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Optic

Optic

Or else the agreement between ideas and their objects was denied, the ideas being explained therefore as illusions. Consequently, it was necessary to deny also the possibility of all knowledge of any objects whatsoever. This was the attitude of certain so-called ‘‘sensational”’ philosophers in England in the eighteenth century. However, it is not my purpose here to undertake an analysis of the opinions of the various philosophical schools on this question. That would be much too extensive a task in this place. I shall confine myself therefore merely to inquiring what I think should be the attitude of an investigator toward these controversies.

Our apperceptions and ideas are effects wrought on our nervous system and our consciousness by the objects that are thus apprehended and conceived. Each effect, as to its nature, quite necessarily depends both on the nature of what causes the effect and on that of the person on whom the effect is produced. To expect to obtain an idea which would reproduce the nature of the thing conceived, that is, which would be true in an absolute sense, would mean to expect an effect which would be perfectly independent of the nature of the thing on which the effect was produced; which would be an obvious absurdity. Our human ideas, therefore, and all ideas of any conceivable intelligent creature, must be images of objects whose mode is essentially codependent on the nature of the consciousness which has the idea, and is conditioned also by its idiosyncrasies.

In my opinion, therefore, there can be no possible sense in speaking of any other truth of our ideas except of a practical truth. Our ideas of things cannot be anything but symbols, natural signs for things which we learn how to use in order to regulate our movements and actions. Having learned correctly how to read those symbols, we are enabled by their help to adjust our actions so as to bring about the desired result; that is, so that the expected new sensations will arise. Not only is there in reality no other comparison at all between ideas and things— all the schools are agreed about this—but any other mode of comparison is entirely unthinkable and has no sense whatever. This latter consideration is the conclusive thing, and must be grasped in order to escape from the labyrinth of conflicting opinions. To ask whether the idea I have of a table, its form, strength, colour, weight, etc., is true per se, apart from any practical use I can make of this idea, and whether it corresponds with the real thing, or is false and due to an illusion, has just as much sense as to ask whether a certain musical note is red, yellow, or blue. Idea and the thing conceived evidently belong to two entirely different worlds, which no more admit of being compared with each other than colours and musical tones or than the letters of a book and the sound of the word they denote.

Were there any sort of similarity of correspondence between the idea in the head of a person A and the thing to which the idea belongs, another intelligent person B, conceiving both the thing itself and A’s idea of it, according to the same laws, might be able to find some similarity between them or at least to suppose so; because the same sort of thing represented (conceived) in the same way would have to give the same kinds of images (ideas). Now I ask, what similarity can be imagined between the process in the brain that is concomitant with the idea of a table and the table itself? Is the form of the table to be supposed to be outlined by electric currents? And when the person with the idea has the idea that he is walking around the table, must the person then be outlined by electric currents? Perspective projections of the external world in the hemispheres of the brain (as they are supposed to be) are evidently not sufficient for representing the idea of a bodily object. And granted that a keen imagination is not frightened away by these and similar hypotheses, such an electrical reproduction of the table in the brain would be simply another bodily object to be perceived, but no idea of the table. However, it is not simply persons with materialistic opinions who try to refute the proposed statement, but also persons with idealistic views. And for the latter I should think the argument would be still more forcible. What possible similarity can there be between the idea, some modification of the incorporeal mind that has no extension in space, and the body of the table that occupies space? As far as I am aware, the idealistic philosophers have never once investigated even a single hypothesis or imagination in order to show this connection. And by the very nature of this view it is something that cannot be investigated at all.

In the next place as to the properties of objects in the external world, a little reflection reveals that all properties attributable to them may be said to be simply effects exerted by them either on our senses or on other natural objects. Colour, sound, taste, smell, temperature, smoothness, and firmness are properties of the first sort, and denote effects on our organs of sense. Smoothness and firmness denote the degree of resistance either to the gliding contact or pressure of the hand. But other natural bodies may be employed instead of the hand. And the same thing is true in testing other mechanical properties such as elasticity and weight. Chemical properties are described by certain reactions, that is, by effects exerted by one natural body on others. It is the same way with any other physical property of a body, optical, electrical, or magnetic. In every case we have to do with the mutual relations between various bodies and with the effects depending on the forces that different bodies exert on each other. For all natural forces are such as are exerted by one body on others. When we try to think of mere matter without force, it is void of properties likewise, except as to its different distribution in space and as to its motion. All properties of bodies in nature are manifested therefore simply by being so situated as to interact with other bodies of nature or with our organs of sense. But as such interaction may occur at any time, particularly too as it may be produced by us voluntarily at any moment, and as then we see invariably the peculiar sort of interaction occurring, we attribute to the objects a permanent capacity for such effects which is always ready to become effective. This permanent capacity is a so-called characteristic property.

The result is that in point of fact the characteristic properties of natural objects, in spite of this name, do not denote something that is peculiar to the individual object by itself, but invariably imply some relation to a second object (including our organs of sense). The kind of effect must, of course, depend always on the peculiarities both of the body producing it and of the body on which it is produced. As to this there is never any doubt even for an instant, provided we have in mind those properties of bodies that are manifested when two bodies belonging to the external world react on each other, as in the case of chemical reactions. But in the case of properties depending on the mutual relations between things and our organs of sense, people have always been disposed to forget that here too we are concerned with the reaction toward a special reagent, namely, our own nervous system; and that colour, smell, and taste, and feeling of warmth or cold are also effects quite essentially depending on the nature of the organ that is affected. Doubtless, the reactions of natural objects to our senses are those that are most frequently and most generally perceived. For both our welfare and convenience they are of the most powerful importance. The reagent by which we have to test them is something we are endowed with by nature, but that does not make any difference in the connection.

Hence there is no sense in asking whether vermilion as we see it, is really red, or whether this is simply an illusion of the senses. The sensation of red is the normal reaction of normally formed eyes to light reflected from vermilion. A person who is red-blind will see vermilion as black or as a dark grey-yellow. This too is the correct reaction for an eye formed in the special way his is. All he has to know is that his eye is simply formed differently from that of other persons. In itself the one sensation is not more correct and not more false than the other, although those who call this substance red are in the large majority. In general, the red colour of vermilion exists merely in so far as there are eyes which are constructed like those of most people. Persons who are red-blind have just as much right to consider that a characteristic property of vermilion is that of being black. As a matter of fact, we should not speak of the light reflected from vermilion as being red, because it is not red except for certain types of eyes. When we speak of the properties of bodies with reference to other bodies in the external world, we do not neglect to name also the body with respect to which the property exists. Thus we say that lead is soluble in nitric acid, but not in sulphuric acid. Were we to say simply that lead is soluble, we should notice at once that the statement is incomplete, and the question would have to be asked immediately, Soluble in what? But when we say that vermilion is red, it is implicitly understood that it is red for our eyes and for other people’s eyes supposed to be made like ours. We think this does not need to be mentioned, and so we neglect to do so, and can be misled into thinking that red is a property belonging to vermilion or to the light reflected from it, entirely independently of our organs of sense. The statement that the waves of light reflected from vermilion have a certain length is something different. That is true entirely without reference to the special nature of our eye. Then we are thinking simply of relations that exist between the substance and the various systems of waves in the aether.

The only respect in which there can be a real agreement between our perceptions and the reality is the time-sequence of the events with thei: various peculiarities. Simultaneity, sequence, the regular recurrence of simultaneity or sequence, may occur likewise in the sensations as well as in the events. The external events, like their perceptions, proceed in time; and so the temporal relations of the latter may be the faithful reproduction of the temporal relations of the former. The sensation of thunder in the ear succeeds the sensation of lightning in the eye, just in the same way as the sound vibrations in the air due to the electrical discharge reach the place where the observer is later than the vibrations of the luminiferous aether. Yet here it certainly should be noted that the time-sequence of the sensations is not quite a faithful reproduction of the time-sequence of the external events, inasmuch as the transmission from the organs of sense to the brain takes time, and in fact a different time for different organs. Moreover, in case of the eye and the ear, the time has to be added that it takes light and sound to reach the organ. Thus at present we see the fixed stars as they were various long periods of years ago.

