Chapter LXIII: Appendix: B (1)

A MORE DETAILED STATEMENT OF KANT’S RELATIONS TO HIS PHILOSOPHICAL PREDECESSORS[1759]

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.

This irreconcilability of the subjectivist consequences of the doctrine with its realist basis was seized upon by Berkeley. To remove the contradiction, he denied the facts from which the doctrine had been developed. That is to say, starting from its results he disproved its premisses. Arguing from the physical and physiological conditions of perception Descartes had concluded that only sensations can be directly apprehended by the mind. Berkeley starts from this conclusion, and virtually adopts it as an assumption which cannot be questioned, and which does not call for proof. Since, he contends, we know only sensations, the assertion that they are due to material causes is mere hypothesis, and is one for which there may be no valid grounds. As Descartes himself had already suggested, there is a second possible method of interpreting the relevant facts. There may exist an all-powerful Being who produces the sensations in our minds from moment to moment; and provided that they are produced in the same order as now, the whole material world might be annihilated without our being in the least aware that so important an event had taken place. Since we can experience only sensations, any hypothesis which will account for the order of their happening is equally legitimate. The whole question becomes one of relative simplicity in the explanation given. The simpler analysis, other things being equal, must hold the field.

Berkeley reinforces this argument by pointing to the many embarrassing consequences to which Descartes’ dualism must lead. We postulate bodies in order to account for the origin of our sensations, and yet are unable to do so by their means. The dualistic theory creates more difficulties than it solves, without a single counter-advantage, save perhaps--so Berkeley argues--that it seems to harmonise better with the traditional prejudices of the philosophic consciousness.

If we grant Berkeley his premisses, the main lines of his argument are fairly cogent, however unconvincing may be his own positive views. The crux, however, of the Berkeleian idealism lies almost exclusively in the establishment of its fundamental assumption, that only ideas (_i.e._ images) can be known by the mind. This assumption Berkeley, almost without argument, takes over from his predecessors. It was currently accepted, and from it, therefore, he believed that he could safely argue. It rests, however, upon the assumption of facts which he himself questions. In rejecting the Cartesian dualism he casts down the ladder by which alone it is possible to climb into his position. For save through the facts of physics and physiology there seems to be no possible method of disproving the belief of ordinary consciousness, that in perception we apprehend independent material bodies. And until that belief can be shown to be false and ungrounded, the Berkeleian idealism is without support. It cannot establish the fundamental assumption upon which its entire argument proceeds. Thus, though Berkeley convincingly demonstrates the internal incoherence of the doctrine of representative perception--the inconsistency of its conclusions with the physical and physiological facts upon which alone it can be based--he cannot himself solve the problem in answer to which that doctrine was propounded. His services, like those of so many other reformers, were such as he did not himself foresee. In simplifying the problem, he prepared the way for the more sceptical treatment of its difficult issues by Hume.

At this point, in the philosophy of Hume, the problem of perception comes into the closest possible connection with the logical problem, referred to above. The question, how mind knowing is related to the objects known, is found to depend upon the question, how in certain crucial cases predicates may legitimately be referred to their subject. This logical problem arises in two forms, a narrower and a wider. The narrower issue concerns only the principle of causality. With what right do we assert that every event must have a cause? What is the ground which justifies us in thus predicating of events a causal character? Obviously, this logical question is fundamental, and must be answered before we can hope to solve the more special problem, as to our right to interpret sensations as effects of material bodies. Hume was the first to emphasise the vital interconnection of these two lines of enquiry.

The wider issue is the generating problem of Kant’s _Critique_: How in a judgment can a predicate be asserted of a subject in which it is not already involved? In other words, what is it that in such a case justifies us in connecting the predicate with the subject? Though this problem was never directly raised by any pre-Kantian thinker, not even by Hume, it is absolutely vital to all the pre-Kantian systems. Thus Descartes’ philosophy is based upon a distinction, nowhere explicitly drawn but everywhere silently assumed, between abstract and fruitful ideas. The former contain just so much content and no more; this content may be explicitly unfolded in a series of judgments, but no addition is thereby made to our knowledge. The latter, on the other hand, are endowed with an extraordinary power of inner growth. To the attentive mind they disclose a marvellous variety of inner meaning. The chief problem of scientific method consists, according to Descartes, in the discovery of these fruitful ideas, and in the separation of them from the irrelevant accompaniments which prevent them from unfolding their inner content. Once they are discovered, the steady progress of knowledge is assured. They are the springs of knowledge, and from them we have only to follow down the widening river of truth.

