The Philosophy of Freedom
7. Are There Limits to Knowledge?
We have established that the elements for the explanation of reality are to be found in the two spheres: perceiving and thinking. It is due, as we have seen, to our organization that the full, complete reality, including our own selves as subjects, appears at first as a duality. The act of knowing overcomes this duality by fusing the two elements of reality, the percept and the concept gained by thinking, into the complete thing. Let us call the manner in which the world presents itself to us, before it has taken on its true nature through our knowing it, “the world of appearance,” in contrast to the unified whole composed of percept and concept. We can then say: The world is given to us as a duality, and knowledge transforms it into a unity. A philosophy which starts from this basic principle may be called a monistic philosophy, or monism. Opposed to this is the two-world theory, or dualism. The latter does not assume just that there are two sides of a single reality which are kept apart merely by our organization, but that there are two worlds absolutely distinct from one another. It then tries to find in one of these two worlds the principles for the explanation of the other.
Dualism rests on a false conception of what we call knowledge. It divides the whole of existence into two spheres, each of which has its own laws, and it leaves these two worlds standing apart and opposed.
It is from a dualism such as this that there arises the distinction between the perceptual object and the thing-in-itself, which Kant introduced into philosophy, and which, to the present day, we have not succeeded in eradicating. According to our line of argument, it is due to the nature of our mental organization that a particular thing can be given to us only as a percept. Thinking then overcomes this particularity by assigning to each percept its rightful place in the world as a whole. As long as we designate the separated parts of the world as percepts, we are simply following, in this separating out, a law of our subjectivity. If, however, we regard the sum of all percepts as the one part, and contrast with this a second part, namely, the things-in-themselves, then we are philosophizing into the blue. We are merely playing with concepts. We construct an artificial pair of opposites, but we can gain no content for the second of these opposites, since such content for a particular thing can be drawn only from perception.
Every kind of existence that is assumed outside the realm of percept and concept must be relegated to the sphere of unjustified hypotheses. To this category belongs the “thing-in-itself”. It is quite natural that a dualistic thinker should be unable to find the connection between the world principle which he hypothetically assumes and the things given in experience. A content for the hypothetical world principle can be arrived at only by borrowing it from the world of experience and then shutting one's eyes to the fact of the borrowing. Otherwise it remains an empty concept, a non-concept which has nothing but the form of a concept. Here the dualistic thinker usually asserts that the content of this concept is inaccessible to our knowledge; we can know only that such a content exists, but not what it is that exists. In both cases it is impossible to overcome dualism. Even though one were to import a few abstract elements from the world of experience into the concept of the thing-in-itself, it would still remain impossible to derive the rich concrete life of experience from these few qualities which are, after all, themselves taken from perception. DuBois-Reymond considers that the imperceptible atoms of matter produce sensation and feeling by means of their position and motion, and then comes to the conclusion that we can never find a satisfactory explanation of how matter and motion produce sensation and feeling, for “it is absolutely and for ever incomprehensible that it should be other than indifferent to a number of atoms of carbon, hydrogen, nitrogen, and so on, 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 abstracted from the rich world of percepts. They are then transferred to the notional world of atoms. And then astonishment arises that real life cannot be evolved out of this self-made principle borrowed from the world of percepts.
That the dualist can reach no explanation of the world, working as he does with a completely empty concept of the “in-itself” of a thing, follows at once from the very definition of his principle given above.
In every case the dualist finds himself compelled to set impassable barriers to our faculty of knowledge. The follower of a monistic world conception knows that everything he needs for the explanation of any given phenomenon in the world must lie within this world itself. What prevents him from reaching it can be only accidental limitations in space and time, or defects of his organization, that is, not of human organization in general, but only of his own particular one.
It follows from the concept of the act of knowing as we have defined it, that one cannot speak of limits to knowledge. Knowing is not a concern of the world in general, but an affair which man must settle for himself. Things demand no explanation. They exist and act on one another according to laws which can be discovered through thinking. They exist in indivisible unity with these laws. Our Egohood confronts them, grasping at first only that part of them we have called percepts. Within our Egohood, however, lies the power to discover the other part of the reality as well. Only when the Egohood has taken the two elements of reality which are indivisibly united in the world and has combined them also for itself, is our thirst for knowledge satisfied — the I has then arrived at the reality once more.
