of this concept is not accessible to our knowledge. We can know only that such a content must be present, but not what it is. In both cases it is impossible to overcome dualism. Even if one brings a few abstract elements from the sphere of experience into the concept of the thing-in-itself, it still remains impossible to derive the rich concrete life of experience from those few qualities which, after all, are themselves taken from perception only. DuBois-Reymond  thinks that the imperceptible atoms of matter produce sensation and feeling by means of their position and motion, and then comes to the conclusion: We can never find a satisfactory explanation of how matter and motion produce sensation and feeling, for
“It is absolutely and forever unintelligible that it should be other than indifferent to a number of atoms of carbon, hydrogen, and nitrogen, etc., how they lie and move, how they lay and moved, or how they will lie and will move. It is impossible to see how consciousness could come into existence through their interaction.”
This conclusion is characteristic of this whole trend of thought. Position and motion are abstractions derived from the rich sphere of perceptions. They are then transferred to the imagined world of atoms. Then astonishment arises that real life cannot be evolved out of this principle which is self-made and borrowed from the sphere of perceptions.
That the dualist who works with a completely empty concept of the “in-itself” of things can reach no explanation of the world, already follows from the definition of his principle indicated above.
A dualist is always compelled to set impassable barriers to our faculty of knowledge. The follower of a monistic world view knows that everything he needs for the explanation of any given phenomenon in the world must lie within this world itself. What hinders him from reaching the explanation can be only contingent limitations in space and time, or shortcomings of his organization. And, indeed, not of the human organization in general, but only of his own particular one.
It follows from the concept of cognition, as defined by us, that one cannot speak of limits to knowledge. Cognition is not a concern of the universe in general, but one which men must settle for themselves. Things claim no explanation. They exist and act on one another according to laws which thinking can discover. They exist in indivisible unity with these laws. Our egohood confronts them, grasping at first only what we have called perceptions. In the inner core of our egohood, however, we find the power to discover the other part of reality also. Only when the egohood has again combined for itself the two elements of reality which are indivisibly united in the world, is the thirst for knowledge satisfied: the I has again come to reality.
Therefore, the conditions required for cognition to arise, come about through and for the I. The I sets itself the problems of cognition. And it takes them from the element of thinking, in itself absolutely clear and transparent. If we ask questions we cannot answer, then the content of the question cannot be clear and distinct in all its details. The world does not set us the questions; it is we ourselves who set them.
I can imagine that it would be quite impossible for me to answer a question which I happened to find written down somewhere, without knowing the sphere from which the content of the question was taken.
In knowledge we are concerned with questions which arise for us through the fact that a sphere of perceptions, conditioned by time, space, and our subjective organization, is confronted by a sphere of concepts pointing to a world which is a unity. My task is to reconcile these two spheres, well known to me. One cannot speak here of a limit of knowledge. It may be that at a particular moment, this or that remains unexplained because, through our place in life, we are prevented from perceiving all that is involved. What is not found to-day, however, may be found tomorrow. The limits due to these causes are only transitory, and can be overcome by the progress of perceiving and thinking.