Descartes professed to give a complete list of the possible fruitful ideas. They are, he claimed, better known than any other concepts. They lie at the basis of all experience, and no one can possibly be ignorant of them; though, owing to their simplicity and omnipresence, their philosophical importance has been overlooked. When, however, Descartes proceeded to classify them, he found that while such ideas as space, triangle, number, motion, contain an inexhaustible content that is progressively unfolded in the mathematical sciences, those ideas, on the other hand, through which we conceive mental existences,--the notions of mind, thought, self--do not by any means prove fruitful upon attentive enquiry. As Malebranche later insisted, we can define mind only in negative terms; its whole meaning is determined through its opposition to the space-world, which alone is truly known. Though it is the function of mind to know, it cannot know itself. And when we remove from our list of ideas those which are not really fruitful, we find that only mathematical concepts remain.[1764] They alone have this apparently miraculous property of inexhaustibly developing before the mind. Scientific knowledge is limited to the material world; and even there, the limits of our mathematical insight are the limits of our knowledge.

Malebranche believed no less thoroughly than Descartes in the asserted power and fruitfulness of mathematical concepts. Under the influence of this belief, he developed, as so many other thinkers from Plato onwards have done, a highly mystical theory of scientific knowledge. It is a revelation of eternal truth, and yet is acquired by inner reflection, not laboriously built up by external observation. It comes by searching of the mind, not by exploration of the outer world. But Malebranche was not content, like Descartes, merely to accept this type of knowledge. He proceeded to account for it in metaphysical terms. The fruitfulness of mathematical ideas is due, he claimed, to the fundamental concept of extension in which they all share. This idea, representing, as it does, an infinite existence, is too great to be contained within the finite mind. Through it the mind is widened to the apprehension of something beyond itself; we know it through consciousness of its archetype in the mind of God. It is the one point at which consciousness transcends its subjective limits. Its fruitfulness is due to, and is the manifestation of, this divine source. The reason why we are condemned to remain ignorant of everything beyond the sphere of quantity is that extension alone holds this unique position. It is the only fruitful idea which the mind possesses, and other concepts, such as triangle, circle, or number, are fruitful only in proportion as they share in it. We can acquire no genuine knowledge even of the nature of the self. Being ignorant of mind, we cannot comprehend the self which is one of its modes. It is as if we sought to comprehend the nature of a triangle, in the absence of any conception of space. Were we in possession of the archetypal idea of mind, we should not only be able to deduce from it those various feelings and emotions which we have already experienced, and those sensations of the secondary qualities which we falsely ascribe to the influence of external objects, but we should also be able to discover by pure contemplation innumerable other emotions and qualities, which entirely transcend our present powers. And all of these would then be experienced in their ideal nature, and not, as now, merely through feeble and confused feeling. If mathematicians destroy their bodily health through absorption in the progressive clarification of the mysteries of space, what might not happen if the archetypal idea of mind were revealed to us? Could we attend to the preservation of a body which would incessantly distract us from the infinite and overwhelming experiences of our divine destiny?

This romantic conception of the possibilities of rational science reveals more clearly than any other Cartesian doctrine the real bearing and perverse character of the rationalistic preconceptions which underlie the Cartesian systems. The Cartesians would fain make rational science, conceived on the analogy of the mathematical disciplines, coextensive with the entire realm of the real. This grotesque enterprise is conceived as abstractly possible even by so cautious a thinker as John Locke. His reason for condemning the physical sciences as logically imperfect is that they fail to conform to this rationalistic ideal. Hence those sentences which sound so strangely in the mouth of Locke, the sensationalist.