As to the representation of space-relations, there certainly is something of this sort in the peripheral nerve terminals in the eye and to a certain extent in the tactile skin, but still only in a limited way; for the eye gives only perspective surface-images, and the hand reproduces the objective area on the surface of a body by shaping itself to it as congruently as possible. A direct image of a portion of space of three dimensions is not afforded either by the eye or by the hand. It is only by comparing the images in the two eyes, or by moving the body with respect to the hand, that the idea of solid bodies is obtained. Now since the brain itself has three dimensions, of course, there is still another conceivable possibility, and that is to fancy by what mechanism in the brain itself images of three dimensions can arise from external objects in space. But I cannot see any necessity for such an assumption nor even any probability for it. The idea of a body in space, of a table, for instance, involves a quantity of separate observations. It comprises the whole series of images which this table would present to me in looking at it from different sides and at different distances; besides the whole series of tactile impressions that would be obtained by touching the surface at various places in succession. Such an idea of a single individual body is, therefore, in fact a conception (Begriff) which grasps and includes an infinite number of single, successive apperceptions, that can all be deduced from it; just as the species ‘‘table’” includes all individual tables and expresses their common peculiarities. The idea of a single individual table which I carry in my mind is correct and exact, provided I can deduce from it correctly the precise sensations I shall have when my eye and my hand are brought into this or that definite relation with respect to the table. Any other sort of similarity between such an idea and the body about which the idea exists, I do not know how to conceive. One is the mental symbol of the other. The kind of symbol was not chosen by me arbitrarily, but was forced on me by the nature of my organ of sense and of my mind. This is what distinguishes this sign-language of our ideas from the arbitrary phonetic signs and alphabetical characters that we use in speaking and writing. A writing is correct when he who knows how to read it forms correct ideas by it. And so the idea of a thing is correct for him who knows how to determine correctly from it in advance what sense-impressions he will get from the thing when he places himself in definite external relations to it. Incidentally, it does not matter at all what sort of mental symbols we employ, provided they constitute a sufficiently varied and ordered system. Nor does it matter either how the words of a language sound, provided there are enough of them, with sufficient means of denoting their grammatical relations to one another.

On this view of the matter, we must be on our guard against saying that all our ideas of things are consequently false, because they are not equal to the things themselves, and that hence we are not able to know anything as to the true nature of things. That they cannot be equal to things, is in the nature of knowledge. Ideas are merely pictures of things. Every image is the image of a thing merely for him who knows how to read it, and who is enabled by the aid of the image to form an idea of the thing. Every image is similar to its object in one respect, and dissimilar in all others, whether it be a painting, a statue, the musical or dramatic representation of a mental mood, etc. Thus the ideas of the external world are images of the regular sequence of natural events, and if they are formed correctly according to the laws of our thinking, and we are able by our actions to translate them back into reality again, the ideas we have are also the only true ones for our mental capacity. All others would be false.

In my opinion, it is a mistake, therefore, to try to find preestablished harmony between the laws of thought and those of nature, an identity between nature and mind, or whatever we may call it. A system of signs may be more or less perfect and convenient. Accordingly, it will be more or less easy to employ, more exact in denoting or more inexact, just as is the case with different languages. But otherwise each system can be adapted to the case more or less well. If there were not a number of similar natural objects in the world, our faculty of forming shades of conception would indeed not be of any use to us. Were there no solid bodies, our geometrical faculties would necessarily remain undeveloped and unused, just as the physical eye would not be of any service to us in a world where there was no light. If in this sense anybody wishes to speak of an adaptation of our laws of mind to the laws of nature, there is no objection to it. Evidently, however, such adaptation does not have to be either perfect or exact. The eye is an extremely useful organ practically, although it cannot see distinctly at all distances, or perceive all sorts of aether vibrations, or concentrate exactly in one point all the rays that issue from a point. Our intellectual faculties are connected with the activities of a material organ, namely the brain, just as the faculty of vision is connected with the eye. Human intelligence is wonderfully effective in the world, and brings it under a strict law of causation. Whether it necessarily must be able to control whatever is in the world or can happen—I can see no guarantee for that.

We must speak now of the manner in which our ideas and perceptions are formed by inductive conclusions. The best analysis of the nature of our conclusions I find in J. 8. Mrux’s Logic. As long as the premise of the conclusion is not an injunction imposed by outside authority for our conduct and belief, but a statement related to reality, which can therefore be only the result of experience, the conclusion, as a matter of fact, does not tell us anything new or something that we did not know already before we made the statement. Thus, for example:

Major: All men are mortal.

Minor: Caius is a man.

Conclusion: Caius is mortal.

The major premise, that all men are mortal, which is a statement of experience, we should scarcely venture to assert without knowing beforehand whether the conclusion is correct, namely, that Caius, who is a man, either is dead or will die. Thus we must be sure of the conclusion before we can state the major premise by which we intend to prove it. That seems to be proceeding in a circle. The real relation evidently is, that, in common with other folks, we have observed heretofore without exception that no person has ever survived beyond a certain age. Observers have learned by experience that Lucius, Flavius and other individuals of their acquaintance, no matter what their names are, have all died; and they have embraced this experience in the general statement, that all men die. Inasmuch as this final result occurred regularly in all the instances they observed, they have felt justified in explaining this general law as being valid also for all those cases which might come up for observation hereafter. Thus we preserve in our memory the store of experiences heretofore accumulated on this subject by ourselves and others in the form of the general statement which constitutes the major premise of the above conclusion.

However, the conviction that Caius would die might obviously have been reached directly also without formulating the general statement in our consciousness, by having compared his case with all those which we knew previously. Indeed, this is the more usual and original method of reasoning by induction. Conclusions of this sort are reached without conscious reflection, because in our memory the same sort of thing in cases previously observed unites and reinforces them; as is shown especially in those cases of inductive reasoning where we cannot succeed in deducing from previous experiences a rule with precisely defined limits to its validity and without any exceptions. This is the case in all complicated processes. For instance, from analogy with previous similar cases, we can sometimes predict with tolerable certainty what one of our acquaintances will do, if under certain circumstances he decides to go into business; because we know his character and that he is, let us say, ambitious or timid. Wemay not be able to say exactly how we have estimated the extent of his ambition or timidity, or why this ambition or timidity of his will be enough to decide that his business will turn out as we expect.

In the case of conclusions properly so-called, which are reached consciously, supposing they are not based on injunctions but on facts of experience, what we do, therefore, is really nothing more than deliberately and carefully to retrace those steps in the inductive generalizations of our experiences which were previously traversed more rapidly and without conscious reflection, either by ourselves or by other observers in whom we have confidence. But although nothing essentially new is added to our previous knowledge by formulating a general principle from our previous experiences, still it is useful in many respects. A definitely stated general principle is much easier to preserve in the memory and to be imparted to others than to have to do this same thing with every individual case as it arises. In formulating it we are led to test accurately every new case that occurs, with reference to the correctness of the generalization. In this way every exception will be impressed on us twice as forcibly. The limits of its validity will be recalled much sooner when we have the principle before us in its general form, instead of having to go over each separate case. By this sort of conscious formulation of inductive reasoning, there is much gain in the convenience and certainty of the process; but nothing essentially new is added that did not exist already in the conclusions which were reached by analogy without reflection. It is by means of these latter that we judge the character of a person from his countenance and movements, or predict what he will do in a given situation from a knowledge of his character.

Now we have exactly the same case in our sense-perceptions. When those nervous mechanisms whose terminals lie on the right-hand portions of the retinas of the two eyes have been stimulated, our usual experience, repeated a million times all through life, has been that a luminous object was over there in front of us on our left. We had to lift the hand toward the left to hide the light or to grasp the luminous object; or we had to move toward the left to get closer to it. Thus while in these cases no particular conscious conclusion may be present, yet the essential and original office of such a conclusion has been performed, and the result of it has been attained; simply, of course, by the unconscious processes of association of ideas going on in the dark background of our memory. Thus too its results are urged on our consclousness, so to speak, as if an external power had constrained us, over which our will has no control.

These inductive conclusions leading to the formation of our sense-perceptions certainly do lack the purifying and scrutinizing work of conscious thinking. Nevertheless, in my opinion, by their peculiar nature they may be classed as conclusions, inductive conclusions unconsciously formed.

There is one circumstance quite characteristic of these conclusions which operates against their being admitted in the realm of conscious thinking and against their being formulated in the normal form of logical conclusions. This is that we are not able to specify more closely what has taken place in us when we have experienced a sensation in a definite nerve fibre, and how it differs from corresponding sensations in other nerve fibres. Thus, suppose we have had a sensation of light in certain fibres of the nervous mechanism of vision. All we know is that we have had a sensation of a peculiar sort which is different from all other sensations, and also from all other visual sensations, and that whenever it occurred, we invariably noticed a luminous object on the left. Naturally, without ever having studied physiology, this is all we can say about the sensation, and even for our own imagination we cannot localize or grasp the sensation except by specifying it in terms of the conditions of its occurrencé. I have to say, ‘‘I see something bright there on my left.’”’ That is the only way I can describe the sensation. After we have pursued scientific studies, we begin to learn that we have nerves, that these nerves have been stimulated, and that their terminals in fact lie on the right-hand side of the retina. Then for the first time we are in a position to define this mode of sensation independently of the mode in which it is ordinarily produced.

It is the same way with most sensations. The sensations of taste and smell usually cannot be described even as to their quality except in terms of the bodies responsible for them; although we do have a few rather vague and more general expressions like ‘‘sweet,” “‘sour,”’ “bitter” and ‘‘sharp.”’

These judgments, in which our sensations in our ordinary state of consciousness are connected with the existence of an external cause, can never once be elevated to the plane of conscious judgments. The inference that there is a luminous object on my left, because the nerve terminals on the right-hand side of my retina are in a state of stimulation, can only be expressed by one who knows nothing about the inner structure of the eye by saying, ‘“There is something bright over there on my left, because I see it there.” And accordingly from the standpoint of everyday experience, the only way of expressing the experience I have when the nerve terminals on the right-hand side of my eyeball are stimulated by exerting pressure there, is by saying, ‘“When I press my eye on the right-hand side, I see a bright glow on the left.”

