All these systems involve the negation of two fundamental scientific principles. The first is that every change must be explained by reference to an antecedent change to which it bears a strict quantitative relation. The second is that no particular change can be referred to another change as its necessary antecedent unless it can be shown by experience that a precisely similar couple of changes are, in fact, always so connected. Let me illustrate these principles by an example. I leave a kettle full of cold water on the fire, and on returning after a sufficient interval of time I find the water boiling. Had I stayed by the fire and watched the process, my kettle would – a popular proverb to the contrary notwithstanding – have certainly boiled as soon, but also no sooner for being helped by my consciousness. The essential thing is that energy of combustion in the fire should be turned into energy of boiling in the water. Now, what is Berkeley's interpretation of the facts? Fire, kettle, water, and ebullition are what in his writings are called "ideas" —i. e., phenomena occasionally in my mind, but always in God's mind. And according to this view the necessary antecedent to the boiling of the water is not the fire's burning, but God's consciousness of its burning, his perception being the essence of the operation. But it is proved by experience that neither my perception nor anyone else's ever made a single drop of water boil. In other words, perception is not in this instance a vera causa. Why, then, should the perception of any other mind, however exalted, have that effect?
Nor is this all. How does Berkeley know that God exists? Because, he says, to exist is to be perceived, and therefore for the universe to exist implies a universal Percipient. But he got the idea of God from other men, who certainly did not come by it as a generalisation from their perceptions; they got it by generalising from their voluntary actions, which do produce the changes that perception cannot produce. It will be said that volitions and the feelings that prompt them exist only in consciousness. In whose consciousness? In that of a spirit. And what is spirit apart from sensation, thought, feeling, and volition? Simply one of those abstract ideas whose existence Berkeley himself denied.
Hume
The next step in the evolution of English thought was to consist in a return to Locke's method, involving a complete breach with seventeenth-century Platonism, and with the Continental metaphysics that it had inspired. This decisive movement was effected by one in whom German criticism has recognised the greatest of all British philosophers. David Hume (1711-1776) was born and bred at Edinburgh, which also seems to have been through life his favourite residence. But his great work, the Treatise on Human Nature, was written during a stay in France, between the ages of twenty-three and twenty-six. Thus his precocity was even greater than Berkeley's. Indeed, such maturity of thought so early reached is without a parallel in history. But Hume's style had not then acquired the perfection – the inimitable charm, Kant calls it – of his later writings; and, whether for this or for other reasons, the book, in his own words, "fell dead-born from the press." In middle life the office of librarian of the Advocates' Library at Edinburgh gave him access to the materials for his History of England, which proved a source of fame and profit. A profound historical scholar, J. S. Brewer, tells us that Hume "possessed in a pre-eminent degree some of the highest excellences of a historian." Other historians have treated their subjects philosophically; he furnishes the sole instance of a great speculative genius who has also produced a historical masterpiece of the first order. But morally it is a blot on his fame. It is sad that a philosopher should have deliberately perverted the truth, that one who has David Hume. performed priceless services to freedom of thought should have made himself the apologist of clericalising absolutism, and, still more, that a master of English played this part to some extent through hatred of the great English people engendered by disappointed literary ambition. It may be mentioned, however, as a possible extenuation that towards the middle of the eighteenth century the highest English ability had thrown itself, with few exceptions, on the Tory side. It must be mentioned also that in private life Hume's character was entirely admirable – cheerful, generous, and gentle, without a frailty and without a stain. His opinions were unpopular; but his life offered no handle for obloquy, although his studious retirement was more than once exchanged for the responsibilities of political office, and the freedom from pedantry so conspicuous in his writings bears witness to habits of well-bred social intercourse.
Hume's philosophy is best understood when we consider it as, in the first place, a criticism of Berkeley, just as Berkeley's had been a criticism of Locke. It will be remembered that the founder of subjective idealism discarded the notion of material substance as an "abstract idea," an unintelligible figment devoid of any sensuous or imaginative content. The only true substances are the subjects of what we call experience communicating through sensation with God, the infinite spirit whose eternal consciousness is reality itself. Hume applied the same tests to spiritual substance, and found that it equally disappeared under his introspective analysis. He begins by dividing the contents of consciousness into two classes, impressions and ideas – the second being copies of the first, and distinguished from them by their relative faintness. Now, from these perceptions (which he called thoughts) Descartes had passed by an immediate inference to the ego or self, which he affirms as the primary fact of consciousness, using it as a basis for sundry other conclusions. But Hume stops him at once, and will not grant the existence of the metaphysical self – that is, a simple and continued substance, as distinguished from particular states of consciousness. We are, he declares, "nothing but a bundle of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement." "There is properly no simplicity in it [the self] at one time, nor identity in different [times]; whatever natural propensity we may have to imagine that simplicity and identity." So much being assumed, Berkeley's whole argument for a new theology founded on subjective idealism is bound to collapse, as also is the argument for natural immortality derived from the supposed simplicity and identity of the thinking substance.
Modern critics have rightly insisted, as against Hume, that isolated perceptions without a self are abstractions not less unintelligible than a self without perceptions. But the metaphysical argument for human immortality has not benefited by this more concrete interpretation of epistemology; and probably Hume was really more interested in destroying this than in maintaining the sceptical paradox which does not recur in his later writings.