Dualism makes the mistake of transferring the antithesis of object and subject, which has significance only within the sphere of perceptions, to purely invented entities outside this sphere. But as the separate things within the field of perception remain separated only as long as the perceiver refrains from thinking, which cancels all separation and shows it to be due to merely subjective factors, so the dualist, in fact, transfers to entities behind the sphere of perceptions definitions which, even for perceptions, have no absolute but only relative validity. In doing this he splits up the two factors concerned in the process of cognition, perception and concept, into four: 1) the object-in-itself, 2) the perception which the subject has of the object, 3) the subject, 4) the concept which relates the perception to the object-in-itself. The relation between object and subject is considered to be real, that is, the subject is considered to be really (dynamically) influenced by the object. This real process is said not to appear in consciousness. But it is supposed to evoke in the subject a response to the stimulation from the object. The result of this response is said to be the perception. This at last enters our consciousness. The object is said to have an objective reality (independent of the subject), the perception a subjective reality. This subjective reality is said to be referred by the subject to the object. This latter reference is said to be an ideal one. The dualist, in other words, splits up the process of cognition into two parts. One part, i.e., the production of the perceptual object out of the thing-in-itself, takes place, according to him, outside of consciousness, the other part, the union of perception with concept and the reference of this to the object, within consciousness. These presuppositions make it clear that the dualist believes he receives in his concepts only something subjective, which represents what confronts his consciousness. The objectively real process in the subject, by means of which the perception comes about, and still more the objective relationships between things-in-themselves, remain inaccessible to direct cognition for such a dualist. In his opinion, man can obtain only concepts that represent the objectively real. The bond of unity which connects things with one another and also objectively with our individual spirit (as thing-in-itself), lies beyond consciousness in a being-in-itself of whom we likewise can have in our consciousness only a concept that represents it.
The dualist believes that the whole world would be nothing but a mere abstract scheme of concepts if he did not insist on “real” connections between the objects beside the conceptual ones. In other words, the ideal principles which can be discovered by thinking seem too airy for the dualist, and he seeks, in addition, “real principles” with which to support them.
Let us examine these “real principles” a little more closely. The naive man (naive realist) regards the objects of external experience as realities. The fact that his hands can grasp and his eyes can see these objects is for him the proof of their reality. “Nothing exists that cannot be perceived” is, in fact, the basic axiom of the naive man, and it is held to be equally valid in its converse: “Everything which can be perceived, exists.” The best proof for this assertion is the naive man's belief in immortality and in ghosts. He thinks of the soul as a fine kind of physical matter which, in special circumstances, may actually become visible to the ordinary man (naive belief in ghosts).
In contrast to this real world of his, the naive realist regards everything else, especially the world of ideas, as unreal, as “merely ideal.” What we add to objects by thinking is mere thoughts about the objects. Thought adds nothing real to perception.
But it is not only with reference to the existence of things that the naive man regards sense perception as the sole proof of reality, but also with reference to happenings. According to him, one thing can act upon another only when a force actually present to sense perception issues from the one and seizes upon the other. The older physicists thought that very fine substances emanate from the objects and penetrate through the sense-organs into the soul. They thought the actual seeing of these substances to be impossible only because of the coarseness of our sense-organs in comparison with the fineness of these substances. In principle, the reason for attributing reality to these substances was the same as that for attributing it to the objects of the physical world, namely, the form of their existence, which was thought to be analogous to that of physical reality.
The self-dependent nature of what can be experienced, not physically but ideally, is not regarded by naive consciousness as being real in the same sense. Something grasped “merely as idea” is regarded as a chimera until sense perception can provide conviction of its reality. In short, in addition to the ideal evidence of his thinking, the naive man demands the real evidence of his senses. This need of naive man is the reason why primitive forms of belief in revelation arise. For naive consciousness, the God who is given through thinking always remains a God merely “thought.” Naive consciousness demands that the manifestation should be through means accessible to physical perception. God must appear in bodily form; little value is attached to the evidence of thinking, but only to the Divine Nature being proved by the changing of water into wine in a way which can be testified by the senses.
The act of cognition, too, is regarded by naive man as a process analogous to sense-perception. Things must make an impression on the soul or send out images which penetrate the senses, etc.
What the naive man can perceive with his senses he regards as real, and that of which he has no such perception (God, soul, cognition, etc.) he regards as analogous to what is perceived.
A science based on naive realism will consist in an exact description of the content of perception. Concepts are only means to this end. They exist to provide ideal counterparts of perceptions. For things themselves, they have no significance. For the naive realist, only the individual tulips which are seen or could be seen, are real. The one idea of the tulip, is to him an abstraction, is to him an unreal thought-picture, which the soul has put together for itself out of the characteristics common to all tulips.