“It is the contemplation of our abstract ideas that alone is able
to afford us general knowledge.”[1765] “The true method of
advancing knowledge is by considering our abstract ideas.”[1766]
”[Did we know the real essence of gold] it would be no more
necessary that gold should exist, and that we should make
experiments upon it, than it is necessary for the knowing of the
properties of a triangle, that a triangle should exist in any
matter: the idea in our minds would serve for the one as well as
for the other.”[1767] “In the knowledge of bodies, we must be
content to glean what we can from particular experiments, since we
cannot, from a discovery of their real essences, grasp at a time
whole sheaves, and in bundles comprehend the nature and the
properties of whole species together.”[4]

Locke’s empirical doctrine of knowledge is thus based upon a rationalistic theory of the real. It is not, he holds, the constitution of reality, but the _de facto_ limitations of our human faculties which make empirical induction the only practicable mode of discovery in natural science. Indeed, Locke gives more extreme expression than even Descartes does, to the mystically conceived mathematical method. Being ignorant of mathematics, and not over well-informed even in the physical sciences, Locke was not checked by any too close acquaintance with the real character and necessary limits of this method; and he accordingly makes statements in that unqualified fashion which seldom fails to betray the writer who is expounding views which he has not developed for himself by first-hand study of the relevant facts.

But though the unique character of mathematical knowledge thus forced itself upon the attention of all the Cartesian thinkers, and in the above manner led even the most level-headed of Descartes’ successors to dream strange dreams, no real attempt was made (save in the neglected writings of Leibniz) to examine, in a sober spirit, the grounds and conditions of its possibility. In the English School, Locke’s eulogy of abstract ideas served only to drive his immediate successors to an opposite extreme. Both Berkeley and Hume attempted to explain away, in an impossible manner, those fundamental differences, which, beyond all questioning, profoundly differentiate mathematical from empirical judgments.[1768] It is not surprising that Kant, who had no direct acquaintance with Hume’s _Treatise_, should have asserted that had Hume realised the bearing of his main teaching upon the theory of mathematical science, he would have hesitated to draw his sceptical conclusions. Such, however, was not the case. Hume’s theory of mathematical reasoning undoubtedly forms the least satisfactory part of his philosophy. He did, however, perceive the general bearing of his central teaching. It was in large degree his ignorance of the mathematical disciplines that concealed from him the thorough unsatisfactoriness of his general position, and which prevented him from formulating the logical problem in its full scope--the problem, namely, how judgments which make additions to our previous knowledge, and yet do not rest upon mere sensation, are possible. He treated it only as it presents itself in those judgments which involve the concept of causality.[1769] But this analysis of causal judgments awoke Kant from his dogmatic slumber, and so ultimately led to the raising of the logical problem in its widest form:--how synthetic _a priori_ judgments, whether mathematical, physical, or metaphysical, are possible.

Hume discussed the causal problem both in regard to the general principle of causality and in its bearing upon our particular judgments of causal relation. The problems concerned in these two discussions are essentially distinct. The first involves immensely wider issues, and so far as can be judged from the existing circumstantial evidence,[1770] it was this first discussion, not as has been so often assumed by Kant’s commentators the second and more limited problem, which exercised so profound an influence upon Kant at the turning-point of his speculations. In stating it, it will be best to take Hume’s own words.

“To begin with the first question concerning the necessity of a
cause: ‘Tis a general maxim in philosophy, that _whatever begins to
exist_, must have a cause of existence. This is commonly taken for
granted in all reasonings, without any proof given or demanded.
‘Tis supposed to be founded on intuition, and to be one of those
maxims, which though they may be deny’d with the lips, ‘tis
impossible for men in their hearts really to doubt of. But if we
examine this maxim by the idea of knowledge above explain’d we
shall discover in it no mark of any such intuitive certainty; but
on the contrary shall find, that ‘tis of a nature quite foreign to
that species of conviction.”[1771]

The principle that every event must have a cause, is neither intuitively nor demonstratively certain. So far from there existing a _necessary_ connection between the idea of an event as something happening in time and the idea of a cause, no connection of any kind is discoverable by us. We can conceive an object to be non-existent at this moment, and existent the next, without requiring to conjoin with it the altogether different idea of a productive source.

This had been implicitly recognised by those few philosophers who had attempted to give demonstrations of the principle. By so doing, however, they only reinforce Hume’s contention that it possesses no rational basis. When Hobbes argues that as all the points of time and place in which we can suppose an object to begin to exist, are in themselves equal, there must be some cause determining an event to happen at one moment rather than at another, he is assuming the very principle which he professes to prove. There is no greater difficulty in supposing the time and place to be fixed without a cause, than in supposing the existence to be so determined. If the denial of a cause is not intuitively absurd in the one case, it cannot be so in the other. If the first demands a proof, so likewise must the second. Similarly with the arguments advanced by Locke and Clarke. Locke argues that if anything is produced without a cause, it is produced by nothing, and that that is impossible, since nothing can never be a cause any more than it can be something, or equal to two right angles. Clarke’s contention that if anything were without a cause, it would produce _itself_, _i.e._ exist before it existed, is of the same character. These arguments assume the only point which is in question.