Thus the conditions necessary for an act of knowledge to take place are there through the I and for the I. The I sets itself the problems of knowledge; and moreover it takes them from an element that is absolutely clear and transparent in itself: the element of thinking. If we set ourselves questions which we cannot answer, it must be because the content of the questions is not in all respects clear and distinct. It is not the world which sets us the questions, but we ourselves.
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 our knowledge we are concerned with questions which arise for us through the fact that a sphere of percepts, conditioned by place, time, and our subjective organization, is confronted by a sphere of concepts pointing to the totality of the universe. My task consists in reconciling these two spheres, with both of which I am well acquainted. Here one cannot speak of a limit to knowledge. It may be that, at any particular moment, this or that remains unexplained because, through our place in life, we are prevented from perceiving the things involved. What is not found today, however, may be found tomorrow. The limits due to these causes are only transitory, and can be overcome by the progress of perception and thinking.
Dualism makes the mistake of transferring the antithesis of object and subject, which has meaning only within the perceptual realm, to purely notional entities outside this realm. But since the separate things within the perceptual field remain separated only so long as the perceiver refrains from thinking (which cancels all separation and shows it to be due to purely subjective factors), the dualist is therefore transferring to entities behind the perceptible realm determining factors which even for this realm have no absolute validity, but only relative. He thus splits up the two factors concerned in the process of knowledge, namely percept and concept, into four: (1) the object in itself; (2) the precept which the subject has of the object; (3) the subject; (4) the concept which relates the precept to the object in itself. The relation between subject and object is a real one; the subject is 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 percept. Only at this stage does it enter our consciousness. The object is said to have an objective (independent of the subject) reality, the percept a subjective reality. This subjective reality is referred by the subject to the object. This reference is called an ideal one. With this the dualist therefore splits up the process of knowledge into two parts. The one part, namely, the production of the perceptual object out of the thing-in-itself, he conceives of as taking place outside consciousness, whereas the other, the combination of percept with concept and the reference of the concept to the object, takes place, according to him, within consciousness.
With these presuppositions, it is clear why the dualist believes his concepts to be merely subjective representatives of what is there prior to his consciousness. The objectively real process in the subject by means of which the percept comes about, and still more the objective relations between things-in-themselves, remain for such a dualist inaccessible to direct knowledge; according to him, man can obtain only conceptual representatives of the objectively real. The bond of unity which connects things with one another and also objectively with the individual mind of each of us (as thing-in-itself) lies beyond our consciousness in a being-in-itself of whom, once more, we can have in our consciousness merely a conceptual representative.
The dualist believes that he would dissolve away the whole world into a mere abstract. scheme of concepts, did he not insist on real connections between the objects besides the conceptual ones. In other words, the ideal principles which thinking discovers 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 naïve man (naïve realist) regards the objects of external experience as realities. The fact that his hands can grasp these objects, and his eyes see them, is for him sufficient proof of their reality. “Nothing exists that cannot be perceived” is, in fact, the first axiom of the naïve man; and it is held to be equally valid in its converse: “Everything which can be perceived exists.” The best evidence for this assertion is the naïve man's belief in immortality and ghosts. He thinks of the soul as refined material substance which may, in special circumstances, become visible even to the ordinary man (naïve belief in ghosts).
In contrast with this real world of his, the naïve realist regards everything else, especially the world of ideas, as unreal or “merely ideal”. What we add to objects by thinking is nothing more than thoughts about the things. Thought adds nothing real to the percept.
But it is not only with reference to the existence of things that the naïve man regards sense perception as the sole proof of reality, but also with reference to events. A thing, according to him, can act on another only when a force actually present to sense perception issues from the one and seizes upon the other. In the older physics it was thought that very fine substances emanate from the objects and penetrate through the sense organs into the soul. The actual seeing of these substances is impossible only because of the coarseness of our sense organs relative to the fineness of these substances. In principle, the reason for attributing reality to these substances was the same as for attributing it to the objects of the sense-perceptible world, namely because of their mode of existence, which was thought to be analogous to that of sense-perceptible reality.