There is no other way of describing the sensation and of identifying it with other previous sensations except by designating the place where the corresponding external object appears to be. Hence, therefore, these cases of experience have the peculiarity that the connection between the sensation and an external object can never be expressed without anticipating it already in the designation of the sensation, and without presupposing the very thing we are trying to describe.

Even when we have learned to understand the physiological origin and connection of the illusions of the senses, it is impossible to get rid of the illusion in spite of our better knowledge. This is because inductive reasoning is the result of an unconscious and involuntary activity of the memory; and for this very reason it strikes our consciousness as a foreign and overpowering force of nature. Incidentally, manifold analogies for it are to be found in all other possible modes of apparition. We might say that all apparition originates in premature, unmeditated inductions, where from previous cases conclusions are deduced as to new ones, and where the tendency to abide by the false conclusions persists in spite of the better insight into the matter based on conscious deliberation. Every evening apparently before our eyes the sun goes down behind the stationary horizon, although we are well aware that the sun is fixed and the horizon moves. An actor who cleverly portrays an old man is for us an old man there on the stage, so long as we let the immediate impression sway us, and do not forcibly recall that the programme states that the person moving about there is the young actor with whom we are acquainted. We consider him as being angry or in pain according as he shows us one or the other mode of countenance and demeanour. He arouses fright or sympathy in us, we trembie for the moment, which we see approaching, when he will perform or suffer something dreadful; and the deep-seated conviction that all this is only show and play does not hinder our emotions at all, provided the actor does not cease to play his part. On the contrary, a fictitious tale of this sort, which we seem to enter into ourselves, grips and tortures us more than a similar true story would do when we read it in a dry documentary report.

The experiences we have that certain aspects, demeanours and modes of speech are indicative of fierce anger, are generally experiences concerning the external signs of certain emotions and peculiarities of character which the actor can portray for us. But they are not nearly so numerous and regular in recurrence as those experiences by which we have ascertained that certain sensations correspond with certain external objects. And so we need not be surprised if the idea of an object which is ordinarily associated with a sensation does not vanish,even when we know that in this particular instance there is no such object.

Finally, the tests we employ by voluntary movements of the body are of the greatest importance in strengthening our conviction of the correctness of the perceptions of our senses. And thus, as contrasted with purely passive observations, the same sort of firmer conviction arises as is derived by the process of experiment in scientific investigations. The peculiar ultimate basis, which gives convincing power to all our conscious inductions, is the law of causation. If two natural phenomena have frequently been observed to occur together, such as thunder and lightning, they seem to be regularly connected together, and we infer that there must be a common basis for both of them. And if this causal connection has invariably acted heretofore, so that thunder and lightning accompany each other, then in the future too like causes must produce like effects, and the result must be the same in the future. However, so long as we are limited to mere observations of such phenomena as occur by themselves without our help, and without our being able to make experiments so as to vary the complexity of causes, it is difficult to be sure that we have really ascertained all the factors that may have some influence on the result. There must be an enormous variety of cases where the law is obeyed, and the law must define the result with great precision, if we are to be satisfied with a case of mere observation. This is the case with the motions of the planetary system. Of course, we cannot experiment with the planets, but the theory of universal gravitation as propounded by Newton gives such a complete and exact explanation of the comparatively complicated apparent motions of the heavenly bodies, that we no longer hesitate about considering it as being sufficiently proved. And yet there are RrIcu’s experiments on the gravitational attraction of lead balls, Foucautt’s experiment on the deviation of the plane of vibration of a pendulum in consequence of the earth’s rotation, and the experimental determinations of the velocity of light in traversing terrestrial distances as made by Foucautr and Fiznav, that are of the utmost value in strengthening our conviction experimentally also.

Probably there is no event of pure observation that has been found to be so unexceptionally correct as the general statement previously used by way of illustration, namely, that all human beings die before they have passed a certain age. In many millions of human beings not a single exception has been found. If one had occurred, we might assume that we should have heard of it. Among those who have died there are individuals who have lived in the most varied climates and on the most various kinds of nourishment, besides having been engaged in the most diverse occupations. Nevertheless, the statement that all men are bound to die, cannot be said to have the same degree of certainty as any law of physics whose consequences have been precisely compared experimentally with experience in manifold modifications. I do not know the causal connection far the death of human beings. I cannot state the causes that inevitably entail old age, in case life has not been terminated sooner by some rougher external injury. I have not been able to verify by experiments that when I allow those causes to operate, old age inevitably occurs, and that it does not occur when I remove those causes of its occurrence. Anyone who tells me that the life of man can be indefinitely prolonged by employing certain means may be treated, of course, with the utmost incredulity, but he cannot be positively contradicted without knowing certainly that individuals have actually lived in the circumstances he describes, and yet have ultimately perished. On the other hand, when I assert that all liquid mercury will expand whenit is heated, if it is free to do so, I know that whenever I have observed the two together, not only higher temperature and expansion of mercury were due to the action of an unknown common third cause, as I might have supposed from pure observation alone, but I know by experiment that the heat by itself was enough to cause the expansion of the mercury. At various times I have often heated mercury. I have deliberately selected the moment when I wished the experiment to begin. If therefore the mercury expanded under these circumstances, the expansion must have been dependent on those conditions that I produced in the experiment. Consequently, I know that the heating by itself was a sufficient cause for the expansion, and that no other latent influences were needed to bring about this result. By comparatively few carefully executed experiments we are enabled to establish the causal conditions of an event with more certainty than can be done by a million observations where we have not been able to vary the conditions as we please. For instance, if I had merely seen mercury expand in a thermometer which was inaccessible to me, and in a place where the air was saturated with moisture at all temperatures, I should have to inquire whether mercury expands on account of heat or on account of the moisture. The only way to determine this would be by experiment, and by finding out whether the volume of mercury changes with change of humidity, when the temperature is kept constant, or with change of temperature, when the humidity is kept constant.

The same great importance which experiment has for the certainty of our scientific convictions, it has also for the unconscious inductions of the perceptions of our senses. It is only by voluntarily bringing our organs of sense in various relations to the objects that we learn to be sure as to our judgments of the causes of our sensations. This kind of experimentation begins in earliest youth and continues all tbroueh life without interruption.

If the objects had simply been passed in review before our eyes by some foreign force without our being able to do anything about them, probably we should never have found our way about amid such an optical phantasmagoria; any more than mankind could interpret the apparent motions of the planets in the firmament before the laws of perspective vision could be applied to them. But when we notice that we can get various images of a table in front of us simply by changing our position; and that we can sometimes have one view and sometimes another, just as we like at any time, by a suitable change of position; and that the table may vanish from sight, and then be there again at any moment we like, simply by turning the eyes toward it; we get the conviction based on experiment, that our movements are responsible for the different views of the table, and that whether we see it just at this moment or do not see it, still we can see it whenever we like. Thus by our movements we find out that it is the stationary form of the table in space which is the cause of the changing image in our eyes. We explain the table as having existence independent of our observation, because at any moment we like, simply by assuming the proper position with respect to it, we can observe it.

The essential thing in this process is just this principle of experimentation. Spontaneously and by our own power, we vary some of the conditions under which the object has been perceived. We know that the changes thus produced in the way that objects look depend solely on the movements we have executed. Thus we obtain a different series of apperceptions of the same object, by which we can be convinced with experimental certainty that they are simply apperceptions, and that it is the common cause of them all. In fact we see children also experimenting with objects in this way. They turn them constantly round and round, and touch them with the hands and the mouth, doing the same things over and over again day after day with the same objects, until their forms are impressed on them; in other words, until they get the various visual and tactile impressions made by observing and feeling the same object on various sides.

In this sort of experimentation with objects some of the changes in the sense-impressions are found to be due to our own will; whereas others, that is, all that depend on the nature of the object directly before us, are urged upon us by a necessity which we cannot alter as we like, and which we feel most when it arouses disagreeable sensations or pain. Thus we come to recognize something independent of our will and imagination, that is, an external cause of our sensations. This is shown by its persisting independently of our instantaneous perception; because at any moment we like, by suitable manipulations and movements, we can cause to recur each one of the series of sensations that can be produced in us by this external cause. Thus this latter is recognized as an object existing independently of our perception.

The idea and the cause here combine, and it is a question whether we have a right to assume this cause in the original perception of the senses. Here again the difficulty is that we are not able to describe the processes except in the language of metaphysics, whereas the reflection of the consciousness in itself is not yet distinctly contained in the original form of the conscious perception.

Natural consciousness, which is entirely absorbed in the interest of observing the external world, and has little inducement to direct its attention to the Ego that appears always the same amid the multicoloured variations of outside objects, is not in the habit of noticing that the properties of the objects that are seen and touched are their effects, partly on other natural bodies, but mainly on our senses. Now as our nervous system and our sensation-faculty, as being the constant reagent on which the effect is exerted, is thus left out of account entirely, and as the difference of the effect is regarded as being simply a difference in the object from which it proceeds, the effect can no longer be recognized as an effect (for every effect must be the effect on something else), and so comes to be considered objectively as being a property of the body and merely as belonging to it. And then as soon as we recall that we perceive these properties, our impression, consequently, seems to us to be a pure image of the external state of affairs reflecting only that external condition and depending solely on it.

