A Commentary to Kant's 'Critique of Pure Reason'

by Norman Kemp Smith

The development of philosophy, prior to Kant, had rendered two problems especially prominent—the problem of sense-perception and the problem of judgment. The one raises the question of the interrelation of mind knowing and objects known; the other treats of the connection holding between subject and predicate in the various forms of judgment. The one enquires how it is possible to know reality; the other seeks to determine the criterion of truth. These two problems are, as Kant discovered, inseparable from one another; and the logical is the more fundamental of the two. Indeed it was Hume’s analysis of the judgment involved in the causal principle that enabled Kant to formulate his Critical solution of the problem of perception. In this Appendix I propose to follow these problems as they rise into view in the systems of Descartes and his successors.

Galileo’s revolutionary teaching regarding the nature of motion was the immediate occasion of Descartes’ restatement of the problem of perception. That teaching necessitated an entirely new view of the nature of matter, and consequently of the interrelation of mind and body. Questions never before seriously entertained now became pressing. The solutions had to be as novel as the situation which they were designed to meet.

These new problems arose in the following manner. According to the medieval view, motion may properly be conceived on the analogy of human activity. It comes into being, exhausts itself in exercise, and ceases to be. It is a fleeting activity; only its “material” and “formal” conditions have any permanence of existence. According to Galileo’s teaching, on the other hand, motion is as different from human activity as matter is from mind. It is ingenerable and indestructible. We know it only through the effect which in some incomprehensible fashion it produces in those bodies into which it enters, namely, their translation from one part of space to another. That this translatory motion is called by the same name as the power which generates it, doubtless in some degree accounts for the fact that our understanding of the one tends to conceal from us our entire ignorance of the other.[1760] We have only to reflect, however, in order to realise that motion is completely mysterious in its intrinsic dynamical nature. We cannot, for instance, profess to comprehend, even in the least degree, how motion, though incapable of existing apart from matter, should yet be sufficiently independent to be able to pass from one body to another.

Descartes, following out some of the chief consequences of this new teaching, concluded that matter is passive and inert, that it is distinguished neither by positive nor by negative properties from the space which it fills, and that it is to motion that all the articulated organisation of animate and inanimate nature is due. Descartes failed, indeed, to appreciate the dynamical character of motion, and by constantly speaking as if it were reducible to the translatory motion, in which it manifests itself, he represented it as known in all its essential features. None the less, the rôles previously assigned to matter and motion are, in Descartes’ system, completely reversed. Matter is subordinated to motion as the instrument to the agency by which it is directed and shaped. On the older view, material bodies had, through the possession of formative and vital forces, all manner of intrinsic powers. By the new view these composite and nondescript existences are resolved into two elements, all the properties of which can be quantitatively defined—into a matter which is uniform and homogeneous, and into motion whose sole effect is the translation of bodies in space. Matter is the passive and inert substance out of which motion, by its mere mechanical powers, can produce the whole range of material forms.

This revolutionary change in the physical standpoint involved restatement of the philosophical issues. But the resulting difficulties were found thoroughly baffling. Though Descartes and his successors were willing to adopt any hypothesis, however paradoxical, which the facts might seem to demand, their theories, however modified and restated, led only deeper into a hopeless impasse. The unsolved problems of the Cartesian systems formed the discouraging heritage to which Kant fell heir. If matter is always purely material, and motion is its sole organising power, there can be no real kinship between body and mind. The formative and vital forces, which in the Scholastic philosophy and in popular thought serve to maintain the appearance of continuity between matter and mind, can no longer be credited. Motion, which alone is left to mediate between the opposites, is purely mechanical, and (on Descartes’ view) is entirely lacking in inner or hidden powers. The animal body is exclusively material, and is therefore as incapable of feeling or consciousness as any machine made by human hands. The bodily senses are not ‘sensitive’; the brain cannot think. Mental experiences do, of course, accompany the brain-motions. But why a sensation should thus arise when a particular motion is caused in the brain, or how a mental resolution can be followed by a brain state, are questions to which no satisfactory answer can be given. The mental and the material, the spiritual and the mechanical, fall entirely apart.