Naive realism, with its fundamental principle of the reality of all perceived things, is contradicted by experience, which shows us that the content of perceptions is of a transitory nature. The tulip I see, is real to-day; in a year it will have vanished into nothingness. What persists is the species tulip. This species, however, for the naive realist is “merely” an idea, not a reality. Thus, this world view finds itself in the position of seeing its realities arise and perish, while what it regards as unreal, in contrast to the real, persists. Hence the naive realist has to allow for the existence of something ideal besides the perceptions. He has to accept entities which he cannot perceive by means of the senses. He justifies this by imagining their existence to be analogous to that of physical objects. Such hypothetically assumed realities are the invisible forces by means of which objects perceptible to the senses act on one another. Heredity is thought of in this way; it goes beyond the individual and is the reason why a new being develops from the individual which is similar to it, and by means of it the species is maintained. The life principle permeating the organic body is also thought of in this way, and so is the soul, for which one always finds in naive consciousness a concept based on an analogy to sense-reality, and finally so, too, the naive man thinks of the Divine Being. This Divine Being is thought of as active in a manner exactly corresponding to what can be perceived as actions of men, that is, the Divine Being is thought of anthropomorphically.
Modern physics traces sense-impressions back to processes in the smallest particles of bodies and to the infinitely fine substance, the ether, or to something similar. For example, what we sense as warmth, is, within the space occupied by the warmth-giving body, movement of its parts. Here again, something imperceptible is thought of on the analogy of what is perceptible. The physical analogon to the concept “body” is, in this sense, something like the interior of a totally enclosed space in which elastic balls are moving in all directions, impinging on one another, bouncing on and off the walls, etc.
Without such assumptions, for naive realism, the world would collapse into a disconnected chaos of perceptions with no mutual relationships to unite them. It is clear, however, that naive realism can arrive at these assumptions only by inconsistency. If it remained true to its fundamental principle that only what is perceived is real, then it would not assume a reality where it perceives nothing. The imperceptible forces which proceed from perceptible things are essentially unjustified hypotheses from the standpoint of naive realism itself. And as the naive realist acknowledges no other realities, he invests his hypothetical forces with perceptual content. In doing this he applies a form of existence (perceptual existence) to a sphere where he lacks the only means that can give any evidence of such existence: perceiving by means of physical senses.
This self-contradictory world view leads to metaphysical realism. Beside the perceptible reality, the metaphysical realist constructs an imperceptible one which he thinks of on the analogy of the former. Metaphysical realism therefore, is of necessity dualistic.
Where the metaphysical realist observes a relation between perceptible things (mutual approach through movement, becoming conscious of an object, etc.), there he regards a reality as existing. But the relation that he notices he can, however, express only by means of thinking; he cannot perceive it. The relation, which is purely ideal, is arbitrarily made into something similar to what is perceptible. Thus, according to this line of thought, the real world is composed of perceptual objects which are in ceaseless flux, arising and disappearing, and of imperceptible forces which are permanent and produce the perceptual objects.
Metaphysical realism is a contradictory mixture of naive realism and idealism. Its hypothetical forces are imperceptible entities endowed with the qualities of perceptions. In addition to the sphere, for the form of existence of which he has a means of cognition in its perceptibility, the metaphysical realist has decided to acknowledge another sphere to which this means is not applicable, a sphere which can be ascertained only by means of thinking. But he cannot at the same time decide also to acknowledge the form of existence which thinking mediates, namely the concept (the idea), as being of equal importance with perceptions. If one is to avoid the contradiction of imperceptible perceptions, then it must be admitted that the relation thinking mediates between perceptions can have no other form of existence for us than that of the concept. When the untenable part of metaphysical realism is rejected, we then have the world before us as the sum of perceptions and their conceptual (ideal) relations. Then metaphysical realism merges into a world view which requires the principle of perceptibility for perceptions and that of “think-ability” for the relations between the perceptions. Side by side with the realm of perceptions and that of concepts, this world view cannot acknowledge a third realm for which both principles, the so-called real principle and the ideal principle, have equal validity.