But if we ponder over the basis of this process, it is obvious that we can never emerge from the world of our sensations to the apperception of an external world, except by inferring from the changing sensation that external objects are the causes of this change. Once the idea of external objects has been formed, we may not be concerned any more as to how we got this idea, especially because the inference appears to be so self-evident that we are not conscious of its being a new result.

Accordingly, the law of causation, by virtue of which we infer the cause from the effect, has to be considered also as being a law of our thinking which is prior to all experience. Generally, we can get no experience from natural objects unless the law of causation is already active in us. Therefore, it cannot be deduced first from experiences which we have had with natural objects.

This statement has been made in many ways. The law of causation was supposed to be a law of nature arrived at by induction. Recently it has been again interpreted in that way by J. S. Miuu. He has even suggested the possibility of its not being valid in other parts of the universe. As opposed to that view, I shall merely say, for what it is worth, that there is good reason to think that the empirical proof of the law is extremely doubtful. For the number of cases in which we think we can trace perfectly the causal connection between natural processes is small as compared with the number of those in which we are absolutely unable to do so at present. The former cases belong almost exclusively to inorganic nature. The cases that are not understood include the larger part of the phenomena of organic nature. In fact, by the evidence of our own consciousness, we positively assume both in beasts and in man a principle of free will, for which we claim most decidedly complete independence of the force of the law of causation. And in spite of all theoretical speculations as to possible mistakes about this conviction, I am of the opinion that our natural consciousness will hardly ever be free from it. Thus the case of conduct itself, which we know best and most accurately, we consider as being an exception to that law. Were therefore the law of causation a law of experience, its inductive proof would seem to be in a very bad shape. The best we could say is that it was not any more valid than rules of meteorology like the law of rotation of the wind, etc. Perhaps, we could not positively controvert the vitalistic physiologists who maintain that the law of causation is valid in inorganic nature; although in the organic world they relegate it to a lower sphere of action.

Finally, the law of causation bears on its face the character of a purely logical law, chiefly because the conclusions derived from it do not concern actual experience, but its interpretation. Hence it cannot be refuted by any possible experience.! For if we founder anywhere in applying the law of causation, we do not conclude that it is false, but simply that we do not yet completely understand the complex of causes mutually interacting in the given phenomenon. And when at length we have succeeded in explaining certain natural processes by the law of causation, the conclusions we derive from it are that certain masses of matter exist and move in space and act on each other with certain motive forces. But the conception of both matter and force are entirely abstract in nature, as is shown by their attributes. Matter without force! is assumed to exist only in space, but not to act or to have any properties. Thus it would be of no importance whatever for all other affairs in the world or for our perceptions. It would be practically non-existent. Force without matter is indeed said to act; but it cannot exist independently, for whatever exists is matter. Thus the two conceptions are inseparable; they are merely abstract modes of regarding the same objects of nature in various aspects. For that very reason neither matter nor force can be direct objects of observation, but are always merely the revealed causes of the facts of experience. Hence, if we conclude by proposing certain abstractions, which can never be objects of experience, as the final and sufficient bases of natural phenomena, how can we say that experience proves that the phenomena have sufficient bases?

The law of sufficient basis amounts simply to the requirement of wishing to understand everything. The process of our comprehension with respect to natural phenomena is that we try to find generic notions and laws of nature. Laws of nature are merely generic notions for the changes in nature. But since we have to assume the laws of nature as being valid and as acting independently of our observation and thinking, whereas as generic notions they would concern at first only the method of our thinking, we call them causes and forces. Hence, when we cannot trace natural phenomena to a law, and therefore cannot make the law objectively responsible as being the cause of the phenomena, the very possibility of comprehending such phenomena ceases.

However, we must try to comprehend them. There is no other method of bringing them under the control of the intellect. And so in investigating them we must proceed on the supposition that they are comprehensible. Accordingly, the law of sufficient reason is really nothing more than the urge of our intellect to bring all our perceptions under its own control. It is not a law of nature. Our intellect is the faculty of forming general conceptions. It has nothing to do with our sense-perceptions and experiences, unless it is able to form general conceptions or laws. These laws are then objectified and designated as causes. But if it is found that the natural phenomena are to be subsumed under a definite causal connection, this is certainly an objectively valid fact, and corresponds to special objective relations between natural phenomena, which we express in our thinking as being their causal connection, simply because we do not know how else to express it.

Just as it is the characteristic function of the eye to have lightsensations, so that we can see the world only as a luminous phenomenon, so likewise it is the characteristic function of the intellect to form general conceptions, that is, to search for causes; and hence it can conceive (begreifen) of the world only as being causal connection. We have other organs besides the eye for comprehending the external world, and thus we can feel or smell many things that we cannot see. Besides our intellect there is no other equally systematized faculty, at any rate for comprehending the external world. Thus if we are unable to conceive a thing, we cannot imagine it as existing.

The earlier history of the theory of the sense-perceptions is practically the same as the history of philosophy, as given at the end of §17. The investigations of the physiologists of the seventeenth and eighteenth centuries generally did not go beyond the image on the retina, for they supposed that when it was formed, everything was settled. Hence they were little troubled by the questions as to why we see objects erect and why we see them single, in spite of two inverted retinal images.

Among philosophers Descartes was the first to take any deep interest in visual perceptions as related to the knowledge of his time. He considered the qualities of sensation as being essentially subjective, but he regarded the ideas of the quantitative relations of size, form, motion, position, duration, number of objects, etc., as something that could be correctly perceived objectively. However, in order to explain the correctness of these ideas, he assumes, as the idealistic philosophers did who came after him, a system of innate ideas which are in harmony with the things. This theory was subsequently developed inits most logical and purest form by LrrBnirz.

BERKELEY made a profound study of the influence of memory on the visual perceptions and their concomitant inductive conclusions. He says concerning them that they take place so quickly that we are not aware of them unless we are deliberately on the watch for them. It is true,this empirical basis led him to assert that not only the qualities of sensation but the perceptions also were mainly merely internal processes having no correspondence with anything outside. What led him into making this false conclusion was the error contained in the proposition that the cause (the object perceived) must be of the same kind as its effect (the idea), that is, must be a mental entity also, and not a real object.

In his theory of the human understanding, Locke denied the existence of innate ideas and attempted to establish an empirical basis for all understanding; but this attempt ended in Humn’s denying all possibility of objective knowledge.

The most essential step for putting the problem in its true light was taken by Kant in his Critique of Pure Reason, in which he derived all real content of knowledge from experience. But he made a distinction between this and whatever in the form of our apperceptions and ideas was conditioned by the peculiar ability of our mind. Pure thinking a prior7 can yield only formally correct propositions, which, while they may certainly appear to be absolutely binding as necessary laws of thought and imagination, are, however, of no real significance for actuality; and hence they can never enable us to form any conclusion about facts of possible experience.

According to this view perception is recognized as an effect produced on our sensitive faculty by the object perceived; this effect, in its minuter determinations, being just as dependent on what causes the effect as on the nature of that on which the effect is produced. This point of view was applied to the empirical relations especially by Jou. MUuuer in his theory of the Specific Energy of the Senses.

The subsequent idealistic systems of philosophy associated with the names of J. G. Ficuts, ScHELLING and Hxcet all emphasized the theory that idea is essentially dependent on the nature of the mind; thus neglecting the influence which the thing causing the effect has on the effect. Consequently, their views have had slight influence on the theory of the sense-perceptions.

Kant had briefly represented space and time as given forms of all apperception, without going farther and investigating how much might be derived from experience in the more minute formation of individual apperceptions of space and time. This investigation was outside of his special work. Thus, for example, he regarded the geometrical axioms as being propositions in spaceapperception which were given to start with;—a view which is not at all settled yet.” His lead was followed by Jon. Mtuumr and the group of physiologists who tried to develop the intuitzon theory of space-apperception. JOH. MU uer himself assumed that the retina might ‘“‘sense’’ itself in its spaceextension by virtue of an innate ability for it, and that the sensations of the two retinas are fused together in this case. The one who has recently tried to carry out this view in its most logical form and to adapt it to newer discoveries is EK. Herine.

Prior to Mier, SteinBucH had tried to explain individual apperceptions of space by means of the movements of the eye and of the body. Among the philosophers, HerBart, Lorzr, Wairz and CorNneELius attacked the same problem. From the empirical side, it was WHEATSTONE especially who, by inventing the stereoscope, gave a powerful incentive to the investigation of the influence of experience on our visual apperceptions. In addition to minor contributions which I myself have made to the solution of this problem in various works, attempts to give an empirical view may be found in the writings of Nace, WunpT and CiassEen. In the succeeding chapters, more will be said with reference to these investigations and the points of controversy.