“When we exclude all causes we really do exclude them, and neither
suppose nothing nor the object itself to be the causes of the
existence, and consequently can draw no argument from the absurdity
of these suppositions to prove the absurdity of that
exclusion.”[1772]

The remaining argument, that every effect must have a cause, since this is implied in the very idea of an effect, is “still more frivolous.”

“Every effect necessarily presupposes a cause; effect being a
relative term, of which cause is the correlative. But this does not
prove that every being must be preceded by a cause; no more than it
follows, because every husband must have a wife, that therefore
every man must be married.”[1773]

The far-reaching conclusion, that the principle of causality has no possible rational basis, Hume extends and reinforces through his other doctrines, viz. that synthetic reason[1774] is merely generalised belief, and that belief is in all cases due to the ultimate instincts and propensities which _de facto_ constitute our human nature. The synthetic principles which lie at the basis of our experience are non-rational in character. Each is due to a ‘blind and powerful instinct,’ which, demanding no evidence, and ignoring theoretical inconsistency for the sake of practical convenience, necessitates belief.

“Nature by an absolute and uncontrollable necessity has determined
us to judge as well as to breathe and feel.”[1775] “All these
operations are a species of natural instincts, which no reasoning
or process of the thought and understanding is able either to
produce or to prevent.”[1776]

Reason is “nothing but a wonderful and unintelligible instinct in our souls.”[1777] It justifies itself by its practical uses, but can afford no standard to which objective reality must conform.

It is from this point of view that Hume states his answer to the problem of perception. Our natural belief in the permanence and identity of objects, as expressed through the principle of substance and attribute, leads us to interpret the objects of sense-perception as independent realities. We interpret our subjective sensations as being qualities of independent substances. Our other natural belief, in the dynamical interdependence of events, as expressed through the principle of causality, leads, however, to the opposite conclusion, that the known objects are merely mental. For by it we are constrained to interpret sensations, not as objective qualities, but only as subjective effects, expressive of the reactions of our psycho-physical organism. The Cartesian problems owe their origin to the mistaken attempt to harmonise, in a theoretical fashion, these two conflicting principles. The conflict is inevitable and the antinomy is insoluble, so long as the two principles are regarded as objectively valid. The only satisfactory solution comes through recognition that reason is unable to account, save in reference to practical ends, even for its own inevitable demands. The principle of substance and attribute and the principle of causality co-operate in rendering possible such organisation of our sense-experience as is required for practical life. But when we carry this organisation further than practical life itself demands, the two principles at once conflict.

Kant shows no interest in this constructive part of Hume’s philosophy; and must, indeed, have been almost entirely ignorant of it, since it finds only very imperfect expression in the _Enquiry_, and is ignored in Beattie’s _Nature of Truth_. Accordingly, Kant does not regard Hume as offering a positive explanation of knowledge, but rather as representing the point of view of thoroughgoing scepticism. But even had he been acquainted at first hand with Hume’s _Treatise_, he would undoubtedly have felt little sympathy with Hume’s naturalistic view of the function of reason. His training in the mathematical sciences would have enabled him to detect the inadequacy of Hume’s treatment of mathematical knowledge, and his strong moral convictions would have led him to rebel against the naturalistic assumptions which underlie Hume’s entire position. The Berkeley-Hume comedy is thus repeated with reversed rôles. Just as Berkeley’s anti-materialistic philosophy was mainly influential as a step towards the naturalism of Hume, and as such still survives in the philosophies of John Stuart Mill, Herbert Spencer, Huxley, Mach and Karl Pearson, so in turn Hume’s anti-metaphysical theory of knowledge was destined to be one of the chief contributory sources of the German speculative movement.

