The unreflective man regards his percepts, such as they appear to his immediate apprehension, as things having a wholly independent existence. When he sees a tree he believes that it stands in the form which he sees, with the colours of all its parts, etc., there on the spot towards which his gaze is directed. When the same man sees the sun in the morning appear as a disc on the horizon, and follows the course of this disc, he believes that the phenomenon exists and occurs (by itself) exactly as he perceives it. To this belief he clings until he meets with further percepts which contradict his former ones. The child who has as yet had no experience of distance grasps at the moon, and does not correct its first impression as to the real distance until a second percept contradicts the first. Every extension of the circle of my percepts compels me to correct my picture of the world. We see this in everyday life, as well as in the mental development of mankind. The picture which the ancients made for themselves of the relation of the earth to the sun and other heavenly bodies, had to be replaced by another when Copernicus found that it contradicted percepts which in those early days were unknown. A man who had been born blind said, when operated on by Dr. Franz, that the idea of the size of objects which he had formed before his operation by his sense of touch was a very different one. He had to correct his tactual percepts by his visual percepts.
How is it that we are compelled to make these continual corrections in our observations?
A single reflection supplies the answer to this question. When I stand at one end of an avenue, the trees at the other end, away from me, seem smaller and nearer together than those where I stand. But the scene which I perceive changes when I change the place from which I am looking. The exact form in which it presents itself to me is, therefore, dependent on a condition which inheres, not in the object, but in me, the percipient. It is all the same to the avenue where I stand. But the picture of it which I receive depends essentially on my standpoint. In the same way, it makes no difference to the sun and the planetary system that human beings happen to perceive them from the earth; but the picture of the heavens which human beings have is determined by the fact that they inhabit the earth. This dependence of our percepts on our points of observation is the easiest kind of dependence to understand. The matter becomes more difficult when we realise further that our perceptual world is dependent on our bodily and mental organisation. The physicist teaches us that within the space in which we hear a sound there are vibrations of the air, and that there are vibrations also in the particles of the body which we regard as the cause of the sound. These vibrations are perceived as sounds only if we have normally constructed ears. Without them the whole world would be for us for ever silent. Again, the physiologist teaches us that there are men who perceive nothing of the wonderful display of colours which surrounds us. In their world there are only degrees of light and dark. Others are blind only to one colour, e.g., red. Their world lacks this colour tone, and hence it is actually a different one from that of the average man. I should like to call the dependence of my perceptual world on my point of observation “mathematical,” and its dependence on my organisation “qualitative.” The former determines proportions of size and mutual distances of my percepts, the latter their quality. The fact that I see a red surface as red—this qualitative determination—depends on the structure of my eye.
My percepts, then, are in the first instance subjective. The recognition of the subjective character of our percepts may easily lead us to doubt whether there is any objective basis for them at all. When we know that a percept, e.g., that of a red colour or of a certain tone, is not possible without a specific structure of our organism, we may easily be led to believe that it has no being at all apart from our subjective organisation, that it has no kind of existence apart from the act of perceiving of which it is the object. The classical representative of this theory is George Berkeley, who held that from the moment we realise the importance of a subject for perception, we are no longer able to believe in the existence of a world apart from a conscious mind. “Some truths there are so near and obvious to the mind that man need only open his eyes to see them. Such I take this important one to be, viz., that all the choir of heaven and the furniture of the earth—in a word, all those bodies which compose the mighty frame of the world—have not any subsistence without a mind; that their being is to be perceived or known; that consequently, so long as they are not actually perceived by me, or do not exist in my mind or that of any other created spirit, they must either have no existence at all or else subsist in the mind of some Eternal Spirit” (Berkeley, Of the Principles of Human Knowledge, Part I, Section 6).
On this view, when we take away the act of perceiving, nothing remains of the percept. There is no colour when none is seen, no sound when none is heard. Extension, form, and motion exist as little as colour and sound apart from the act of perception. We never perceive bare extension or shape. These are always joined with colour or some other quality, which are undoubtedly dependent on the subject. If these latter disappear when we cease to perceive, the former, being connected with them, must disappear likewise.
If it is urged that, even though figure, colour, sound, etc., have no existence except in the act of perception, yet there must be things which exist apart from perception and which are similar to the percepts in our minds, then the view we have mentioned would answer, that a colour can be similar only to a colour, a figure to a figure. Our percepts can be similar only to our percepts and to nothing else. Even what we call a thing is nothing but a collection of percepts which are connected in a definite way. If I strip a table of its shape, extension, colour, etc.—in short, of all that is merely my percepts—then nothing remains over. If we follow this view to its logical conclusion, we are led to the assertion that the objects of my perceptions exist only through me, and only in as far as, and as long as, I perceive them. They disappear with my perceiving and have no meaning apart from it. Apart from my percepts I know of no objects and cannot know of any.
No objection can be made to this assertion as long as we take into account merely the general fact that the percept is determined in part by the organisation of the subject. The matter would be far otherwise if we were in a position to say what part exactly is played by our perceiving in the occurrence of a percept. We should know then what happens to a percept whilst it is being perceived, and we should also be able to determine what character it must possess before it comes to be perceived.