Or else the agreement between ideas and their objects was denied, the ideas being explained therefore as illusions. Consequently, it was necessary to deny also the possibility of all knowledge of any objects whatsoever. This was the attitude of certain so-called ‘‘sensational”’ philosophers in England in the eighteenth century. However, it is not my purpose here to undertake an analysis of the opinions of the various philosophical schools on this question. That would be much too extensive a task in this place. I shall confine myself therefore merely to inquiring what I think should be the attitude of an investigator toward these controversies.

Our apperceptions and ideas are effects wrought on our nervous system and our consciousness by the objects that are thus apprehended and conceived. Each effect, as to its nature, quite necessarily depends both on the nature of what causes the effect and on that of the person on whom the effect is produced. To expect to obtain an idea which would reproduce the nature of the thing conceived, that is, which would be true in an absolute sense, would mean to expect an effect which would be perfectly independent of the nature of the thing on which the effect was produced; which would be an obvious absurdity. Our human ideas, therefore, and all ideas of any conceivable intelligent creature, must be images of objects whose mode is essentially codependent on the nature of the consciousness which has the idea, and is conditioned also by its idiosyncrasies.

In my opinion, therefore, there can be no possible sense in speaking of any other truth of our ideas except of a practical truth. Our ideas of things cannot be anything but symbols, natural signs for things which we learn how to use in order to regulate our movements and actions. Having learned correctly how to read those symbols, we are enabled by their help to adjust our actions so as to bring about the desired result; that is, so that the expected new sensations will arise. Not only is there in reality no other comparison at all between ideas and things— all the schools are agreed about this—but any other mode of comparison is entirely unthinkable and has no sense whatever. This latter consideration is the conclusive thing, and must be grasped in order to escape from the labyrinth of conflicting opinions. To ask whether the idea I have of a table, its form, strength, colour, weight, etc., is true per se, apart from any practical use I can make of this idea, and whether it corresponds with the real thing, or is false and due to an illusion, has just as much sense as to ask whether a certain musical note is red, yellow, or blue. Idea and the thing conceived evidently belong to two entirely different worlds, which no more admit of being compared with each other than colours and musical tones or than the letters of a book and the sound of the word they denote.

Were there any sort of similarity of correspondence between the idea in the head of a person A and the thing to which the idea belongs, another intelligent person B, conceiving both the thing itself and A’s idea of it, according to the same laws, might be able to find some similarity between them or at least to suppose so; because the same sort of thing represented (conceived) in the same way would have to give the same kinds of images (ideas). Now I ask, what similarity can be imagined between the process in the brain that is concomitant with the idea of a table and the table itself? Is the form of the table to be supposed to be outlined by electric currents? And when the person with the idea has the idea that he is walking around the table, must the person then be outlined by electric currents? Perspective projections of the external world in the hemispheres of the brain (as they are supposed to be) are evidently not sufficient for representing the idea of a bodily object. And granted that a keen imagination is not frightened away by these and similar hypotheses, such an electrical reproduction of the table in the brain would be simply another bodily object to be perceived, but no idea of the table. However, it is not simply persons with materialistic opinions who try to refute the proposed statement, but also persons with idealistic views. And for the latter I should think the argument would be still more forcible. What possible similarity can there be between the idea, some modification of the incorporeal mind that has no extension in space, and the body of the table that occupies space? As far as I am aware, the idealistic philosophers have never once investigated even a single hypothesis or imagination in order to show this connection. And by the very nature of this view it is something that cannot be investigated at all.

In the next place as to the properties of objects in the external world, a little reflection reveals that all properties attributable to them may be said to be simply effects exerted by them either on our senses or on other natural objects. Colour, sound, taste, smell, temperature, smoothness, and firmness are properties of the first sort, and denote effects on our organs of sense. Smoothness and firmness denote the degree of resistance either to the gliding contact or pressure of the hand. But other natural bodies may be employed instead of the hand. And the same thing is true in testing other mechanical properties such as elasticity and weight. Chemical properties are described by certain reactions, that is, by effects exerted by one natural body on others. It is the same way with any other physical property of a body, optical, electrical, or magnetic. In every case we have to do with the mutual relations between various bodies and with the effects depending on the forces that different bodies exert on each other. For all natural forces are such as are exerted by one body on others. When we try to think of mere matter without force, it is void of properties likewise, except as to its different distribution in space and as to its motion. All properties of bodies in nature are manifested therefore simply by being so situated as to interact with other bodies of nature or with our organs of sense. But as such interaction may occur at any time, particularly too as it may be produced by us voluntarily at any moment, and as then we see invariably the peculiar sort of interaction occurring, we attribute to the objects a permanent capacity for such effects which is always ready to become effective. This permanent capacity is a so-called characteristic property.

The result is that in point of fact the characteristic properties of natural objects, in spite of this name, do not denote something that is peculiar to the individual object by itself, but invariably imply some relation to a second object (including our organs of sense). The kind of effect must, of course, depend always on the peculiarities both of the body producing it and of the body on which it is produced. As to this there is never any doubt even for an instant, provided we have in mind those properties of bodies that are manifested when two bodies belonging to the external world react on each other, as in the case of chemical reactions. But in the case of properties depending on the mutual relations between things and our organs of sense, people have always been disposed to forget that here too we are concerned with the reaction toward a special reagent, namely, our own nervous system; and that colour, smell, and taste, and feeling of warmth or cold are also effects quite essentially depending on the nature of the organ that is affected. Doubtless, the reactions of natural objects to our senses are those that are most frequently and most generally perceived. For both our welfare and convenience they are of the most powerful importance. The reagent by which we have to test them is something we are endowed with by nature, but that does not make any difference in the connection.

Hence there is no sense in asking whether vermilion as we see it, is really red, or whether this is simply an illusion of the senses. The sensation of red is the normal reaction of normally formed eyes to light reflected from vermilion. A person who is red-blind will see vermilion as black or as a dark grey-yellow. This too is the correct reaction for an eye formed in the special way his is. All he has to know is that his eye is simply formed differently from that of other persons. In itself the one sensation is not more correct and not more false than the other, although those who call this substance red are in the large majority. In general, the red colour of vermilion exists merely in so far as there are eyes which are constructed like those of most people. Persons who are red-blind have just as much right to consider that a characteristic property of vermilion is that of being black. As a matter of fact, we should not speak of the light reflected from vermilion as being red, because it is not red except for certain types of eyes. When we speak of the properties of bodies with reference to other bodies in the external world, we do not neglect to name also the body with respect to which the property exists. Thus we say that lead is soluble in nitric acid, but not in sulphuric acid. Were we to say simply that lead is soluble, we should notice at once that the statement is incomplete, and the question would have to be asked immediately, Soluble in what? But when we say that vermilion is red, it is implicitly understood that it is red for our eyes and for other people’s eyes supposed to be made like ours. We think this does not need to be mentioned, and so we neglect to do so, and can be misled into thinking that red is a property belonging to vermilion or to the light reflected from it, entirely independently of our organs of sense. The statement that the waves of light reflected from vermilion have a certain length is something different. That is true entirely without reference to the special nature of our eye. Then we are thinking simply of relations that exist between the substance and the various systems of waves in the aether.

The only respect in which there can be a real agreement between our perceptions and the reality is the time-sequence of the events with thei: various peculiarities. Simultaneity, sequence, the regular recurrence of simultaneity or sequence, may occur likewise in the sensations as well as in the events. The external events, like their perceptions, proceed in time; and so the temporal relations of the latter may be the faithful reproduction of the temporal relations of the former. The sensation of thunder in the ear succeeds the sensation of lightning in the eye, just in the same way as the sound vibrations in the air due to the electrical discharge reach the place where the observer is later than the vibrations of the luminiferous aether. Yet here it certainly should be noted that the time-sequence of the sensations is not quite a faithful reproduction of the time-sequence of the external events, inasmuch as the transmission from the organs of sense to the brain takes time, and in fact a different time for different organs. Moreover, in case of the eye and the ear, the time has to be added that it takes light and sound to reach the organ. Thus at present we see the fixed stars as they were various long periods of years ago.