The self-contained nature of what can be experienced through ideas is not regarded by the naïve mind as being real in the same way that sense experience is. An object grasped in “mere idea” is regarded as a chimera until conviction of its reality can be given through sense perception. In short, the naïve man demands the real evidence of his senses in addition to the ideal evidence of his thinking. In this need of the naïve man lies the original ground for primitive forms of the belief in revelation. The God who is given through thinking remains to the naïve mind always a merely “notional” God. The naïve mind demands a manifestation that is accessible to sense perception. God must appear in the flesh, and little value is attached to the testimony of thinking, but only to proof of divinity such as changing water into wine in a way that can be testified by the senses.
Even the act of knowing itself is pictured by the naïve man as a process analogous to sense perception. Things, it is thought, make an impression on the soul, or send out images which enter through our senses, and so on.
What the naïve man can perceive with his senses he regards as real, and what he cannot thus perceive (God, soul, knowledge, etc.) he regards as analogous to what he does perceive.
A science based on naïve realism would have to be nothing but an exact description of the content of perception. For naïve realism, concepts are only the means to an end. They exist to provide ideal counterparts of percepts, and have no significance for the things themselves. For the naïve realist, only the individual tulips which he sees (or could see) are real; the single idea of the tulip is to him an abstraction, the unreal thought-picture which the soul has put together 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 teaches us that the content of percepts is of a transitory nature. The tulip I see is real today; in a year it will have vanished into nothingness. What persists is the species tulip. For the naïve realist, however, this species is “only” an idea, not a reality. Thus this theory of the world find itself in the position of seeing its realities arise and perish, while what it regards as unreal, in contrast with the real, persists. Hence naïve realism is compelled to acknowledge, in addition to percepts, the existence of something ideal. It must admit entities which cannot be perceived by the senses. In doing so, it justifies itself by conceiving their existence as being analogous to that of sense-perceptible objects. Just such hypothetical realities are the invisible forces by means of which the sense-perceptible objects act on one another. Another such thing is heredity, which works on beyond the individual and is the reason why a new being which develops from the individual is similar to it, thereby serving to maintain the species. Such a thing again is the life-principle permeating the organic body, the soul for which the naïve mind always finds a concept formed in analogy with sense realities, and finally the naïve man's Divine Being. This Divine Being is thought of as acting in a manner exactly corresponding to the way in which man himself is seen to act; that is, anthropomorphically.
Modern physics traces sensations back to processes of the smallest particles of bodies and of an infinitely fine substance, called ether, or to other such things. For example, what we experience as warmth is, within the space occupied by the warmth-giving body, the movement of its parts. Here again something imperceptible is conceived in analogy with what is perceptible. In this sense, the perceptual analogue to the concept “body” would be, shall we say, the interior of a totally enclosed space, in which elastic spheres are moving in all directions, impinging one on another, bouncing on and off the walls, and so on. 1hat is, movements of a kind similar to those which can be perceived are supposed to occur imperceptibly within the body and to account for the warmth which is perceived directly but as something quite different. — Translator's footnote
Without such assumptions the world would fall apart for the naïve realist into an incoherent aggregate of percepts without mutual relationships and with no tendency to unite. It is clear, however, that naïve realism can make these assumptions only by an inconsistency. If it would remain true to its fundamental principle that only what is perceived is real, then it ought not to assume a reality where it perceives nothing. The imperceptible forces which proceed from the perceptible things are in fact unjustified hypotheses from the standpoint of naïve realism. And because naïve realism knows no other realities, it invests its hypothetical forces with perceptual content. It thus ascribes a form of existence (perceptible existence) to a sphere where the only means of making any assertion about such existence, namely, sense perception, is lacking.
This self-contradictory theory leads to metaphysical realism. This constructs, in addition to the perceptible reality, an imperceptible reality which it conceives on the analogy of the perceptible one. Therefore metaphysical realism is of necessity dualistic.
Wherever the metaphysical realist observes a relationship between perceptible things (such as when two things move towards each other, or when something objective enters consciousness), there he sees a reality. However, the relationship which he notices can only be expressed by means of thinking; it cannot be perceived. The purely ideal relationship is then arbitrarily made into something similar to a perceptible one. Thus, according to this theory, the real world is composed of the objects of perception which are in ceaseless flux, arising and disappearing, and of imperceptible forces which produce the objects of perception, and are the things that endure.