This leads us to turn our attention from the object of a perception to the subject of it. I am aware not only of other things but also of myself. The content of my perception of myself consists, in the first instance, in that I am something stable in contrast with the ever coming and going flux of percepts. The awareness of myself accompanies in my consciousness the awareness of all other percepts. When I am absorbed in the perception of a given object I am, for the time being, aware only of this object. Next I become aware also of myself. I am then conscious, not only of the object, but also of my Self as opposed to and observing the object. I do not merely see a tree, I know also that it is I who see it. I know, moreover, that some process takes place in me when I observe a tree. When the tree disappears from my field of vision, an after-effect of this process remains, viz., an image of the tree. This image has become associated with my Self during my perception. My Self has become enriched; to its content a new element has been added. This element I call my idea of the tree. I should never have occasion to talk of ideas, were I not aware of my own Self. Percepts would come and go; I should let them slip by. It is only because I am aware of my Self, and observe that with each perception the content of the Self is changed, that I am compelled to connect the perception of the object with the changes in the content of my Self, and to speak of having an idea.
That I have ideas is in the same sense matter of observation to me as that other objects have colour, sound, etc. I am now also able to distinguish these other objects, which stand over against me, by the name of the outer world, whereas the contents of my perception of my Self form my inner world. The failure to recognise the true relation between idea and object has led to the greatest misunderstandings in modern philosophy. The fact that I perceive a change in myself, that my Self undergoes a modification, has been thrust into the foreground, whilst the object which causes these modifications is altogether ignored. In consequence it has been said that we perceive, not objects, but only our ideas. I know, so it is said, nothing of the table in itself, which is the object of my perception, but only of the changes which occur within me when I perceive a table. This theory should not be confused with the Berkeleyan theory mentioned above. Berkeley maintains the subjective nature of my perceptual contents, but he does not say that I can know only my own ideas. He limits my knowledge to my ideas because, on his view, there are no objects other than ideas. What I perceive as a table no longer exists, according to Berkeley, when I cease to look at it. This is why Berkeley holds that our percepts are created directly by the omnipotence of God. I see a table because God causes this percept in me. For Berkeley, therefore, nothing is real except God and human spirits. What we call the “world” exists only in spirits. What the naïve man calls the outer world, or material nature, is for Berkeley non-existent. This theory is confronted by the now predominant Kantian view which limits our knowledge of the world to our ideas, not because of any conviction that nothing beyond these ideas exists, but because it holds that we are so organised that we can have knowledge only of the changes within our own selves, not of the things-in-themselves which are the causes of these changes. This view concludes from the fact that I know only my own ideas, not that there is no reality independent of them, but only that the subject cannot have direct knowledge of such reality. The mind can merely “through the medium of its subjective thoughts imagine it, conceive it, know it, or perhaps also fail to know it” (O. Liebmann, Zur Analysis der Wirklichkeit, p. 28). Kantians believe that their principles are absolutely certain, indeed immediately evident, without any proof. “The most fundamental principle which the philosopher must begin by grasping clearly, consists in the recognition that our knowledge, in the first instance, does not extend beyond our ideas. Our ideas are all that we immediately have and experience, and just because we have immediate experience of them the most radical doubt cannot rob us of this knowledge. On the other hand, the knowledge which transcends my ideas—taking ideas here in the widest possible sense, so as to include all psychical processes—is not proof against doubt. Hence, at the very beginning of all philosophy we must explicitly set down all knowledge which transcends ideas as open to doubt.” These are the opening sentences of Volkelt’s book on Kant’s Theory of Knowledge. What is here put forward as an immediate and self-evident truth is, in reality, the conclusion of a piece of argument which runs as follows. Naïve common sense believes that things, just as we perceive them, exist also outside our minds. Physics, Physiology, and Psychology, however, teach us that our percepts are dependent on our organisation, and that therefore we cannot know anything about external objects except what our organisation transmits to us. The objects which we perceive are thus modifications of our organisation, not things-in-themselves. This line of thought has, in fact, been characterised by Ed. von Hartmann as the one which leads necessarily to the conviction that we can have direct knowledge only of our own ideas (cp. his Grundproblem der Erkenntnistheorie, pp. 16–40). Because outside our organisms we find vibrations of particles and of air, which are perceived by us as sounds, it is concluded that what we call sound is nothing more than a subjective reaction of our organisms to these motions in the external world. Similarly, colour and heat are inferred to be merely modifications of our organisms. And, further, these two kinds of percepts are held to be the effects of processes in the external world which are utterly different from what we experience as heat or as colour. When these processes stimulate the nerves in the skin of my body, I perceive heat; when they stimulate the optical nerve I perceive light and colour. Light, colour, and heat, then, are the reactions of my sensory nerves to external stimuli. Similarly, the sense of touch reveals to me, not the objects of the outer world, but only states of my own body. The physicist holds that bodies are composed of infinitely small particles called molecules, and that these molecules are not in direct contact with one another, but have definite intervals between them. Between them, therefore, is empty space. Across this space they act on one another by attraction and repulsion. If I put my hand on a body, the molecules of my hand by no means touch those of the body directly, but there remains a certain distance between body and hand, and what I experience as the body’s resistance is nothing but the effect of the force of repulsion which its molecules exert on my hand. I am absolutely external to the body and experience only its effects on my organism.
The theory of the so-called Specific Nervous Energy, which has been advanced by J. Müller, supplements these speculations. It asserts that each sense has the peculiarity that it reacts to all external stimuli in only one definite way. If the optic nerve is stimulated, light sensations result, irrespective of whether the stimulation is due to what we call light, or to mechanical pressure, or an electrical current. On the other hand, the same external stimulus applied to different senses gives rise to different sensations. The conclusion from these facts seems to be, that our sense-organs can give us knowledge only of what occurs in themselves, but not of the external world. They determine our percepts, each according to its own nature.