The Doctrine of Perception or Knowledge of The Understanding

With all transcendental ideality the objective world retains empirical reality; the object is indeed not the thing in itself, but as an empirical object it is real. It is true that space is only in my head; but empirically my head is in space. The law of causality can certainly never enable us to get quit of idealism by building a bridge between things in themselves and our knowledge of them, and thus certifying the absolute reality of the world, which exhibits itself in consequence of its application; but this by no means does away with the causal relation of objects to each other, thus it does not abolish the causal relation which unquestionably exists between the body of each knowing person and all other material objects. But the law of causality binds together only phenomena, and does not lead beyond them. With that law we are and remain in the world of objects, i.e., the world of phenomena, or more properly the world of ideas. Yet the whole of such a world of experience is primarily conditioned by the knowledge of a subject in general as its necessary presupposition, and then by the special forms of our perception and apprehension, thus necessarily belongs to the merely phenomenal, and has no claim to pass for the world of things in themselves. Indeed the subject itself (so far as it is merely the knowing subject) belongs to the merely phenomenal, of which it constitutes the complementary half.

Without application of the law of causality, however, perception of an objective world could never be arrived at; for this perception is, as I have often explained, essentially matter of the intellect, and not merely of the senses. The senses afford us mere sensation, which is far from being perception. The part played by sensations of the senses in perception was distinguished by Locke under the name secondary qualities, which he rightly refused to ascribe to things in themselves. But Kant, carrying Locke’s method further, distinguished also, and refused to ascribe to things in themselves what belongs to the working up of this material (the sensations) by the brain. The result was, that in this was included all that Locke had left to things in themselves as primary qualities—extension, form, solidity, &c.—so that with Kant the thing in itself was reduced to a completely unknown quantity = x. With Locke accordingly the thing in itself is certainly without colour, sound, smell, taste, neither warm nor cold, neither soft nor hard, neither smooth nor rough; yet it has still extension and form, it is impenetrable, at rest or in motion, and has mass and number. With Kant, on the other hand, it has laid aside all these latter qualities also, because they are only possible by means of time, space, and causality, and these spring from an intellect (brain), just as colours, tones, smells, &c., originate in the nerves of the organs of sense. The thing in itself has with Kant become spaceless, unextended, and incorporeal. Thus what the mere senses bring to the perception, in which the objective world exists, stands to what is supplied by the functions of the brain (space, time, causality) as the mass of the nerves of sense stand to the mass of the brain, after subtracting that part of the latter which is further applied to thinking proper, i.e., to abstract ideas, and is therefore not possessed by the brutes. For as the nerves of the organs of sense impart to the phenomenal objects colour, sound, taste, smell, temperature, &c., so the brain imparts to them extension, form, impenetrability, the power of movement, &c., in short all that can only be presented in perception by means of time, space, and causality. How small is the share of the senses in perception, compared with that of the intellect, is also shown by a comparison of the nerve apparatus for receiving impressions with that for working them up. The mass of the nerves of sensation of the whole of the organs of sense is very small compared with that of the brain, even in the case of the brutes, whose brain, since they do not, properly speaking, i.e., in the abstract, think, is merely177 used for effecting perception, and yet when this is complete, thus in the case of mammals, has a very considerable mass, even after the cerebellum, whose function is the systematic guidance of movements, has been taken away. That excellent book by Thomas Reid, the “Inquiry into the Human Mind” (first edition, 1764; 6th edition, 1810), as a negative proof of the Kantian truths, affords us a very thorough conviction of the inadequacy of the senses to produce the objective perception of things, and also of the non-empirical origin of the perception of space and time. Reid refutes Locke’s doctrine that perception is a product of the senses, by a thorough and acute demonstration that the collective sensations of the senses do not bear the least resemblance to the world as known in perception, and especially that the five primary qualities of Locke (extension, form, solidity, movement, and number) absolutely could not be afforded us by any sensation of the senses. Accordingly he gives up the question as to the mode of origination and the source of perception as completely insoluble; and although altogether unacquainted with Kant, he gives us, as it were, according to the regula falsi, a thorough proof of the intellectual nature of perception (really first explained by me as a consequence of the Kantian doctrine), and also of the a priori source, discovered by Kant, of its constituent elements, space, time, and causality, from which those primary qualities of Locke first proceed, but by means of which they are easily constructed. Thomas Reid’s book is very instructive and well worth reading—ten times more so than all the philosophy together that has been written since Kant. Another indirect proof of the same doctrine, though in the way of error, is afforded by the French sensational philosophers, who, since