Metaphysical realism is a contradictory mixture of naïve realism and idealism. Its hypothetical forces are imperceptible entities endowed with the qualities of percepts. The metaphysical realist has made up his mind to acknowledge, in addition to the sphere which he is able to know through perception, another sphere for which this means of knowledge fails him and which can be known only by means of thinking. But he cannot make up his mind at the same time to acknowledge that the mode of existence which thinking reveals, namely, the concept (idea), is just as important a factor as the percept. If we are to avoid the contradiction of imperceptible percepts, we must admit that the relationships which thinking establishes between the percepts can have no other mode of existence for us than that of concepts. If we reject the untenable part of metaphysical realism, the world presents itself to us as the sum of percepts and their conceptual (ideal) relationships. Metaphysical realism would then merge into a view of the world which requires the principle of perceivability for percepts and that of conceivability for the relationships between the percepts. This view of the world can admit no third sphere — in addition to the world of percepts and the world of concepts — in which both the so-called “real” and “ideal” principles are simultaneously valid.
When the metaphysical realist asserts that, besides the ideal relationship between the percept of the object and the percept of the subject, there must also exist a real relationship between the “thing-in-itself” of the percept and the “thing-in-itself” of the perceptible subject (that is, of the so-called individual spirit), he is basing his assertion on the false assumption of a real process, analogous to the processes in the sense world but imperceptible. Further, when the metaphysical realist asserts that we enter into a conscious ideal relationship to our world of percepts, but that to the real world we can have only a dynamic (force) relationship, he repeats the mistake we have already criticized. One can talk of a dynamic relationship only within the world of percepts (in the sphere of the sense of touch), but not outside that world.
Let us call the view which we have characterized above, into which metaphysical realism merges when it discards its contradictory elements, monism, because it combines one-sided realism with idealism into a higher unity.
For naïve realism, the real world is an aggregate of perceived objects (percepts); for metaphysical realism, not only percepts but also imperceptible forces are real; monism replaces forces by ideal connections which are gained through thinking. The laws of nature are just such connections. A law of nature is in fact nothing but the conceptual expression of the connection between certain percepts.
Monism never finds it necessary to ask for any principles of explanation for reality other than percepts and concepts. It knows that in the whole field of reality there is no occasion for this question. In the perceptual world, as it presents itself directly to perception, it sees one half of the reality; in the union of this world with the world of concepts it finds the full reality.
The metaphysical realist may object to the adherent of monism: It may be that for your organization, your knowledge is complete in itself, with no part lacking; but you do not know how the world is mirrored in an intelligence organized differently from your own. To this the monist will reply: If there are intelligences other than human, and if their percepts are different from ours, all that concerns me is what reaches me from them through perception and concept. Through my perceiving, that is, through this specifically human mode of perceiving, I, as subject, am confronted with the object. The connection of things is thereby interrupted. The subject restores this connection by means of thinking. In doing so it puts itself back into the context of the world as a whole. Since it is only through the subject that the whole appears cut in two at the place between our percept and our concept, the uniting of those two gives us true knowledge. For beings with a different perceptual world (for example, if they had twice our number of sense organs), the continuum would appear broken in another place, and the reconstruction would accordingly have to take a form specific for such beings. The question concerning the limits of knowledge exists only for naïve and metaphysical realism, both of which see in the contents of the soul only an ideal representation of the real world. For these theories, what exists outside the subject is something absolute, founded in itself, and what is contained within the subject is a picture of this absolute, but quite external to it. The completeness of knowledge depends on the greater or lesser degree of resemblance between the picture and the absolute object. A being with fewer senses than man will perceive less of the world, one with more senses will perceive more. The former will accordingly have a less complete knowledge than the latter.
For monism, the situation is different. The manner in which the world continuum appears to be rent asunder into subject and object depends on the organization of the perceiving being. The object is not absolute, but merely relative, with reference to this particular subject. Bridging over the antithesis, therefore, can again take place only in the quite specific way that is characteristic of the particular human subject. As soon as the I, which is separated from the world in the act of perceiving, fits itself back into the world continuum through thoughtful contemplation, all further questioning ceases, having been but a consequence of the separation.