We may now turn to Hume’s treatment of the narrower problem--that of justifying our _particular_ causal judgments. Hume’s attitude towards this question is predetermined by the more fundamental argument, above stated, which precedes it in the _Treatise_, but which is entirely omitted from the corresponding chapters of the _Enquiry_. As the general principle of causality is of an irrational character, the same must be true of those particular judgments which are based upon it. Much of Hume’s argument on this question is, indeed, merely a restatement of what had already been pointed out by his predecessors. There is no necessary connection discoverable between _any_ cause and its effect. This is especially evident as regards the connection between brain states and mental experiences. No explanation can be given why a motion in the brain should produce sensations in the mind, or why a mental resolution should produce movements in the body. Such sequences may be empirically verified; they cannot be rationally understood. That this likewise holds, though in less obvious fashion, of the causal interrelations of material bodies, had been emphasised by Geulincx, Malebranche, Locke, and Berkeley. The fact that one billiard ball should communicate motion to another by impact is, when examined, found to be no less incomprehensible than the interaction of mind and body. Hume, in the following passage, is only reinforcing this admitted fact, in terms of his own philosophy.

“We fancy that were we brought on a sudden into this world we could
at first have inferred that one billiard ball would communicate
motion to another upon impulse; and that we needed not to have
waited for the event, in order to pronounce with certainty upon it.
Such is the influence of custom, that, where it is strongest, it
not only covers our natural ignorance, but even conceals itself,
and seems not to take place merely because it is found in the
highest degree.”[1778]

Nor are we conscious of any causal power within the self. When Berkeley claims that mind has the faculty of producing images at will, he is really ascribing to it creative agency. And such creation, as Malebranche had already pointed out, is not even conceivable.

“I deny that my will produces in me my ideas, for I cannot even
conceive how it could produce them, since my will, not being able
to act or will without knowledge, presupposes my ideas and does not
make them.”[1779] “Is there not here,” Hume asks, “either in a
spiritual or material substance, or both, some secret mechanism or
structure of parts, upon which the effect depends, and which, being
entirely unknown to us, renders the power or energy of the will
equally unknown and incomprehensible?”[1780]

But the fact that Hume thus restates conclusions already emphasised by his predecessors will not justify us in contending (as certain historians of philosophy seem inclined to do) that in his treatment of the causal problem he failed to make any important advance upon the teaching of the Occasionalists. Hume was the first to perceive the essential falsity of the Cartesian, rationalistic view of the causal nexus. For Descartes, an effect is that which can be deduced with logical necessity from the concept of its cause. The Occasionalists similarly argued that because natural events can never be deduced from one another they must in all cases be due to supernatural agency; like Descartes, they one and all failed to comprehend that since by an effect we mean that which follows _in time_ upon its cause, and since, therefore, the principle of causality is the law of _change_, the nature of causality cannot be expressed in logical terms. Hume was the first to appreciate the significance of this fundamental fact; and an entirely new set of problems at once came into view. If causal connection is not, as previous thinkers had believed, logical in character, if it does not signify logical dependence of the so-called effect upon its cause, its true connotation must lie elsewhere; and until this has been traced to its hidden source, any attempted solution of metaphysical problems is certain to involve many false assumptions. The answer that is given to the problem of the origin and content of the causal concept must determine our interpretation alike of sense-experience and of pure thought.

The problem presents on examination, however, a most paradoxical aspect. As Hume has already shown, every effect is an event distinct from its cause, and there is never any connection, beyond that of mere sequence, discoverable between them. We observe only sequence; we assert necessary connection. What, then, is in our minds when this latter assertion is made? And how, if the notion of necessitated connection cannot be gained through observation of the external events, is it acquired by us? Hume again propounds a naturalistic solution. Causation, _i.e._ _necessitated_ sequence in time, is not in any sense a conception; it is not a comprehended relation between events, but a misunderstood feeling in our minds. We cannot form any, even the most remote, conception of how one event can produce another. Neither imagination nor pure thought, however freely they may act, are capable of inventing any such notion. But nature, by the manner in which it has constituted our minds, deludes us into the belief that we are in actual possession of this idea. The repeated sequence of events, in fixed order, generates in us the feeling of a tendency to pass from the perception or idea of the one to the idea of the other. This feeling, thus generated by custom, and often in somewhat confused fashion combined with the feeling of ‘animal nisus,’ which is experienced in bodily effort, is mistaken by the mind for a definite concept of force, causality, necessary connection. As mere feeling it can afford no insight into the relation holding between events, and as merely subjective can justify no inference in regard to that relation. The terms force, causality, necessitated sequence in time, have a practical value, as names for our instinctive, natural expectations; but when employed as instruments for the _theoretical_ interpretation of experience, they lead us off on a false trail.