When the metaphysical realist maintains that beside the ideal relation between the perceptual object and the perceiving subject, there must also exist a real relation between the “thing-in-itself” of the perception and the “thing-in-itself” of the perceptible subject (of the so called individual spirit), then this assertion is due to the mistaken assumption of the existence of a process, analogous to a process in the sense-world, but imperceptible. Further, when the metaphysical realist says: I have a conscious ideal relationship with my world of perceptions, but with the real world I can have only a dynamic (force) relationship, he then makes the above mistake to an even greater degree. One can only speak of a force-relationship within the world of perceptions (in the sphere of the sense of touch), not outside that sphere.
Let us call the world view characterized above, into which metaphysical realism merges if it discards its contradictory elements, monism, because it unites one-sided realism with idealism in a higher unity.
For the naive realist, the real world is an aggregate of objects of perception; for the metaphysical realist also the imperceptible forces are realities. Instead of forces, the monist has ideal connections which he attains by means of his thinking. The laws of nature are such connections. For a law of nature is nothing other than the conceptual expression for the connection of certain perceptions.
The monist never has any need to ask for factors other than perceptions and concepts, with which to explain reality. He knows that in the whole sphere of reality there is no need to ask for this. In the sphere of perceptions, directly accessible to his perceiving, he sees half of a reality; in the union of this sphere with the sphere of concepts, he finds the full reality. The metaphysical realist may make the objection to the adherent of monism: It could be that for your organization your knowledge is complete in itself, that no part is lacking; but what you do not know is how the world is mirrored in an intelligence organized differently from your own. To this the monist would reply: If there are intelligences other than human, if their perceptions have a different form than ours, then all that would be of significance for me would be what reaches me from them by means of perceptions and concepts. By means of my perceiving and, in fact, by means of this specifically human manner of perceiving, as subject I am placed over against the object. The connection of things is thereby broken. The subject restores this connection by means of thinking. In doing so, things are re-inserted into the world whole. Since it is only through our subject that this whole appears rent in two at the place between our perception and our concept, so likewise the union of these two factors gives us a true knowledge. For beings with a different world of perceptions (if, for example, they had twice as many sense-organs), the connection would appear broken in another place, and the restoration would, accordingly, have a form specific for such beings. The question concerning limits of knowledge exists only for the naive and metaphysical realists, both of whom see in the content of the soul only an ideal representation of the world. For them, what exists outside the subject is something absolute, something self-dependent, and the content of the subject is a picture of this absolute and is completely external to it. How complete is knowledge of this absolute would depend on the greater or lesser degree of resemblance between the picture and the absolute object. A being with fewer senses than man would perceive less of the world, one with more senses would perceive more. The former's knowledge would therefore be less complete than that of the latter.
For the monist, things are different. It is the organization of the perceiving being that determines how the world unity appears to be torn apart into subject and object. The object is not something absolute, but is only something relative in relation to this particular subject. The bridging of the contrasting entities can, therefore, take place again only in the quite specific way that is characteristic of the human subject. As soon as the I, which, in perceiving, is separated from the world, reinserts itself into the connection of things through thinking investigation, all further questioning ceases, since all questions arose only as a result of the separation.
A differently constituted being would have a differently constituted knowledge. Our knowledge suffices to answer the questions asked by our nature.
The metaphysical realist should ask: How does what is given as perception come to be the given; what is it that affects the subject?
For the monist, the perception is determined by the subject. But in thinking, the subject has, at the same time, the means for canceling this determination, caused through the subject itself.