A differently constituted being would have a differently constituted knowledge. Our own knowledge suffices to answer the questions put by our own nature.
Metaphysical realism has to ask: By what means are our percepts given? What is it that affects the subject?
Monism holds that percepts are determined through the subject. But at the same time, the subject has in thinking the means for canceling this self-produced determination.
The metaphysical realist is faced by a further difficulty when he seeks to explain the similarity between the world pictures of different human individuals. He has to ask himself: How is it that the picture of the world which I build up out of my subjectively determined percepts and my concepts turns out to be the same as the one which another individual is also building up out of the same two subjective factors? How can I, in any case, draw conclusions from my own subjective picture of the world about that of another human being? The fact that people can understand and get on with one another in practical life leads the metaphysical realist to conclude that their subjective world pictures must be similar. From the similarity of these world pictures he then further concludes that the “individual spirits” behind the single human subjects as percepts, or the “I-in-itself” behind the subjects, must also be like one another.
This is an inference from a sum of effects to the character of the underlying causes. We believe that we can understand the situation well enough from a sufficiently large number of instances to know how the inferred causes will behave in other instances. Such an inference is called an inductive inference. We shall be obliged to modify its results if further observation yields some unexpected element, because the character of our conclusion is, after all, determined only by the particular form of our actual observations. The metaphysical realist asserts that this knowledge of causes, though conditional, is nevertheless quite sufficient for practical life.
Inductive inference is the method underlying modern metaphysical realism. At one time it was thought that we could evolve something out of concepts that is no longer a concept. It was thought that the metaphysical realities, which metaphysical realism after all requires, could be known by means of concepts. This kind of philosophizing is now out of date. Instead it is thought that one can infer from a sufficiently large number of perceptual facts the character of the thing-in-itself which underlies these facts. Whereas formerly it was from concepts, now it is from percepts that people seek to evolve the metaphysical. Since one has concepts before oneself in transparent clearness, it was thought that one might be able to deduce the metaphysical from them with absolute certainty. Percepts are not given with the same transparent clearness. Each subsequent one is a little different from others of the same kind which preceded it. Basically, therefore, anything inferred from past percepts will be somewhat modified by each subsequent percept. The character of the metaphysical thus obtained can, therefore, be only relatively true, since it is subject to correction by further instances. Eduard von Hartmann's metaphysics has a character determined by this basic method, as expressed in the motto on the title page of his first important book: “Speculative results following the inductive method of Natural Science.”
The form which the metaphysical realist nowadays gives to his things-in-themselves is obtained by inductive inferences. Through considerations of the process of knowledge he is convinced of the existence of an objectively real world continuum, over and above the “subjective” world continuum which we know through percepts and concepts. The nature of this reality he thinks he can determine by inductive inferences from his percepts.
Author's addition, 1918
For the unprejudiced observation of what is experienced through percept and concept, as we have tried to describe it in the foregoing pages, certain ideas which originate in the field of natural science are repeatedly found to be disturbing. Thus it is said that in the spectrum of light the eye perceives colors from red to violet. But in the space beyond the violet there are forces of radiation for which there is no corresponding color-perception in the eye, but instead there is a definite chemical effect; in the same way, beyond the limit of the red there are radiations having only an effect of warmth. By studying these and other similar phenomena, one is led to the view that the range of man's perceptual world is determined by the range of his senses, and that he would be confronted by a very different world if he had additional, or altogether different, senses. Anyone who chooses to indulge in the extravagant flights of fancy for which the brilliant discoveries of recent scientific research offer such tempting opportunities, may well arrive at the conclusion that nothing enters man's field of observation except what can affect the senses which his bodily organization has evolved. He has no right to regard what is perceived, limited as it is by his organization, as in any way setting a standard for reality. Every new sense would confront him with a different picture of reality.