As to the representation of space-relations, there certainly is something of this sort in the peripheral nerve terminals in the eye and to a certain extent in the tactile skin, but still only in a limited way; for the eye gives only perspective surface-images, and the hand reproduces the objective area on the surface of a body by shaping itself to it as congruently as possible. A direct image of a portion of space of three dimensions is not afforded either by the eye or by the hand. It is only by comparing the images in the two eyes, or by moving the body with respect to the hand, that the idea of solid bodies is obtained. Now since the brain itself has three dimensions, of course, there is still another conceivable possibility, and that is to fancy by what mechanism in the brain itself images of three dimensions can arise from external objects in space. But I cannot see any necessity for such an assumption nor even any probability for it. The idea of a body in space, of a table, for instance, involves a quantity of separate observations. It comprises the whole series of images which this table would present to me in looking at it from different sides and at different distances; besides the whole series of tactile impressions that would be obtained by touching the surface at various places in succession. Such an idea of a single individual body is, therefore, in fact a conception (Begriff) which grasps and includes an infinite number of single, successive apperceptions, that can all be deduced from it; just as the species ‘‘table’” includes all individual tables and expresses their common peculiarities. The idea of a single individual table which I carry in my mind is correct and exact, provided I can deduce from it correctly the precise sensations I shall have when my eye and my hand are brought into this or that definite relation with respect to the table. Any other sort of similarity between such an idea and the body about which the idea exists, I do not know how to conceive. One is the mental symbol of the other. The kind of symbol was not chosen by me arbitrarily, but was forced on me by the nature of my organ of sense and of my mind. This is what distinguishes this sign-language of our ideas from the arbitrary phonetic signs and alphabetical characters that we use in speaking and writing. A writing is correct when he who knows how to read it forms correct ideas by it. And so the idea of a thing is correct for him who knows how to determine correctly from it in advance what sense-impressions he will get from the thing when he places himself in definite external relations to it. Incidentally, it does not matter at all what sort of mental symbols we employ, provided they constitute a sufficiently varied and ordered system. Nor does it matter either how the words of a language sound, provided there are enough of them, with sufficient means of denoting their grammatical relations to one another.

On this view of the matter, we must be on our guard against saying that all our ideas of things are consequently false, because they are not equal to the things themselves, and that hence we are not able to know anything as to the true nature of things. That they cannot be equal to things, is in the nature of knowledge. Ideas are merely pictures of things. Every image is the image of a thing merely for him who knows how to read it, and who is enabled by the aid of the image to form an idea of the thing. Every image is similar to its object in one respect, and dissimilar in all others, whether it be a painting, a statue, the musical or dramatic representation of a mental mood, etc. Thus the ideas of the external world are images of the regular sequence of natural events, and if they are formed correctly according to the laws of our thinking, and we are able by our actions to translate them back into reality again, the ideas we have are also the only true ones for our mental capacity. All others would be false.

In my opinion, it is a mistake, therefore, to try to find preestablished harmony between the laws of thought and those of nature, an identity between nature and mind, or whatever we may call it. A system of signs may be more or less perfect and convenient. Accordingly, it will be more or less easy to employ, more exact in denoting or more inexact, just as is the case with different languages. But otherwise each system can be adapted to the case more or less well. If there were not a number of similar natural objects in the world, our faculty of forming shades of conception would indeed not be of any use to us. Were there no solid bodies, our geometrical faculties would necessarily remain undeveloped and unused, just as the physical eye would not be of any service to us in a world where there was no light. If in this sense anybody wishes to speak of an adaptation of our laws of mind to the laws of nature, there is no objection to it. Evidently, however, such adaptation does not have to be either perfect or exact. The eye is an extremely useful organ practically, although it cannot see distinctly at all distances, or perceive all sorts of aether vibrations, or concentrate exactly in one point all the rays that issue from a point. Our intellectual faculties are connected with the activities of a material organ, namely the brain, just as the faculty of vision is connected with the eye. Human intelligence is wonderfully effective in the world, and brings it under a strict law of causation. Whether it necessarily must be able to control whatever is in the world or can happen—I can see no guarantee for that.

We must speak now of the manner in which our ideas and perceptions are formed by inductive conclusions. The best analysis of the nature of our conclusions I find in J. 8. Mrux’s Logic. As long as the premise of the conclusion is not an injunction imposed by outside authority for our conduct and belief, but a statement related to reality, which can therefore be only the result of experience, the conclusion, as a matter of fact, does not tell us anything new or something that we did not know already before we made the statement. Thus, for example:

Major: All men are mortal.

Minor: Caius is a man.

Conclusion: Caius is mortal.

The major premise, that all men are mortal, which is a statement of experience, we should scarcely venture to assert without knowing beforehand whether the conclusion is correct, namely, that Caius, who is a man, either is dead or will die. Thus we must be sure of the conclusion before we can state the major premise by which we intend to prove it. That seems to be proceeding in a circle. The real relation evidently is, that, in common with other folks, we have observed heretofore without exception that no person has ever survived beyond a certain age. Observers have learned by experience that Lucius, Flavius and other individuals of their acquaintance, no matter what their names are, have all died; and they have embraced this experience in the general statement, that all men die. Inasmuch as this final result occurred regularly in all the instances they observed, they have felt justified in explaining this general law as being valid also for all those cases which might come up for observation hereafter. Thus we preserve in our memory the store of experiences heretofore accumulated on this subject by ourselves and others in the form of the general statement which constitutes the major premise of the above conclusion.

However, the conviction that Caius would die might obviously have been reached directly also without formulating the general statement in our consciousness, by having compared his case with all those which we knew previously. Indeed, this is the more usual and original method of reasoning by induction. Conclusions of this sort are reached without conscious reflection, because in our memory the same sort of thing in cases previously observed unites and reinforces them; as is shown especially in those cases of inductive reasoning where we cannot succeed in deducing from previous experiences a rule with precisely defined limits to its validity and without any exceptions. This is the case in all complicated processes. For instance, from analogy with previous similar cases, we can sometimes predict with tolerable certainty what one of our acquaintances will do, if under certain circumstances he decides to go into business; because we know his character and that he is, let us say, ambitious or timid. Wemay not be able to say exactly how we have estimated the extent of his ambition or timidity, or why this ambition or timidity of his will be enough to decide that his business will turn out as we expect.

In the case of conclusions properly so-called, which are reached consciously, supposing they are not based on injunctions but on facts of experience, what we do, therefore, is really nothing more than deliberately and carefully to retrace those steps in the inductive generalizations of our experiences which were previously traversed more rapidly and without conscious reflection, either by ourselves or by other observers in whom we have confidence. But although nothing essentially new is added to our previous knowledge by formulating a general principle from our previous experiences, still it is useful in many respects. A definitely stated general principle is much easier to preserve in the memory and to be imparted to others than to have to do this same thing with every individual case as it arises. In formulating it we are led to test accurately every new case that occurs, with reference to the correctness of the generalization. In this way every exception will be impressed on us twice as forcibly. The limits of its validity will be recalled much sooner when we have the principle before us in its general form, instead of having to go over each separate case. By this sort of conscious formulation of inductive reasoning, there is much gain in the convenience and certainty of the process; but nothing essentially new is added that did not exist already in the conclusions which were reached by analogy without reflection. It is by means of these latter that we judge the character of a person from his countenance and movements, or predict what he will do in a given situation from a knowledge of his character.

Now we have exactly the same case in our sense-perceptions. When those nervous mechanisms whose terminals lie on the right-hand portions of the retinas of the two eyes have been stimulated, our usual experience, repeated a million times all through life, has been that a luminous object was over there in front of us on our left. We had to lift the hand toward the left to hide the light or to grasp the luminous object; or we had to move toward the left to get closer to it. Thus while in these cases no particular conscious conclusion may be present, yet the essential and original office of such a conclusion has been performed, and the result of it has been attained; simply, of course, by the unconscious processes of association of ideas going on in the dark background of our memory. Thus too its results are urged on our consclousness, so to speak, as if an external power had constrained us, over which our will has no control.

These inductive conclusions leading to the formation of our sense-perceptions certainly do lack the purifying and scrutinizing work of conscious thinking. Nevertheless, in my opinion, by their peculiar nature they may be classed as conclusions, inductive conclusions unconsciously formed.

There is one circumstance quite characteristic of these conclusions which operates against their being admitted in the realm of conscious thinking and against their being formulated in the normal form of logical conclusions. This is that we are not able to specify more closely what has taken place in us when we have experienced a sensation in a definite nerve fibre, and how it differs from corresponding sensations in other nerve fibres. Thus, suppose we have had a sensation of light in certain fibres of the nervous mechanism of vision. All we know is that we have had a sensation of a peculiar sort which is different from all other sensations, and also from all other visual sensations, and that whenever it occurred, we invariably noticed a luminous object on the left. Naturally, without ever having studied physiology, this is all we can say about the sensation, and even for our own imagination we cannot localize or grasp the sensation except by specifying it in terms of the conditions of its occurrencé. I have to say, ‘‘I see something bright there on my left.’”’ That is the only way I can describe the sensation. After we have pursued scientific studies, we begin to learn that we have nerves, that these nerves have been stimulated, and that their terminals in fact lie on the right-hand side of the retina. Then for the first time we are in a position to define this mode of sensation independently of the mode in which it is ordinarily produced.

It is the same way with most sensations. The sensations of taste and smell usually cannot be described even as to their quality except in terms of the bodies responsible for them; although we do have a few rather vague and more general expressions like ‘‘sweet,” “‘sour,”’ “bitter” and ‘‘sharp.”’

These judgments, in which our sensations in our ordinary state of consciousness are connected with the existence of an external cause, can never once be elevated to the plane of conscious judgments. The inference that there is a luminous object on my left, because the nerve terminals on the right-hand side of my retina are in a state of stimulation, can only be expressed by one who knows nothing about the inner structure of the eye by saying, ‘“There is something bright over there on my left, because I see it there.” And accordingly from the standpoint of everyday experience, the only way of expressing the experience I have when the nerve terminals on the right-hand side of my eyeball are stimulated by exerting pressure there, is by saying, ‘“When I press my eye on the right-hand side, I see a bright glow on the left.”