This is one of the fundamental points upon which Hume reveals a deeper speculative insight than either Malebranche, Geulincx, or Locke. Though these latter insist upon our ignorance of the relation holding between events, they still assume that causation and natural necessity are concepts which have a quite intelligible meaning; and in consequence they fail to draw the all-important conclusion, that the general principle of causality has neither intuitive nor demonstrative validity. For that is the revolutionary outcome of Hume’s analysis of the notion of necessitated connection. The principle of causality is a synthetic judgment in which no connection is discoverable between its subject and its predicate. That is the reason why it is neither self-evident nor capable of being established upon more ultimate grounds.

As has already been stated, the wider problem concerning the _principle_ of causality is developed only in the _Treatise_; the problem regarding the _concept_ of causality is discussed both in the _Treatise_ and in the _Enquiry_. An appreciation of the wider problem is required, however, in order to set this second problem in its true light, for it is only through its connection with the wider issue that Hume’s reduction of the concept of causality to a merely instinctive, non-rational expectation acquires its full significance. Hume’s analysis then amounts, as Kant was the first to realise, to an attack upon the objective validity of all constructive thinking. Not only rationalism, but even such metaphysics as may claim to base its conclusions upon the teaching of experience, is thereby rendered altogether impossible. The issue is crucial, and must be honestly faced, before metaphysical conclusions, no matter what their specific character may be, whether _a priori_ or empirical, can legitimately be drawn. If we may not assert that an event must have some cause, even the right to enquire for a cause must first be justified. And if so fundamental a principle as that of causality is not self-evident, are there any principles which can make this claim?

The account which we have so far given of Hume’s argument covers only that part of it which is directed against the rationalist position, and which was therefore so influential in turning Kant on to the line of his Critical speculations. But Hume attacked with equal vigour the empiricist standpoint; and as this aspect of his teaching, constituting as it did an integral part of Kant’s own philosophy, must undoubtedly have helped to confirm Kant in his early rationalist convictions, we may profitably dwell upon it at some length. In opposition to the empiricists, Hume argues that experience is incapable of justifying any inference in regard to matters of fact. It cannot serve as a basis from which we can inductively extend our knowledge of facts beyond what the senses and memory reveal. Inductive inference, when so employed, necessarily involves a _petitio principii_; we assume the very point we profess to have proved.

The argument by which Hume establishes this important contention is as follows. All inductive reasoning from experience presupposes the validity of belief in causal connection. For when we have no knowledge of causes, we have no justification for asserting the continuance of uniformities. Now it has been shown that we have no experience of any necessary relation between so-called causes and their effects. The most that experience can supply are sequences which repeat themselves. In regarding the sequences as causal, and so as universally constant, we make an assertion for which experience gives no support, and to which no amount of repeated experience, recalled in memory, can add one jot of real evidence. To argue that because the sequences have remained constant in a great number of repeated experiences, they are therefore more likely to remain constant, is to assume that constancy in the past is a ground for inferring it in the future; and that is the very point which demands proof. In drawing the conclusion we virtually assume that there is a necessary connection, _i.e._ an absolutely constant relation, between events. But since no _single_ experience of causal sequence affords ground for inferring that the sequence will continue in the future, no number of repeated experiences, recalled in memory, can contribute to the strengthening of the inference. It is meaningless to talk even of likelihood or probability. The fact that the sun has without a single known exception arisen each day in the past does not (if we accept the argument disproving all knowledge of necessary connection) constitute _proof_ that it will rise to-morrow.

“None but a fool or a madman will be unaffected in his expectations
or natural beliefs by this constancy, but he is no philosopher who
accepts this as in the nature of evidence.”[1781]

Since, for all that we know to the contrary, bodies may change their nature and mode of action at any moment, it is vain to pretend that we are scientifically assured of the future because of the past.