The difficulties arising out of this incomprehensibility of the causal interrelations of mind and body are not, however, in themselves a valid argument against a dualistic interpretation of the real. The difficulties of accounting for the causal relation are, in essential respects, equally great even when the interaction is between homogeneous existences. The difficulties are due to the nature of causal action as such, not to the character of the bodies between which it holds. This, indeed, was clearly recognised by Descartes, and was insisted upon by his immediate successors. The transference of motion by impact is no less incomprehensible than the interaction of soul and body. If motion can exist only in matter, there is no possible method of conceiving how it can make the transition from one discrete portion of matter to another. Causal action is thus a problem which no philosophy can pretend to solve, and which every philosophy, whether monistic or dualistic, must recognise as transcending the scope of our present knowledge.

It is in another and more special form that Descartes’ dualism first reveals its fatal defects, namely, in its bearing upon the problem of sense-perception. Descartes can solve the problem of knowledge only by first postulating the doctrine of representative perception. That doctrine is rendered necessary by the dualism of mind and body. Objects can be known only mediately by means of their action upon the sense-organs, and through the sense-organs upon the brain. The resulting brain states are in themselves merely forms of motion. They lead, however, in a manner which Descartes never professes to explain,[1761] to the appearance of sensations in the mental field. Out of these sensations the mind then constructs mental images of the distant bodies; and it is these mental images alone which are directly apprehended. Material bodies are invisible and intangible; they are knowable only through their mental duplicates. Thus, according to the doctrine of representative perception, each mind is segregated in a world apart. It looks out upon a landscape which is as mental and as truly inward as are its feelings and desires. The apparently ultimate relation of mind knowing and object known is rendered complex and problematic through the distinction between mental objects and real things. Mental objects are in all cases images merely. They exist only so long as they are apprehended; and they are numerically and existentially distinct in each individual mind. Real things are not immediately perceived; they are hypothetically inferred. To ordinary consciousness the body which acts on the sense-organ is the object known; when reflective consciousness is philosophically enlightened, the object immediately known is recognised as a merely mental image, and the external object sinks to the level of an assumed cause.

The paradoxical character of this doctrine is accentuated by Galileo’s distinction between primary and secondary qualities.[1762] Those physical processes, which are entitled light and heat, bear no resemblance to the sensations through which they become known. The many-coloured world of ordinary consciousness is an illusory appearance which can exist only in the human mind. We must distinguish between the sensible world which, though purely mental, appears, through an unavoidable illusion, to be externally real, and that very different world of matter and motion which reveals its independent nature only to reflective thinking. In the latter world the rich variety of sensuous appearance can find no place. There remain only the quantitative, mechanical properties of extension, figure and motion; and even these have to be interpreted in the revolutionary fashion of physical science.

The doctrine of representative perception cannot, however, defend successfully the positions which it thus involves. It wavers in unstable equilibrium. The facts, physical and physiological, upon which it is based, are in conflict with the conclusions in which it results. This has been very clearly demonstrated by many writers in recent times.[1763] The conflict manifested itself in the period between Descartes and Kant only through the uneasy questionings of Locke and Berkeley. The problem, fundamental though it be, is almost completely ignored by Spinoza, Leibniz, and Wolff.

Stated in modern terms, the inherently contradictory character of the doctrine consists in its unavoidable alternation between the realist attitude to which it owes its origin, and the idealist conclusion in which it issues. Such oscillation is due to the twofold simultaneous relation in which it regards ideas as standing to the objects that they are supposed to represent. The function of sensations is cognitive; their origin is mechanical. As cognitive they stand to objects in a relation of inclusion; they reveal the objects, reduplicating them in image before the mind. Yet in their origin they are effects, mechanically generated by the action of material bodies upon the sense-organs and brain. As they are effects mechanically generated, there is no guarantee that they resemble their causes; and if we may argue from other forms of mechanical causation, there is little likelihood that they do. They stand to their first causes in a relation of exclusion, separated from them by a large number of varying intermediate processes. There is thus a conflict between the function of sensations and their origin. Their origin in the external objects is supposed to confer upon them a representative power; and yet the very nature of this origin invalidates any such claim.