Within its proper limits this view is entirely justified. But if anyone allows this view to confuse him in his unprejudiced observation of the relationship of percept and concept as set out in these chapters, then he will bar his own way to any realistic knowledge of man and of the world. To experience the essential nature of thinking, that is, to work one's way into the world of concepts through one's own activity, is an entirely different thing from experiencing something perceptible through the senses. Whatever senses man might possibly have, not one would give him reality if his thinking did not permeate with concepts whatever he perceived by means of it. And every sense, however constructed, would, if thus permeated, enable him to live within reality. This question of how he stands in the world of reality is untouched by any speculations he may have as to how the perceptual world might appear to him if he had different senses. We must clearly understand that every perceptual picture of the world owes its form to the organization of the perceiving being, but also that the perceptual picture which has been thoroughly permeated by the experience of thinking leads us into reality. What causes us to enquire into our relationship to the world is not the fanciful pictures of how different the world would appear to other than human senses, but the realization that every percept gives us only a part of the reality concealed within it, in other words, that it directs us away from its inherent reality. Added to this is the further realization that thinking leads us into that part of the reality which the percept conceals within itself.
Another difficulty in the way of the unprejudiced observation of the relationship between the percept and the concept wrought by thinking, as here described, arises when, for example, in the field of experimental physics it becomes necessary to speak not of immediately perceptible elements, but of non-perceptible quantities as in the case of lines of electric or magnetic force. It may seem as if the elements of reality of which physicists speak had no connection either with what is perceptible or with the concepts which active thinking has wrought. Yet such a view would be based on self-deception. The main point is that all the results of physical research, apart from unjustifiable hypotheses which ought to be excluded, have been obtained through percept and concept. Elements which are seemingly non-perceptible are placed by the physicist's sound instinct for knowledge into the field where percepts lie, and they are thought of in terms of concepts commonly used in this field. The strengths of electric or magnetic fields and such like are arrived at, in the very nature of things, by no other process of knowledge than the one which occurs between percept and concept.
An increase or a modification of human senses would yield a different perceptual picture, an enrichment or a modification of human experience. But even with this experience one could arrive at real knowledge only through the interplay of concept and percept. The deepening of knowledge depends on the powers of intuition which express themselves in thinking (see Chapter 5). In the living experience which develops within thinking, this intuition may dive down to greater or to lesser depths of reality. An extension of the perceptual picture may provide stimulation for this diving down of intuition, and thus indirectly promote it. But under no circumstances should this diving into the depths to reach reality be confused with being confronted by a perceptual picture of greater or lesser breadth, which in any case can only contain half the reality, as determined by the organization of the cognizing being. If one does not lose oneself in abstractions, one will realize that for a knowledge of human nature it is a relevant fact that in physics one has to infer the existence of elements in the perceptual field for which no sense organ is tuned as it is for color or sound. Man's being, quite concretely, is determined not only by what his organization presents to him as immediate percept, but also by the fact that from this immediate perception other things are excluded. Just as it is necessary for life that in addition to the conscious waking state there should be an unconscious sleeping state, so for man's experience of himself it is necessary that in addition to the sphere of his sense perception there should be another sphere — in fact a far larger one — of elements not perceptible to the senses but belonging to the same field from which the sense percepts come. All this was already implied in the original presentation of this work. The author adds these extensions to the argument because he has found by experience that many a reader has not read accurately enough.
It is to be remembered, too, that the idea of percept developed in this book is not to be confused with the idea of external sense percept which is but a special instance of it. The reader will gather from what has gone before, but even more from what will follow, that “percept” is here taken to be everything that approaches man through the senses or through the spirit, before it has been grasped by the actively elaborated concept. “Senses”, as we ordinarily understand the term, are not necessary in order to have percepts in soul- or spirit-experience. It might be said that this extension of our ordinary usage is not permissible. But such extension is absolutely necessary if we are not to be prevented by the current sense of a word from enlarging our knowledge in certain fields. Anyone who uses “perception” to mean only “sense perception” will never arrive at a concept fit for the purposes of knowledge — even knowledge of this same sense perception. One must sometimes enlarge a concept in order that it may get its appropriate meaning in a narrower field. Sometimes one must also add to the original content of a concept in order that the original concept may be justified or, perhaps, readjusted. Thus we find it said here in this book (see Chapter 6): “The mental picture is an individualized concept.” It has been objected that this is an unusual use of words. But this use is necessary if we are to find out what a mental picture really is. How can we expect any progress in knowledge if everyone who finds himself compelled to readjust concepts is to be met by the objection, “This is an unusual use of words”?