KANT’S RELATION TO HUME AND TO LEIBNIZ

Kant’s manner of formulating his fundamental problem—How are synthetic a priori judgments possible?—may well seem to the modern reader to imply an unduly scholastic and extremely rationalistic method of approach. Kant’s reasons for adopting it have, unfortunately, been largely obscured, owing to the mistaken interpretation which has usually been given to certain of his personal utterances. They have been supposed to prove that the immediate occasion of the above formula was Hume’s discussion of the problem of causality in the Enquiry into the Human Understanding. Kant, it is argued, could not have been acquainted with Hume’s earlier and more elaborate Treatise on Human Nature, of which there was then no translation; and his references to Hume must therefore concern only the later work.

Vaihinger has done valuable service in disputing this reading of Kant’s autobiographical statements. Kant does not himself make direct mention of the Enquiry, and the passages in the Critique and in the Prolegomena[9] in which Hume’s teaching is under consideration seem rather to point to the wider argument of the Treatise. This is a matter of no small importance; for if Vaihinger’s view can be established, it will enable us to appreciate, in a manner otherwise impossible, how Kant should have come to regard the problem of a priori synthesis as being the most pressing question in the entire field of speculative philosophy.

The essential difference between the Treatise and the Enquiry, from the standpoint of their bearing upon Critical issues, lies in the wider scope and more radical character of the earlier work. The Enquiry discusses the problem of causality only in the form in which it emerges in particular causal judgments, i.e. as to our grounds for asserting that this or that effect is due to this or that cause. In the Treatise, Hume raises the broader question as to our right to postulate that events must always be causally determined. In other words, he there questions the validity of the universal causal principle, that whatever begins to exist must have a cause of existence; and he does so on the explicit ground that it demands as necessary the connecting of two concepts, that of an event and that of an antecedent cause, between which no connection of any kind can be detected by the mind. The principle, that is to say, is not self-evident; it is synthetic. The concept of an event and the concept of a cause are quite separate and distinct ideas. Events can be conceived without our requiring to think antecedent events upon which they are dependent. Nor is the principle capable of demonstration. For if it be objected that in questioning its validity we are committing ourselves to the impossible assertion that events arise out of nothing, such argument is only applicable if the principle be previously granted. If events do not require a cause, it is as little necessary to seek their source in a generation out of nothing as in anything positive. Similarly, when it is argued that as all the parts of time and space are uniform, there must be a cause determining an event to happen at one moment and in one place rather than at some other time or place, the principle is again assumed. 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. The principle, Hume concludes, is non-rational in character. It is an instrument useful for the organisation of experience; and for that reason nature has determined us to its formation and acceptance. Properly viewed, it expresses a merely instinctive belief, and is explicable only in the naturalistic manner of our other propensities, as necessary to the fulfilling of some practical need. “Nature has determined us to judge as well as to breathe and feel.”

From this naturalistic position Hume makes a no less vigorous attack upon the empirical philosophies which profess to establish general principles by inductive inference from the facts of experience. If the principles which lie at the basis of our experience are non-rational in character, the same must be true of our empirical judgments. They may correctly describe the uniformities that have hitherto occurred in the sequences of our sensations, and may express the natural expectations to which they spontaneously give rise; but they must never be regarded as capable of serving as a basis for inference. In eliminating a priori principles, and appealing exclusively to sense-experience, the empiricist removes all grounds of distinction between inductive inference and custom-bred expectation. And since from this standpoint the possibility of universal or abstract concepts—so Hume argues—must also be denied, deductive inference must likewise be eliminated from among the possible instruments at the disposal of the mind. So-called inference is never the source of our beliefs; it is our fundamental natural beliefs, as determined by the constitution of our nature in its reaction upon external influences, that generate those expectations which, however they may masquerade in logical costume, have as purely natural a source as our sensations and feelings. Such, briefly and dogmatically stated, is the sum and substance of Hume’s teaching.[10]

Now it was these considerations that, as it would seem, awakened Kant to the problem of a priori synthesis. He was, and to the very last remained, in entire agreement with Hume’s contention that the principle of causality is neither self-evident nor capable of logical demonstration, and he at once realised that what is true of this principle must also hold of all the other principles fundamental to science and philosophy. Kant further agreed that inductive inference from the data of experience is only possible upon the prior acceptance of rational principles independently established; and that we may not, therefore, look to experience for proof of their validity. Thus with the rejection of self-evidence as a feature of the a priori, and with the consequent admission of its synthetic character, Kant is compelled to acquiesce in the inevitableness of the dilemma which Hume propounds. Either Hume’s sceptical conclusions must be accepted, or we must be able to point to some criterion which is not subject to the defects of the rationalist and empirical methods of proof, and which is adequate to determine the validity or invalidity of general principles. Is there any such alternative? Such is Kant’s problem as expressed in the formula: How are synthetic a priori judgments possible?

