Physiology shows, further, that there can be no direct knowledge even of the effects which objects produce on our sense-organs. Through his study of the processes which occur in our own bodies, the physiologist finds that, even in the sense-organs, the effects of the external process are modified in the most diverse ways. We can see this most clearly in the case of eye and ear. Both are very complicated organs which modify the external stimulus considerably, before they conduct it to the corresponding nerve. From the peripheral end of the nerve the modified stimulus is then conducted to the brain. Here the central organs must in turn be stimulated. The conclusion is, therefore, drawn that the external process undergoes a series of transformations before it reaches consciousness. The brain processes are connected by so many intermediate links with the external stimuli, that any similarity between them is out of the question. What the brain ultimately transmits to the soul is neither external processes, nor processes in the sense-organs, but only such as occur in the brain. But even these are not apprehended immediately by the soul. What we finally have in consciousness are not brain processes at all, but sensations. My sensation of red has absolutely no similarity with the process which occurs in the brain when I sense red. The sensation, again, occurs as an effect in the mind, and the brain process is only its cause. This is why Hartmann (Grundproblem der Erkenntnistheorie, p. 37) says, “What the subject experiences is therefore only modifications of his own psychical states and nothing else.” However, when I have sensations, they are very far as yet from being grouped in those complexes which I perceive as “things.” Only single sensations can be transmitted to me by the brain. The sensations of hardness and softness are transmitted to me by the organ of touch, those of colour and light by the organ of sight. Yet all these are found united in one object. This unification must, therefore, be brought about by the soul itself; that is, the soul constructs things out of the separate sensations which the brain conveys to it. My brain conveys to me singly, and by widely different paths, the visual, tactual, and auditory sensations which the soul then combines into the idea of a trumpet. Thus, what is really the result of a process (i.e., the idea of a trumpet), is for my consciousness the primary datum. In this result nothing can any longer be found of what exists outside of me and originally stimulated my sense-organs. The external object is lost entirely on the way to the brain and through the brain to the soul.
It would be hard to find in the history of human speculation another edifice of thought which has been built up with greater ingenuity, and which yet, on closer analysis, collapses into nothing. Let us look a little closer at the way it has been constructed. The theory starts with what is given in naïve consciousness, i.e., with things as perceived. It proceeds to show that none of the qualities which we find in these things would exist for us, had we no sense-organs. No eye—no colour. Therefore, the colour is not, as yet, present in the stimulus which affects the eye. It arises first through the interaction of the eye and the object. The latter is, therefore, colourless. But neither is the colour in the eye, for in the eye there is only a chemical, or physical, process which is first conducted by the optic nerve to the brain, and there initiates another process. Even this is not yet the colour. That is only produced in the soul by means of the brain process. Even then it does not yet appear in consciousness, but is first referred by the soul to a body in the external world. There I finally perceive it, as a quality of this body. We have travelled in a complete circle. We are conscious of a coloured object. That is the starting-point. Here thought begins its construction. If I had no eye, the object would be, for me, colourless. I cannot, therefore, attribute the colour to the object. I must look for it elsewhere. I look for it, first, in the eye—in vain; in the nerve—in vain; in the brain—in vain once more; in the soul—here I find it indeed, but not attached to the object. I recover the coloured body only on returning to my starting-point. The circle is completed. The theory leads me to identify what the naïve man regards as existing outside of him, as really a product of my mind.