“My practice, you say, refutes my doubts.[1782] But you mistake the
purport of my question. As an agent, I am quite satisfied in the
point; but as a philosopher, who has some share of curiosity, I
will not say scepticism, I want to learn the foundation of this
inference. No reading, no enquiry has yet been able to remove my
difficulty or give me satisfaction in a matter of such importance.
Can I do better than propose the difficulty to the public, even
though, perhaps, I have small hopes of obtaining a solution? We
shall at least, by this means, be sensible of our ignorance, if we
do not augment our knowledge.”[1783]

Kant was the first, after thirty years, to take up this challenge. Experience is no source of evidence until the causal postulate has been _independently_ proved. Only if the principle of causality can be established prior to all specific experience, only if we can predetermine experience as necessarily conforming to it, are empirical arguments valid at all. Hume’s enquiry thus directly leads to the later, no less than to the earlier form of Kant’s epoch-making question.[1784] In its earlier formulation it referred only to _a priori_ judgments; in its wider application it was found to arise with equal cogency in connection with empirical judgments. And as thus extended, it generated the problem: How is sense-experience, regarded as a form of _knowledge_, possible at all?[1785] By showing that the principle of causality has neither intuitive nor demonstrative validity, Hume cuts the ground from under the rationalists; by showing that sense-experience cannot by itself yield conclusions which are objectively valid, he at the same time destroys the empiricist position. In this latter contention Kant stands in complete agreement with Hume. That the sensuously given is incapable of grounding even probable inferences, is a fundamental presupposition (never discussed, but always explicitly assumed) of the Critical philosophy. It was by challenging the sufficiency of Hume’s other line of argument, that which is directed against the rationalists, that Kant discovered a way of escape from the sceptical dilemma. The conditions of experience can be proved by a transcendental method, which, though _a priori_ in character, does not lie open to Hume’s sceptical objections. Each single experience involves rational principles, and consequently even a single empirical observation may suffice to justify an inductive inference. Experience conforms to the demands of pure _a priori_ thought; and can legitimately be construed in accordance with them.

We may now pass to the philosophy in which Kant was educated. It gave to his thinking that rationalist trend, to which, in spite of all counter-influences, he never ceased to remain true.[1786] It also contributed to his philosophy several of its constructive principles. Only two rationalist systems need be considered, those of Leibniz and of Wolff. Kant, by his own admission,[1787] had been baffled in his attempts (probably not very persevering) to master Spinoza’s philosophy. It was with Wolff’s system that he was most familiar; but both directly and indirectly, both in his early years and in the ’seventies, the incomparably deeper teaching of Leibniz must have exercised upon him a profoundly formative influence. In defining the points of agreement and of difference between Hume and Leibniz,[1788] we have already outlined Leibniz’s general view of the nature and powers of pure thought, and may therefore at once proceed to the relevant detail of his main tenets.

Upon two fundamental points Leibniz stands in opposition to Spinoza. He seeks to maintain the reality of the contingent or accidental. These terms are indeed, as he conceives them, synonymous with the actual. Necessity rules only in the sphere of the possible. Contingency or freedom is the differentiating characteristic of the real. This point of view is bound up with his second contention, namely, that the real is a kingdom of ends. It is through divine choice of the best among the possible worlds that the actual present order has arisen. There are thus two principles which determine the real: the principle of contradiction which legislates with absolute universality, and the principle of the best, or, otherwise formulated, of sufficient reason, which differentiates reality from truth, limiting thought, in order that, without violating logic, it may freely satisfy the moral needs. Leibniz thus vindicates against Spinoza the reality of freedom and the existence of ends.

Though Leibniz agrees with Spinoza that the philosophically perfect method would be to start from an adequate concept of the Divine Being, and to deduce from His attributes the whole nature of finite reality, he regards our concept of God as being too imperfect to allow of such procedure. We are compelled to resort to experience, and by analysis to search out the various concepts which it involves. By the study of these concepts and their interrelations, we determine, in obedience to the law of contradiction, the nature of the possible. The real, in contradistinction from the possible, involves, however, the notion of ends. The existence of these ends can never be determined by logical, but only by moral considerations. The chief problem of philosophical method is, therefore, to discover the exact relation in which the logical and the teleological, the necessary and the contingent, stand to one another.