It is a very remarkable historical fact that notwithstanding the clearness and cogency of Hume’s argument, and the appearance of such competent thinkers as Thomas Reid in Scotland, Lambert and Crusius in Germany, no less than thirty years should have elapsed before Hume found a single reader capable of appreciating the teaching of the Treatise at its true value.[11] Even Kant himself was not able from his reading of the Enquiry in 1756-1762 to realise the importance and bearing of the main problem.[12] Though in the Enquiry the wider issue regarding the general principle of causality is not raised, the bearing of Hume’s discussion, when interpreted in the light of Kant’s own teaching, is sufficiently clear; and accordingly we cannot be absolutely certain that it was not a re-reading of the Enquiry or a recalling of its argument[13] that suggested to Kant the central problem of his Critical philosophy. The probability, however, is rather that this awakening took place only indirectly through his becoming acquainted with the wider argument of the Treatise as revealed in James Beattie’s extremely crude and unsympathetic criticism of Hume’s philosophy.[14] Beattie had great natural ability, and considerable literary power. His prose writings have a lucidity, a crispness, and a felicity of illustration which go far to explain their widespread popularity in the latter half of the eighteenth century. Their literary quality is, however, more than counterbalanced by the absence of any genuine appreciation of the deeper, speculative implications and consequences of the problems discussed. And this being so, he is naturally at his worst in criticising Hume. In insisting, as he does, upon the absurd practical results[15] that would follow from the adoption of Hume’s sceptical conclusions, he is merely exploiting popular prejudice in the philosophical arena. That, however, may be forgiven him, if, as would seem to be the case, the quotations which he gives verbatim from Hume’s Treatise really first revealed to Kant the scope and innermost meaning of Hume’s analysis of the causal problem.

The evidence in support of this contention is entirely circumstantial. The German translation of Beattie’s Essay on the Nature and Immutability of Truth was published at Easter 1772, i.e. in the year in which Kant, in the process of his own independent development, came, as is shown by his famous letter to Herz,[16] to realise the mysterious, problematic character of a priori knowledge of the independently real. He was then, however, still entirely unconscious of the deeper problem which at once emerges upon recognition that a priori principles, quite apart from all question of their objective validity, are synthetic in form. We know that Kant was acquainted with Beattie’s work; for he twice refers to Beattie’s criticism of Hume.[17] What more probable than that he read the translation in the year of its publication, or at least at some time not very long subsequent to the date of the letter to Herz? The passages which Beattie quotes from the Treatise are exactly those that were necessary to reveal the full scope of Hume’s revolutionary teaching in respect to the general principle of causality. There seems, indeed, little doubt that this must have been the channel through which Hume’s influence chiefly acted. Thus at last, by a circuitous path, through the quotations of an adversary, Hume awakened philosophy from its dogmatic slumber,[18] and won for his argument that appreciation which despite its cogency it had for thirty years so vainly demanded.

Let us now turn our attention to the rationalist philosophy in which Kant was educated. Hume’s contention that experience cannot by itself justify any inductive inference, forms the natural bridge over which we can best pass to the contrasting standpoint of Leibniz. Hume and Leibniz find common ground in denouncing empiricism. Both agree in regarding it as the mongrel offspring of conflicting principles. If rationalism cannot hold its own, the alternative is not the finding of firm foothold in concrete experience, but only such consolation as a sceptical philosophy may afford.[19] The overthrow of rationalism means the destruction of metaphysics in every form. Even mathematics and the natural sciences will have to be viewed as fulfilling a practical end, not as satisfying a theoretical need. But though Leibniz’s criticism of empiricism is, in its main contention, identical with that of Hume, it is profoundly different both in its orientation and in the conclusions to which it leads. While Hume maintains that induction must be regarded as a non-rational process of merely instinctive anticipation, Leibniz argues to the self-legislative character of pure thought. Sense-experience reveals reality only in proportion as it embodies principles derived from the inherent character of thought itself. Experience conforms to a priori principles, and so can afford an adequate basis for scientific induction.