There is no other way of describing the sensation and of identifying it with other previous sensations except by designating the place where the corresponding external object appears to be. Hence, therefore, these cases of experience have the peculiarity that the connection between the sensation and an external object can never be expressed without anticipating it already in the designation of the sensation, and without presupposing the very thing we are trying to describe.

Even when we have learned to understand the physiological origin and connection of the illusions of the senses, it is impossible to get rid of the illusion in spite of our better knowledge. This is because inductive reasoning is the result of an unconscious and involuntary activity of the memory; and for this very reason it strikes our consciousness as a foreign and overpowering force of nature. Incidentally, manifold analogies for it are to be found in all other possible modes of apparition. We might say that all apparition originates in premature, unmeditated inductions, where from previous cases conclusions are deduced as to new ones, and where the tendency to abide by the false conclusions persists in spite of the better insight into the matter based on conscious deliberation. Every evening apparently before our eyes the sun goes down behind the stationary horizon, although we are well aware that the sun is fixed and the horizon moves. An actor who cleverly portrays an old man is for us an old man there on the stage, so long as we let the immediate impression sway us, and do not forcibly recall that the programme states that the person moving about there is the young actor with whom we are acquainted. We consider him as being angry or in pain according as he shows us one or the other mode of countenance and demeanour. He arouses fright or sympathy in us, we trembie for the moment, which we see approaching, when he will perform or suffer something dreadful; and the deep-seated conviction that all this is only show and play does not hinder our emotions at all, provided the actor does not cease to play his part. On the contrary, a fictitious tale of this sort, which we seem to enter into ourselves, grips and tortures us more than a similar true story would do when we read it in a dry documentary report.

The experiences we have that certain aspects, demeanours and modes of speech are indicative of fierce anger, are generally experiences concerning the external signs of certain emotions and peculiarities of character which the actor can portray for us. But they are not nearly so numerous and regular in recurrence as those experiences by which we have ascertained that certain sensations correspond with certain external objects. And so we need not be surprised if the idea of an object which is ordinarily associated with a sensation does not vanish,even when we know that in this particular instance there is no such object.

Finally, the tests we employ by voluntary movements of the body are of the greatest importance in strengthening our conviction of the correctness of the perceptions of our senses. And thus, as contrasted with purely passive observations, the same sort of firmer conviction arises as is derived by the process of experiment in scientific investigations. The peculiar ultimate basis, which gives convincing power to all our conscious inductions, is the law of causation. If two natural phenomena have frequently been observed to occur together, such as thunder and lightning, they seem to be regularly connected together, and we infer that there must be a common basis for both of them. And if this causal connection has invariably acted heretofore, so that thunder and lightning accompany each other, then in the future too like causes must produce like effects, and the result must be the same in the future. However, so long as we are limited to mere observations of such phenomena as occur by themselves without our help, and without our being able to make experiments so as to vary the complexity of causes, it is difficult to be sure that we have really ascertained all the factors that may have some influence on the result. There must be an enormous variety of cases where the law is obeyed, and the law must define the result with great precision, if we are to be satisfied with a case of mere observation. This is the case with the motions of the planetary system. Of course, we cannot experiment with the planets, but the theory of universal gravitation as propounded by Newton gives such a complete and exact explanation of the comparatively complicated apparent motions of the heavenly bodies, that we no longer hesitate about considering it as being sufficiently proved. And yet there are RrIcu’s experiments on the gravitational attraction of lead balls, Foucautt’s experiment on the deviation of the plane of vibration of a pendulum in consequence of the earth’s rotation, and the experimental determinations of the velocity of light in traversing terrestrial distances as made by Foucautr and Fiznav, that are of the utmost value in strengthening our conviction experimentally also.

Probably there is no event of pure observation that has been found to be so unexceptionally correct as the general statement previously used by way of illustration, namely, that all human beings die before they have passed a certain age. In many millions of human beings not a single exception has been found. If one had occurred, we might assume that we should have heard of it. Among those who have died there are individuals who have lived in the most varied climates and on the most various kinds of nourishment, besides having been engaged in the most diverse occupations. Nevertheless, the statement that all men are bound to die, cannot be said to have the same degree of certainty as any law of physics whose consequences have been precisely compared experimentally with experience in manifold modifications. I do not know the causal connection far the death of human beings. I cannot state the causes that inevitably entail old age, in case life has not been terminated sooner by some rougher external injury. I have not been able to verify by experiments that when I allow those causes to operate, old age inevitably occurs, and that it does not occur when I remove those causes of its occurrence. Anyone who tells me that the life of man can be indefinitely prolonged by employing certain means may be treated, of course, with the utmost incredulity, but he cannot be positively contradicted without knowing certainly that individuals have actually lived in the circumstances he describes, and yet have ultimately perished. On the other hand, when I assert that all liquid mercury will expand whenit is heated, if it is free to do so, I know that whenever I have observed the two together, not only higher temperature and expansion of mercury were due to the action of an unknown common third cause, as I might have supposed from pure observation alone, but I know by experiment that the heat by itself was enough to cause the expansion of the mercury. At various times I have often heated mercury. I have deliberately selected the moment when I wished the experiment to begin. If therefore the mercury expanded under these circumstances, the expansion must have been dependent on those conditions that I produced in the experiment. Consequently, I know that the heating by itself was a sufficient cause for the expansion, and that no other latent influences were needed to bring about this result. By comparatively few carefully executed experiments we are enabled to establish the causal conditions of an event with more certainty than can be done by a million observations where we have not been able to vary the conditions as we please. For instance, if I had merely seen mercury expand in a thermometer which was inaccessible to me, and in a place where the air was saturated with moisture at all temperatures, I should have to inquire whether mercury expands on account of heat or on account of the moisture. The only way to determine this would be by experiment, and by finding out whether the volume of mercury changes with change of humidity, when the temperature is kept constant, or with change of temperature, when the humidity is kept constant.

The same great importance which experiment has for the certainty of our scientific convictions, it has also for the unconscious inductions of the perceptions of our senses. It is only by voluntarily bringing our organs of sense in various relations to the objects that we learn to be sure as to our judgments of the causes of our sensations. This kind of experimentation begins in earliest youth and continues all tbroueh life without interruption.

If the objects had simply been passed in review before our eyes by some foreign force without our being able to do anything about them, probably we should never have found our way about amid such an optical phantasmagoria; any more than mankind could interpret the apparent motions of the planets in the firmament before the laws of perspective vision could be applied to them. But when we notice that we can get various images of a table in front of us simply by changing our position; and that we can sometimes have one view and sometimes another, just as we like at any time, by a suitable change of position; and that the table may vanish from sight, and then be there again at any moment we like, simply by turning the eyes toward it; we get the conviction based on experiment, that our movements are responsible for the different views of the table, and that whether we see it just at this moment or do not see it, still we can see it whenever we like. Thus by our movements we find out that it is the stationary form of the table in space which is the cause of the changing image in our eyes. We explain the table as having existence independent of our observation, because at any moment we like, simply by assuming the proper position with respect to it, we can observe it.

The essential thing in this process is just this principle of experimentation. Spontaneously and by our own power, we vary some of the conditions under which the object has been perceived. We know that the changes thus produced in the way that objects look depend solely on the movements we have executed. Thus we obtain a different series of apperceptions of the same object, by which we can be convinced with experimental certainty that they are simply apperceptions, and that it is the common cause of them all. In fact we see children also experimenting with objects in this way. They turn them constantly round and round, and touch them with the hands and the mouth, doing the same things over and over again day after day with the same objects, until their forms are impressed on them; in other words, until they get the various visual and tactile impressions made by observing and feeling the same object on various sides.

In this sort of experimentation with objects some of the changes in the sense-impressions are found to be due to our own will; whereas others, that is, all that depend on the nature of the object directly before us, are urged upon us by a necessity which we cannot alter as we like, and which we feel most when it arouses disagreeable sensations or pain. Thus we come to recognize something independent of our will and imagination, that is, an external cause of our sensations. This is shown by its persisting independently of our instantaneous perception; because at any moment we like, by suitable manipulations and movements, we can cause to recur each one of the series of sensations that can be produced in us by this external cause. Thus this latter is recognized as an object existing independently of our perception.

The idea and the cause here combine, and it is a question whether we have a right to assume this cause in the original perception of the senses. Here again the difficulty is that we are not able to describe the processes except in the language of metaphysics, whereas the reflection of the consciousness in itself is not yet distinctly contained in the original form of the conscious perception.

Natural consciousness, which is entirely absorbed in the interest of observing the external world, and has little inducement to direct its attention to the Ego that appears always the same amid the multicoloured variations of outside objects, is not in the habit of noticing that the properties of the objects that are seen and touched are their effects, partly on other natural bodies, but mainly on our senses. Now as our nervous system and our sensation-faculty, as being the constant reagent on which the effect is exerted, is thus left out of account entirely, and as the difference of the effect is regarded as being simply a difference in the object from which it proceeds, the effect can no longer be recognized as an effect (for every effect must be the effect on something else), and so comes to be considered objectively as being a property of the body and merely as belonging to it. And then as soon as we recall that we perceive these properties, our impression, consequently, seems to us to be a pure image of the external state of affairs reflecting only that external condition and depending solely on it.