Condillac trod in the footsteps of Locke, have laboured to show once for all that the whole of our perception and thinking can be referred to mere sensations (penser c’est sentir), which, after Locke’s example, they call idées simples, and through the mere coming together [187]178 The World As Will And Idea (Vol. 2 of 3) and comparison of which the whole objective world is supposed to build itself up in our heads. These gentlemen certainly have des idées bien simples. It is amusing to see how, lacking alike the profundity of the German and the honesty of the English philosopher, they turn the poor material of sensation this way and that way, and try to increase its importance, in order to construct out of it the deeply significant phenomena of the world of perception and thought. But the man constructed by them would necessarily be an Anencephalus, a Tête de crapaud, with only organs of sense and without a brain. To take only a couple of the better attempts of this sort out of a multitude of others, I may mention as examples Condorcet at the beginning of his book, “Des Progrès de l’Esprit Humain,” and Tourtual on Sight, in the second volume of the “Scriptures Ophthalmologici Minores,” edidit Justus Radius (1828). [188] The feeling of the insufficiency of a purely sensationalistic explanation of perception is in like manner shown in the assertion which was made shortly before the appearance of the Kantian philosophy, that we not only have ideas of things called forth by sensation, but apprehend the things themselves directly, although they lie outside us—which is certainly inconceivable. And this was not meant in some idealistic sense, but was said from the point of view of common realism. This assertion is well and pointedly put by the celebrated Euler in his “Letters to a German Princess,” vol. ii. p. 68. He says: “I therefore believe that the sensations (of the senses) contain something more than philosophers imagine. They are not merely empty perceptions of certain impressions made in the brain. They do not give the soul mere ideas of things, but actually place before it objects which exist outside it, although we cannot conceive how this really happens.” This opinion is explained by the following facts. Although, as I have fully proved, perception is brought about by application of the law of causality, of which we are conscious a priori, yet in sight the act of the understanding, by means of179 which we pass from the effect to the cause, by no means appears distinctly in consciousness; and therefore the sensation does not separate itself clearly from the idea which is constructed out of it, as the raw material, by the understanding. Still less can a distinction between object and idea, which in general does not exist, appear in consciousness; but we feel the things themselves quite directly, and indeed as lying outside us, although it is certain that what is immediate can only be the sensation, and this is confined to the sphere of the body enclosed by our skin. This can be explained from the fact that outside us is exclusively a spatial determination. But space itself is a form of our faculty of perception, i.e., a function of our brain. Therefore that externality to us to which we refer objects, on the occasion of sensations of sight, is itself really within our heads; for that is its whole sphere of activity. Much as in the theatre we see the mountains, the woods, and the sea, but yet everything is inside the house. From this it becomes intelligible that we perceive things in the relation of externality, and yet in every respect immediately, but have not within us an idea of the things which lie outside us, different from these things. For things are in space, and consequently also external to us only in so far as we perceive them. Therefore those things which to this extent we perceive directly, and not mere images of them, are themselves only our ideas, and as such exist only in our heads. Therefore we do not, as Euler says, directly perceive the things themselves which are external to us, but rather the things which are perceived by us as external to us are only our ideas, and consequently are apprehended by us immediately. The whole observation given above in Euler’s words, and which is quite correct, affords a fresh proof of Kant’s Transcendental Æsthetic, and of my theory of perception which is founded upon it, as also of idealism in general. The directness and unconsciousness referred to above, with which in perception we make the transition from the sensation to its cause, may be illustrated by an analogous procedure in the use of abstract ideas [189]180 The World As Will And Idea (Vol. 2 of 3) or thinking. When we read or hear we receive mere words, but we pass from these so immediately to the conceptions denoted by them, that it is as if we received the conceptions directly; for we are absolutely unconscious of the transition from the words to the conceptions. Therefore it sometimes happens that we do not know in what language it was that we read something yesterday which we now remember. Yet that such a transition always takes place becomes apparent if it is once omitted, that is, if in a fit of abstraction we read without thinking, and then become aware that we certainly have taken in all the words but no conceptions. Only when we pass from abstract conceptions to pictures of the imagination do we become conscious of the transposition we have made. [190] Further, it is really only in perception in the narrowest sense, that is, in sight, that in empirical apprehension the transition from the sensation to its cause takes place quite unconsciously. In every other kind of sense perception, on the contrary, the transition takes place with more or less distinct consciousness; therefore, in the case of apprehension through the four coarser senses, its reality is capable of being established as an immediate fact. Thus in the dark we feel a thing for a