The absence of contradiction is in itself a sufficient guarantee of possibility, _i.e._ even of the possibility of real existence. How very far Leibniz is willing to go on this line is shown by his acceptance of the ontological argument. The whole weight of his system rests, indeed, upon this proof. The notion of God is, he maintains, the sole concept which can determine itself in a purely logical manner not only as possible but also as real. If we are to avoid violating the principle of contradiction, the _Ens perfectissimum_ must be regarded as possessing the perfection of real existence. And since God is perfect in moral as in all other attributes, His actions must be in conformity with moral demands. In creating the natural order God must therefore have chosen that combination of possibilities which constitutes the best of all possible worlds. By means of this conceptual bridge we are enabled to pass by pure _a priori_ thinking from the logically possible to the factually real.

Pure logical thinking is thus an instrument whereby ultimate reality can be defined in a valid manner. _Pure thought is speculative and metaphysical in its very essence._ It uncovers to us what no experience can reveal, the wider universe which exists eternally in the mind of God. Every concept (whether mathematical, dynamical, or moral), provided only that it is not self-contradictory, is an eternal essence, with the intrinsic nature of which even God must reckon in the creation of things. When, therefore, we are determining the unchanging nature of the eternally possible, there is no necessary reference to Divine existence. The purely logical criterion suffices as a test of truth. Every judgment which is made in regard to such concepts must express only what their content involves. All such judgments must be analytic in order to be true.

When, however, we proceed from the possible to the real, that is to say, from the necessary to the contingent, the logical test is no longer sufficient; and only by appeal to the second principle, that of sufficient reason, can judgments about reality be logically justified. Whether or not the principle of sufficient reason is deducible, as Wolff sought to maintain, from the principle of contradiction, is a point of quite secondary importance. That is a question which does not deserve the emphasis which has been laid upon it. What is chiefly important is that for Leibniz, as for Wolff, both principles are principles of analysis. The principle of sufficient reason is not an instrument for determining necessary relations between independent substances. The sufficient ground of a valid predicate must in all cases be found in the concept of the subject to which it is referred. The difference between the two principles lies elsewhere, namely, in the character of the connection established between subject and predicate. In the one case the denial of the proposition involves a direct self-contradiction. In the other the opposite of the judgment is perfectly conceivable; our reason for asserting it is a moral (employing the term in the eighteenth-century sense), not a logical ground. The subject is so constituted, that in the choice of ends, in pursuit of the good, it must by its very nature so behave. The principle of sufficient reason, which represents in our finite knowledge the divine principle of the best, compels us to recognise the predicate as involved in the subject--as involved through a ground which inclines without necessitating. Often the analysis cannot be carried sufficiently far to enable us thus to transform a judgment empirically given into one which is adequately grounded. None the less, in recognising it as true, we postulate that the predicate is related to the subject in this way. There are not for Leibniz two methods of establishing truth, sense-perception to reveal contingent fact, and general reasoning to establish necessary truth. A proposition can be accepted as true only in so far as we can at least _postulate_, through absence of contradiction and through sufficient reason, its analytic character. It must express some form of identity. The proposition, Caesar crossed the Rubicon, is given us as historical fact. The more complete our knowledge of Caesar and of his time, the further we can carry the analysis; and that analysis if completely executed would displace the merely factual validity of the judgment by insight into its metaphysical truth. Thus experience, with its assertions of the here and now about particulars inexhaustibly concrete, sets to rational science an inexhaustible task. We can proceed in our analysis indefinitely, pushing out the frontiers of thought further and further into the empirical realm. Only by the Divine Mind can the task be completed, and all things seen as ordered in complete obedience to the two principles of thought.

Leibniz, in propounding this view, develops a genuinely original conception of the relation holding between appearance and reality. Only monads, that is, spiritual beings, exist. Apart from the representative activity of the monads there are no such existences as space and time, as matter and motion. The mathematical and physical sciences, in their present forms, therefore, cannot be interpreted as revealing absolute existences. But, if ideally developed, they would emancipate themselves from mechanical and sensuous notions; and would consist of a body of truths, which, as thus perfected, would be discovered to constitute the very being of thought. Pure thought or reason consists in the apprehension of such truths. To discover and to prove them thought does not require to issue out beyond itself. It creates this conceptual world in the very act of apprehending it; and as this realm of truth thus expresses the necessary character of all thought, whether divine or human, it is universal and unchanging. Each mind apprehends the same eternal truth; but owing to imperfection each finite being apprehends it with some degree of obscurity and confusion, fragmentarily, in terms of sense, and so falls prey to the illusion that the self stands in mechanical relations to a spatial and temporal world of matter and motion.