As long as one stops here everything seems to fit beautifully. But we must go over the argument once more from the beginning. Hitherto I have used, as my starting-point, the object, i.e., the external percept of which up to now, from my naïve standpoint, I had a totally wrong conception. I thought that the percept, just as I perceive it, had objective existence. But now I observe that it disappears with my act of perception, that it is only a modification of my mental state. Have I, then, any right at all to start from it in my arguments? Can I say of it that it acts on my soul? I must henceforth treat the table of which formerly I believed that it acted on me, and produced an idea of itself in me, as itself an idea. But from this it follows logically that my sense-organs, and the processes in them are also merely subjective. I have no right to talk of a real eye but only of my idea of an eye. Exactly the same is true of the nerve paths, and the brain processes, and even of the process in the soul itself, through which things are supposed to be constructed out of the chaos of diverse sensations. If assuming the truth of the first circle of argumentation, I run through the steps of my cognitive activity once more, the latter reveals itself as a tissue of ideas which, as such, cannot act on one another. I cannot say that my idea of the object acts on my idea of the eye, and that from this interaction results my idea of colour. But it is necessary that I should say this. For as soon as I see clearly that my sense-organs and their activity, my nerve- and soul-processes, can also be known to me only through perception, the argument which I have outlined reveals itself in its full absurdity. It is quite true that I can have no percept without the corresponding sense-organ. But just as little can I be aware of a sense-organ without perception. From the percept of a table I can pass to the eye which sees it, or the nerves in the skin which touches it, but what takes place in these I can, in turn, learn only from perception. And then I soon perceive that there is no trace of similarity between the process which takes place in the eye and the colour which I see. I cannot get rid of colour sensations by pointing to the process which takes place in the eye whilst I perceive a colour. No more can I re-discover the colour in the nerve- or brain-processes. I only add a new percept, localised within the organism, to the first percept which the naïve man localises outside of his organism. I only pass from one percept to another.
Moreover, there is a break in the whole argument. I can follow the processes in my organism up to those in my brain, even though my assumptions become more and more hypothetical as I approach the central processes of the brain. The method of external observation ceases with the process in my brain, more particularly with the process which I should observe, if I could treat the brain with the instruments and methods of Physics and Chemistry. The method of internal observation, or introspection, begins with the sensations, and includes the construction of things out of the material of sense-data. At the point of transition from brain process to sensation, there is a break in the sequence of observation.
The theory which I have here described, and which calls itself Critical Idealism, in contrast to the standpoint of naïve common sense which it calls Naïve Realism, makes the mistake of characterising one group of percepts as ideas, whilst taking another group in the very same sense as the Naïve Realism which it apparently refutes. It establishes the ideal character of percepts by accepting naïvely, as objectively valid facts, the percepts connected with one’s own body; and, in addition, it fails to see that it confuses two spheres of observation, between which it can find no connecting link.
Critical Idealism can refute Naïve Realism only by itself assuming, in naïve-realistic fashion, that one’s own organism has objective existence. As soon as the Idealist realises that the percepts connected with his own organism stand on exactly the same footing as those which Naïve Realism assumes to have objective existence, he can no longer use the former as a safe foundation for his theory. He would, to be consistent, have to regard his own organism also as a mere complex of ideas. But this removes the possibility of regarding the content of the perceptual world as a product of the mind’s organisation. One would have to assume that the idea “colour” was only a modification of the idea “eye.” So-called Critical Idealism can be established only by borrowing the assumptions of Naïve Realism. The apparent refutation of the latter is achieved only by uncritically accepting its own assumptions as valid in another sphere.
This much, then, is certain: Analysis within the world of percepts cannot establish Critical Idealism, and, consequently, cannot strip percepts of their objective character.
Still less is it legitimate to represent the principle that “the perceptual world is my idea” as self-evident and needing no proof. Schopenhauer begins his chief work, The World as Will and Idea, with the words: “The world is my idea—this is a truth which holds good for everything that lives and knows, though man alone can bring it into reflective and abstract consciousness. If he really does this, he has attained to philosophical wisdom. It then becomes clear and certain to him that what he knows is not a sun and an earth, but only an eye that sees a sun, a hand that feels an earth; that the world which surrounds him is there only in idea, i.e., only in relation to something else, the consciousness which is himself. If any truth can be asserted a priori, it is this: for it is the expression of the most general form of all possible and thinkable experience, a form which is more general than time, or space, or causality, for they all presuppose it …” (The World as Will and Idea, Book I, par. 1). This whole theory is wrecked by the fact, already mentioned above, that the eyes and the hand are just as much percepts as the sun and the earth. Using Schopenhauer’s vocabulary in his own sense, I might maintain against him that my eye which sees the sun, and my hand which feels the earth, are my ideas just like the sun and the earth themselves. That, put in this way, the whole theory cancels itself, is clear without further argument. For only my real eye and my real hand, but not my ideas “eye” and “hand,” could own the ideas “sun” and “earth” as modifications. Yet it is only in terms of these ideas that Critical Idealism has the right to speak.
Critical Idealism is totally unable to gain an insight unto the relation of percept to idea. It cannot make the separation, mentioned on p. 58, between what happens to the percept in the process of perception and what must be inherent in it prior to perception. We must therefore attempt this problem in another way.