long time on all sides until from the different effects upon our hands we are able to construct its definite form as their cause. Further, if something feels smooth we sometimes reflect whether we may not have fat or oil upon our hands; and again, if something feels cold we ask ourselves whether it may not be that we have very warm hands. When we hear a sound we sometimes doubt whether it was really an affection of our sense of hearing from without or merely an inner affection of it; then whether it sounded near and weak or far off and strong, then from what direction it came, and finally whether it was the voice of a man or of a brute, or the sound of an instrument; thus we investigate the cause of each effect we experience. In the case of smell and taste uncertainty as to the objective nature of the cause of the effect felt is of the181 commonest occurrence, so distinctly are the two separated here. The fact that in sight the transition from the effect to the cause occurs quite unconsciously, and hence the illusion arises that this kind of perception is perfectly direct, and consists simply in the sensation alone without any operation of the understanding—this has its explanation partly in the great perfection of the organ of vision, and partly in the exclusively rectilineal action of light. On account of the latter circumstance the impression itself leads directly to the place of the cause, and since the eye is capable of perceiving with the greatest exactness and at a glance all the fine distinctions of light and shade, colour and outline, and also the data in accordance with which the understanding estimates distance, it thus happens that in the case of impressions of this sense the operation of the understanding takes place with such rapidity and certainty that we are just as little conscious of it as of spelling when we read. Hence arises the delusion that the sensation itself presents us directly with the objects. Yet it is just in sight that the operation of the understanding, consisting in the knowledge of the cause from the effect, is most significant. By means of it what is felt doubly, with two eyes, is perceived as single; by means of it the impression which strikes the retina upside down, in consequence of the crossing of the rays in the pupils, is put right by following back the cause of this in the same direction, or as we express ourselves, we see things upright although their image in the eye is reversed; and finally by means of the operation of the understanding magnitude and distance are estimated by us in direct perception from five different data, which are very clearly and beautifully described by Dr. Thomas Reid. I expounded all this, and also the proofs which irrefutably establish the intellectual nature of perception, as long ago as 1816, in my essay “On Sight and Colour” (second edition, 1854; third edition, 1870), and with important additions fifteen years later in the revised Latin version of it which is given under the title, “Theoria Colorum Physiologica Eademque Primaria,” in [191]182 [192] The World As Will And Idea (Vol. 2 of 3) the third volume of the “Scriptores Ophthalmologici Minores,” published by Justus Radius in 1830; yet most fully and thoroughly in the second (and third) edition of my essay “On the Principle of Sufficient Reason,” § 21. Therefore on this important subject I refer to these works, so as not to extend unduly the present exposition. On the other hand, an observation which trenches on the province of æsthetics may find its place here. It follows from the proved intellectual nature of perception that the sight of beautiful objects—for example, of a beautiful view—is also a phenomenon of the brain. Its purity and completeness, therefore, depends not merely on the object, but also upon the quality of the brain, its form and size, the fineness of its texture, and the stimulation of its activity by the strength of the pulse of the arteries which supply it. Accordingly the same view appears in different heads, even when the eyes are equally acute, as different as, for example, the first and last impressions of a copper plate that has been much used. This is the explanation of the difference of capacity for enjoying natural beauty, and consequently also for reproducing it, i.e., for occasioning a similar phenomenon of the brain by means of an entirely different kind of cause, the arrangement of colours on a canvas. The apparent immediacy of perception, depending on its entire intellectuality, by virtue of which, as Euler says, we apprehend the thing itself, and as external to us, finds an analogy in the way in which we feel the parts of our own bodies, especially when they suffer pain, which when we do feel them is generally the case. Just as we imagine that we perceive things where they are, while the perception really takes place in the brain, we believe that we feel the pain of a limb in the limb itself, while in reality it also is felt in the brain, to which it is conducted by the nerve of the affected part. Therefore, only the affections of those parts whose nerves go to the brain are felt, and not those of the parts whose nerves belong to the sympathetic system, unless it be that183 an unusually strong affection of these parts penetrates by some roundabout way to the brain, where yet for the most part it only makes itself known as a dull sense of discomfort, and always without definite determination of its locality. Hence, also, it is that we do not feel injuries to a limb whose nerve-trunk has been severed or ligatured. And hence, finally, the man who has lost a limb still sometimes feels pain in it, because the nerves which go to the brain are still there. Thus, in the two phenomena here compared, what goes on in the brain is apprehended as outside of it; in the case of perception, by means of the understanding, which extends its feelers into the outer world; in the case of the feeling of our limbs, by means of the nerves.