But if we ponder over the basis of this process, it is obvious that we can never emerge from the world of our sensations to the apperception of an external world, except by inferring from the changing sensation that external objects are the causes of this change. Once the idea of external objects has been formed, we may not be concerned any more as to how we got this idea, especially because the inference appears to be so self-evident that we are not conscious of its being a new result.

Accordingly, the law of causation, by virtue of which we infer the cause from the effect, has to be considered also as being a law of our thinking which is prior to all experience. Generally, we can get no experience from natural objects unless the law of causation is already active in us. Therefore, it cannot be deduced first from experiences which we have had with natural objects.

This statement has been made in many ways. The law of causation was supposed to be a law of nature arrived at by induction. Recently it has been again interpreted in that way by J. S. Miuu. He has even suggested the possibility of its not being valid in other parts of the universe. As opposed to that view, I shall merely say, for what it is worth, that there is good reason to think that the empirical proof of the law is extremely doubtful. For the number of cases in which we think we can trace perfectly the causal connection between natural processes is small as compared with the number of those in which we are absolutely unable to do so at present. The former cases belong almost exclusively to inorganic nature. The cases that are not understood include the larger part of the phenomena of organic nature. In fact, by the evidence of our own consciousness, we positively assume both in beasts and in man a principle of free will, for which we claim most decidedly complete independence of the force of the law of causation. And in spite of all theoretical speculations as to possible mistakes about this conviction, I am of the opinion that our natural consciousness will hardly ever be free from it. Thus the case of conduct itself, which we know best and most accurately, we consider as being an exception to that law. Were therefore the law of causation a law of experience, its inductive proof would seem to be in a very bad shape. The best we could say is that it was not any more valid than rules of meteorology like the law of rotation of the wind, etc. Perhaps, we could not positively controvert the vitalistic physiologists who maintain that the law of causation is valid in inorganic nature; although in the organic world they relegate it to a lower sphere of action.

Finally, the law of causation bears on its face the character of a purely logical law, chiefly because the conclusions derived from it do not concern actual experience, but its interpretation. Hence it cannot be refuted by any possible experience.! For if we founder anywhere in applying the law of causation, we do not conclude that it is false, but simply that we do not yet completely understand the complex of causes mutually interacting in the given phenomenon. And when at length we have succeeded in explaining certain natural processes by the law of causation, the conclusions we derive from it are that certain masses of matter exist and move in space and act on each other with certain motive forces. But the conception of both matter and force are entirely abstract in nature, as is shown by their attributes. Matter without force! is assumed to exist only in space, but not to act or to have any properties. Thus it would be of no importance whatever for all other affairs in the world or for our perceptions. It would be practically non-existent. Force without matter is indeed said to act; but it cannot exist independently, for whatever exists is matter. Thus the two conceptions are inseparable; they are merely abstract modes of regarding the same objects of nature in various aspects. For that very reason neither matter nor force can be direct objects of observation, but are always merely the revealed causes of the facts of experience. Hence, if we conclude by proposing certain abstractions, which can never be objects of experience, as the final and sufficient bases of natural phenomena, how can we say that experience proves that the phenomena have sufficient bases?

The law of sufficient basis amounts simply to the requirement of wishing to understand everything. The process of our comprehension with respect to natural phenomena is that we try to find generic notions and laws of nature. Laws of nature are merely generic notions for the changes in nature. But since we have to assume the laws of nature as being valid and as acting independently of our observation and thinking, whereas as generic notions they would concern at first only the method of our thinking, we call them causes and forces. Hence, when we cannot trace natural phenomena to a law, and therefore cannot make the law objectively responsible as being the cause of the phenomena, the very possibility of comprehending such phenomena ceases.

However, we must try to comprehend them. There is no other method of bringing them under the control of the intellect. And so in investigating them we must proceed on the supposition that they are comprehensible. Accordingly, the law of sufficient reason is really nothing more than the urge of our intellect to bring all our perceptions under its own control. It is not a law of nature. Our intellect is the faculty of forming general conceptions. It has nothing to do with our sense-perceptions and experiences, unless it is able to form general conceptions or laws. These laws are then objectified and designated as causes. But if it is found that the natural phenomena are to be subsumed under a definite causal connection, this is certainly an objectively valid fact, and corresponds to special objective relations between natural phenomena, which we express in our thinking as being their causal connection, simply because we do not know how else to express it.

Just as it is the characteristic function of the eye to have lightsensations, so that we can see the world only as a luminous phenomenon, so likewise it is the characteristic function of the intellect to form general conceptions, that is, to search for causes; and hence it can conceive (begreifen) of the world only as being causal connection. We have other organs besides the eye for comprehending the external world, and thus we can feel or smell many things that we cannot see. Besides our intellect there is no other equally systematized faculty, at any rate for comprehending the external world. Thus if we are unable to conceive a thing, we cannot imagine it as existing.

The earlier history of the theory of the sense-perceptions is practically the same as the history of philosophy, as given at the end of §17. The investigations of the physiologists of the seventeenth and eighteenth centuries generally did not go beyond the image on the retina, for they supposed that when it was formed, everything was settled. Hence they were little troubled by the questions as to why we see objects erect and why we see them single, in spite of two inverted retinal images.

Among philosophers Descartes was the first to take any deep interest in visual perceptions as related to the knowledge of his time. He considered the qualities of sensation as being essentially subjective, but he regarded the ideas of the quantitative relations of size, form, motion, position, duration, number of objects, etc., as something that could be correctly perceived objectively. However, in order to explain the correctness of these ideas, he assumes, as the idealistic philosophers did who came after him, a system of innate ideas which are in harmony with the things. This theory was subsequently developed inits most logical and purest form by LrrBnirz.

BERKELEY made a profound study of the influence of memory on the visual perceptions and their concomitant inductive conclusions. He says concerning them that they take place so quickly that we are not aware of them unless we are deliberately on the watch for them. It is true,this empirical basis led him to assert that not only the qualities of sensation but the perceptions also were mainly merely internal processes having no correspondence with anything outside. What led him into making this false conclusion was the error contained in the proposition that the cause (the object perceived) must be of the same kind as its effect (the idea), that is, must be a mental entity also, and not a real object.

In his theory of the human understanding, Locke denied the existence of innate ideas and attempted to establish an empirical basis for all understanding; but this attempt ended in Humn’s denying all possibility of objective knowledge.

The most essential step for putting the problem in its true light was taken by Kant in his Critique of Pure Reason, in which he derived all real content of knowledge from experience. But he made a distinction between this and whatever in the form of our apperceptions and ideas was conditioned by the peculiar ability of our mind. Pure thinking a prior7 can yield only formally correct propositions, which, while they may certainly appear to be absolutely binding as necessary laws of thought and imagination, are, however, of no real significance for actuality; and hence they can never enable us to form any conclusion about facts of possible experience.

According to this view perception is recognized as an effect produced on our sensitive faculty by the object perceived; this effect, in its minuter determinations, being just as dependent on what causes the effect as on the nature of that on which the effect is produced. This point of view was applied to the empirical relations especially by Jou. MUuuer in his theory of the Specific Energy of the Senses.

The subsequent idealistic systems of philosophy associated with the names of J. G. Ficuts, ScHELLING and Hxcet all emphasized the theory that idea is essentially dependent on the nature of the mind; thus neglecting the influence which the thing causing the effect has on the effect. Consequently, their views have had slight influence on the theory of the sense-perceptions.

Kant had briefly represented space and time as given forms of all apperception, without going farther and investigating how much might be derived from experience in the more minute formation of individual apperceptions of space and time. This investigation was outside of his special work. Thus, for example, he regarded the geometrical axioms as being propositions in spaceapperception which were given to start with;—a view which is not at all settled yet.” His lead was followed by Jon. Mtuumr and the group of physiologists who tried to develop the intuitzon theory of space-apperception. JOH. MU uer himself assumed that the retina might ‘“‘sense’’ itself in its spaceextension by virtue of an innate ability for it, and that the sensations of the two retinas are fused together in this case. The one who has recently tried to carry out this view in its most logical form and to adapt it to newer discoveries is EK. Herine.

Prior to Mier, SteinBucH had tried to explain individual apperceptions of space by means of the movements of the eye and of the body. Among the philosophers, HerBart, Lorzr, Wairz and CorNneELius attacked the same problem. From the empirical side, it was WHEATSTONE especially who, by inventing the stereoscope, gave a powerful incentive to the investigation of the influence of experience on our visual apperceptions. In addition to minor contributions which I myself have made to the solution of this problem in various works, attempts to give an empirical view may be found in the writings of Nace, WunpT and CiassEen. In the succeeding chapters, more will be said with reference to these investigations and the points of controversy.