The metaphysical realist is faced by a further difficulty when he seeks to explain the similarity of the world picture, of different human individuals. He cannot but ask himself: How is it that the world picture which I build up out of my subjectively determined perceptions and out of my concepts, turns out to be like that which another individual builds up out of the same two subjective factors? How, from my subjective world picture, can I infer anything about that of another human being? The metaphysical realist believes he can infer, from the fact that people come to terms with one another in practical life, that their subjective world pictures must be similar. From the similarity of these world pictures he then further infers that the “individual spirits” behind the single perceiving human subjects, or the “I-in-itself” behind the subjects, must also be similar.
Therefore this inference is drawn from a sum of effects to the nature of their underlying causes. It is believed that from a sufficiently large number of instances, the situation can be so recognized that one can know how the inferred causes will behave in other instances. Such an inference is called an inductive inference. It will be necessary to modify the results if, from further observation, some unexpected element is discovered, because the result, after all, is determined only by the particular form of the earlier observation. The metaphysical realist maintains that this stipulated knowledge of causes is quite sufficient for practical life.
Inductive inference is the methodical foundation of modern metaphysical realism. At one time it was believed that out of concepts could be evolved something that is no longer a concept. It was believed that from concepts could be derived the metaphysical realities which of necessity, metaphysical realism must have. This kind of philosophizing is now superseded. Instead, it is believed that from a sufficiently large number of perceptual facts one can infer the character of the thing-in-itself which underlies these facts. Just as in the past one tried to derive the metaphysical from concepts, so to-day one tries to derive it from perceptions. As concepts are transparent in their clarity, it was believed that one could also deduce the metaphysical from them with absolute certainty. Perceptions are not of such transparency. Each later perception is always a little different from those of the same kind that preceded it. Therefore, anything inferred from the earlier perception is, in reality, somewhat modified by each following one. The aspect of the metaphysical arrived at in this way, therefore, can be said to be only relatively correct, for it is subject to correction by future instances. Eduard von Hartmann's metaphysics is of a kind that is determined by this methodical principle. This is expressed in the motto he gave on the title-page of his first major work: “Speculative results according to the inductive method of natural science.”
The form which the metaphysical realist gives to his things-in-themselves today is obtained by inductive inferences. His consideration of the process of knowledge has convinced him that a connection of things, which is objectively real, exists side by side with the “subjective” connection that can be known through perception and concept. The nature of this objective reality he believes he can determine by inductive inferences from his perceptions.
Addition to the Revised Edition, (1918): Certain representations which arise from investigations of natural phenomena tend, again and again, to disturb unprejudiced observation — as the effort has been made to describe it above — of how we experience concepts and perceptions. Such investigations show that in the light-spectrum the eye perceives colors from red to violet. However, within the spectrum's sphere of radiation, but beyond the violet there are forces to which corresponds no color perception of the eye, but a chemical effect and, similarly, beyond the limit of the red there exist radiations which have only effects of warmth. Investigation of these and similar phenomena has led to the opinion that the range of man's sphere of perceptions is determined by the range of his senses, and that he would have before him a very different world if he had more or altogether different senses. Those who are inclined to flights of imagination, for which the glittering discoveries of recent scientific research in particular offer such tempting opportunities, may come to the conclusion: Nothing can enter man's field of observation except what is able to affect the senses of his bodily organization, and he has no right to regard what he perceives, by means of his limited organization, as being in any way a standard for ascertaining reality. Every new sense would give him a different picture of reality. — Within its proper limits, this opinion is entirely correct. But one who allows this opinion to prevent him from observing without prejudice the relationship between concept and perception, as explained here, will put obstacles in the way to any realistic knowledge of man and world. To experience thinking in its own nature, that is, to experience the active working-out of the sphere of concepts, is something entirely different from the experience of something perceptible through the senses. Whatever senses man might possibly have, not one would give him reality if through the activity of thinking, he did not permeate with concepts the perceptions they conveyed to him; and indeed, every sense, of whatever kind, if thus permeated, gives man the possibility to live within