There is a passage in Hume’s Enquiry which may be employed to illustrate the boldly speculative character of Leibniz’s interpretation of the nature and function of human thought. “Nothing ... [seems] more unbounded than the thought of man, which not only escapes all human power and authority, but is not even restrained within the limits of nature and reality.... While the body is confined to one planet, along which it creeps with pain and difficulty, the thought can in an instant transport us into the most distant regions of the universe.... What never was seen, or heard of, may yet be conceived; nor is anything beyond the power of thought, except what implies an absolute contradiction.” This passage in which Hume means to depict a false belief, already sufficiently condemned by the absurdity of its claims, expresses for Leibniz the wonderful but literal truth. Thought is the revealer of an eternal unchanging reality, and its validity is in no way dependent upon its verification through sense. When Voltaire in his Ignorant Philosopher remarks that “it would be very singular that all nature, all the planets, should obey eternal laws, and that there should be a little animal, five feet high, who, in contempt of these laws, could act as he pleased, solely according to his caprice,”[20] he is forgetting that this same animal of five feet can contain the stellar universe in thought within himself, and has therefore a dignity which is not expressible in any such terms as his size may seem, for vulgar estimation, to imply. Man, though dependent upon the body and confined to one planet, has the sun and stars as the playthings of his mind. Though finite in his mortal conditions, he is divinely infinite in his powers.

Leibniz thus boldly challenges the sceptical view of the function of reason. Instead of limiting thought to the translating of sense-data into conceptual forms, he claims for it a creative power which enables it out of its own resources to discover for itself, not only the actual constitution of the material world, but also the immensely wider realm of possible entities. The real, he maintains, is only one of the many kingdoms which thought discovers for itself in the universe of truth. It is the most comprehensive and the most perfect, but still only one out of innumerable others which unfold themselves to the mind in pure thought. Truth is not the abstracting of the universal aspects in things, not a copy of reality, dependent upon it for meaning and significance. Truth is wider than reality, is logically prior to it, and instead of being dependent upon the actual, legislates for it. Leibniz thus starts from the possible, as discovered by pure thought, to determine in an a priori manner the nature of the real.

This Leibnizian view of thought may seem, at first sight, to be merely the re-emergence of the romantic, rationalistic ideal of Descartes and Malebranche. So to regard it would, however, be a serious injustice. It was held with full consciousness of its grounds and implications, and reality was metaphysically reinterpreted so as to afford it a genuine basis. There was nothing merely mystical and nothing undefined in its main tenets. Leibniz differs from Malebranche in being himself a profound mathematician, the co-discoverer with Newton of the differential calculus. He also differs from Descartes in possessing an absorbing interest in the purely logical aspects of the problem of method; and was therefore equipped in a supreme degree for determining in genuinely scientific fashion the philosophical significance and value of the mathematical disciplines.

Hume and Leibniz are thus the two protagonists that dwarf all others. They realised as neither Malebranche, Locke, nor Berkeley, neither Reid, Lambert, Crusius, nor Mendelssohn ever did, the really crucial issues which must ultimately decide between the competing possibilities. Each maintained, in the manner prescribed by his general philosophy, one of what then appeared to be the only two possible views of the function of thought. The alternatives were these: (a) Thought is merely a practical instrument for the convenient interpretation of our human experience; it has no objective or metaphysical validity of any kind; (b) Thought legislates universally; it reveals the wider universe of the eternally possible; and prior to all experience can determine the fundamental conditions to which that experience must conform. Or to interpret this opposition in logical terms: (a) The fundamental principles of experience are synthetic judgments in which no relation is discoverable between subject and predicate, and which for that reason can be justified neither a priori nor by experience; (b) all principles are analytic, and can therefore be justified by pure thought.

The problem of Kant’s Critique, broadly stated, consists in the examination and critical estimate of these two opposed views. There is no problem, scientific, moral, or religious, which is not vitally affected by the decision which of these alternatives we are to adopt, or what reconciliation of their conflicting claims we hope to achieve. Since Kant’s day, largely owing to the establishment of the evolution theory, this problem has become only the more pressing. The naturalistic, instrumental view of thought seems to be immensely reinforced by biological authority. Thought would seem to be reduced to the level of sense-affection, and to be an instrument developed through natural processes for the practical purposes of adaptation. Yet the counter-view has been no less powerfully strengthened by the victorious march of the mathematical sciences. They have advanced beyond the limits of Euclidean space, defining possibilities such as no experience reveals to us. The Leibnizian view has also been reinforced by the successes of physical science in determining what would seem to be the actual, objective character of the independently real. Kant was a rationalist by education, temperament, and conviction. Consequently his problem was to reconcile Leibniz’s view of the function of thought with Hume’s proof of the synthetic character of the causal principle. He strives to determine how much of Leibniz’s belief in the legislative power of pure reason can be retained after full justice has been done to Hume’s damaging criticisms. The fundamental principles upon which all experience and all knowledge ultimately rest are synthetic in nature: how is it possible that they should also be a priori? Such is the problem that was Kant’s troublous inheritance from his philosophical progenitors, Hume and Leibniz.[21]