reality. Speculations about quite different perceptual pictures conveyed by other senses, has nothing to do with the question concerning man's relation to reality. It is essential to recognize that every perceptual picture derives its form from the organization of the perceiving being, but the perceptual picture when permeated by thinking which is livingly experienced leads man into reality. A fanciful description of how different the world would appear to other than human senses cannot act as an incentive to man to seek for knowledge concerning his relationship to the world; rather will this happen through the insight that every perception gives us only a part of the reality it conceals, that, therefore, it leads away from its reality. This then brings us to the further insight that it is thinking which leads into that part of reality which the perception conceals within itself. An unprejudiced observation of the relation between perceptions, and concepts worked out by thinking, as here described, may also be disturbed by the fact that in the sphere of applied physics it becomes necessary to speak not at all of directly perceptible elements, but of non-perceptible magnitudes, such as lines of electric or magnetic force, etc. It may appear as if the elements of reality, spoken of in physics, had nothing to do either with what is perceptible or with concepts actively worked out by thinking. But such a view is based on self-deception. What matters is that all that is worked out in physics — as long as it is not based on unjustifiable hypotheses which must be excluded — is obtained by means of perceptions and concepts. By a correctly working instinct for knowledge in the physicist, what is apparently a non-perceptible content will always be placed into the field of perceptions, and will be thought of in concepts belonging to this field. The magnitudes in electric and magnetic fields, etc., are attained, owing to their nature, by no other process of cognition than the one which takes place between perception and concept. — An increase or a transformation of the human senses would give a different perceptual picture; it would be an enrichment or a transformation of human experience. But a real knowledge of this experience also could be attained only through the interplay of concept and perception. A deepening of knowledge depends upon the active power of intuition contained in thinking (see p. 30). In the living experience within thinking, this intuition can dive down into lesser or greater depths of reality. Through extension of the perceptual picture this diving down of intuition can receive stimulation and thus be indirectly strengthened. But never should this diving into the depths to attain reality be confused with being confronted with a wider or narrower perceptual picture, in which there would always be contained only a half-reality determined by the organization of the cognizing being. If one avoids getting lost in abstractions, it will be recognized how significant, also for knowledge of the being of man, is the fact that in physics one has to include the existence, in the field of perceptions, of elements for which no sense organ is directly tuned as for color or sound. The essential being of man is determined not only by what confronts him through his organization as direct perception, but also by the fact that he excludes something else from this direct perception. Just as life needs, in addition to the conscious waking state, an unconscious sleeping state, so, for man's self-experience is needed besides the sphere of his sense-perceptions, another sphere also — indeed, a much larger one — of elements not perceptible to the senses, but existing within the same field where sense-perceptions originate. All this was already indirectly indicated in the first edition of this book. The author here adds these amplifications to the content because he has found by experience that many readers have not read accurately enough. — Another thing to be considered is that the idea of perception, as presented in this book, is not to be confused with the idea of external sense-perception, which is but a special instance of perception. The reader will gather from what has already been said, but even more from what will follow, that here perception includes everything that man meets, physically or spiritually, before he has grasped it in actively worked out concepts. We do not need what we usually mean by senses in order to have perceptions of a soul or spiritual kind. It may be said that such extension of the ordinary use of a word is inadmissible. Yet such extension is absolutely necessary if one is not to be barred by the current use of a word from enlarging the knowledge of certain fields. If the word perception is applied to physical perception only, then one cannot arrive at a concept that can be of use for attaining knowledge even of this (physical) perception. Often it is necessary to enlarge a concept in order that it may preserve in a narrower field the meaning appropriate to it. Or it is sometimes necessary to add something different to the previous content of a concept in order that its first content may be justified or even readjusted. For example, it is said in this book (p. 32) “A representation, therefore, is an individualized concept.” It has been objected that this is an unusual use of the word. But this use of the word is necessary if we are to find out what a representation really is. What would become of the progress of knowledge if, when compelled to readjust concepts, one is always to be met with the objection: “This is an unusual use of the word”?