A word must be added about Hume's division of perceptions into impressions and ideas. The point left out of sight in this analysis is that impressions of sense habitually find their reflexes not in revived sensations, but in expressions, in motor reactions which, with human beings, mostly take the form of words uttered or thought. These, no doubt, are associated to some small extent with revived sensations; but they are more commonly grouped with other words, with movements of the limbs, and with actions on the material or human environment of the percipient. Such expressions are incomparably easier to revive in memory, imagination, or expectation than the impressions that originally excited them; and, indeed, it is in connection with them that such revivals of sensation as we actually experience take place. And it is probable that to this active side of our consciousness that we may trace those associative processes which Hume studies next in his analysis of human knowledge.
Putting aside principles of doubtful or secondary value, the relations between states of consciousness that first offer themselves to view are, according to Hume, Co-existence and Succession (united under the name of Contiguity), Resemblance, and Causation. It is with the account he gives of this last category that his name is inseparably associated, for from it all subsequent speculation has taken rise. Yet primarily he seems to have had no other object in view than to simplify the laws of knowledge by resolving one of them into a particular case of another, and thus reducing his three categories to two. The relation of cause and effect, he tells us, is no more than a certain relation between antecedent and consequent in time where the sequence is so habitual as to establish in our minds a custom of expecting the one whenever the other occurs. The sequence is not necessary, for one can think, without any self-contradiction, of a change which has not been preceded by another change; nor is it, like the truths of geometry, something that can be known à priori. Without experience no one could tell that bread will nourish a man and not nourish a lion, nor even predict how a billiard-ball will behave when another ball strikes it. Should it be objected that the à priori knowledge of a general principle need not involve an equal knowledge of nature's operations in particular cases, Hume would doubtless reply by saying that there is no abstract idea of causation apart from its concrete exemplifications.
It is possible to accept Hume's theory in principle without pledging oneself to all his incidental contentions. Causation, as a general law, may be known only by experience, whether we can or cannot think of it as a pure abstraction. And we may interpret it in terms of unconditional antecedence and consequence, while discarding his apparent assumption of an inscrutable connection between the two; a mysterious necessity for the production of the one by the other, for which it is felt that a reason exists, but for which our reason cannot account. It is inconceivable that our knowledge of any given sequence could be increased, except by the disclosure of intermediate sequences, making their continuity, in space and time, more absolute than we had before perceived, until the whole process has been resolved into a transference of momentum from one molecule to another – a change for which, according to Hume, no reason can be given. Nor, on his principles, would it help us to explain such transferences by bringing them under the law of the Conservation of Energy. For, although this would be a great triumph for science, his philosophy demands a reason why the quantity of energy should remain unalterable for ever.
It is a mistake, shared by Hume with his opponents, to suppose that the common sense of mankind ever saw more than invariable sequence in the relation of cause and effect, or ever interpolated a mysterious power between them. In the famous verse, "Let there be light, and there was light," it is the instantaneity of succession, not the interpolation of any exerted effort, that so impresses the imagination. And when Shakespeare wants to illustrate logical compulsion in conduct, his reference is to an instance of invariable succession: —
This above all, – to thine own self be true;
And it must follow, as the night the day,
Thou canst not then be false to any man.
Indeed, I think it will be found on examination that when we associate the idea of power, or of necessity, with causal sequences, it is not in connection with a case of causation here and now, but rather in reference to similar effects that may be expected from the same cause elsewhere or at another time. And that "custom," by which Hume seeks to explain our belief in the "power" of the cause to produce its effect as well as the "necessity" of the connection between them, rather acts negatively by eliminating all other antecedents as possible causes than positively by setting up a habit of thinking about a particular antecedent and consequent at the same time. And that is why a burnt child needs no repetition of the experiment to be convinced that contact with fire was the cause of its pain. The very novelty of the experiment was enough to eliminate any explanation other than that of contact with the flame.
The child, as it grows older, may learn to speak of the fire as having a power to burn. But that merely means, "if I touch it, it will burn me – or light paper if I hold the paper to it." Power, in fact, is incomplete causation, the presence of every condition but that one which, in Aristotelian phrase, turns potency into act. And it is in contradistinction to that idea of possibility that the idea of necessary connection comes in. When all the elements of the causal antecedent are combined the effect necessarily supervenes. Furthermore, the causal antecedent is thought of as necessary in contrast with the contingency of other antecedents whose connection with the effect is merely accidental. Finally, the idea of production has been quoted as vitally distinguishing true causation from invariable sequence. But various myths, of which the story of Œdipus is the best known, show that primitive folk regard day and night as alternately producing one another, just as Polonius quotes their sequence as a type of logical necessity.
Hume professed himself a Deist, but probably with no more seriousness than when he, or when Gibbon, called Christianity "our religion." At any rate, his philosophy destroys every argument for the existence of a Creator advanced in his own or in the preceding century. Nor need his particular theory of causation be invoked for the purpose. The most telling attack is on the argument from design. The apparent adaptation of means to ends in living organisms is quoted as evidence of their having been planned by a conscious intelligence. But, answers Hume, such an intelligence would itself exhibit marks of design, and so on for ever. Why not, then, stop at the animal organism as an ultimate fact? It was Shelley's unlucky demand for a solution of this difficulty that led to his expulsion from Oxford.
It has been shown how the new analysis of mind cut the ground from under Berkeley's theism, and from under the metaphysical argument for human immortality. By denying the substantiality of the ego it also confirmed the necessitarianism of Spinoza. Hume seemed to think he could abate the unpopularity of this doctrine by interpreting the constant motivation of human actions as a mere relation of antecedence and consequence. But the decisive point was that he assimilated sequences in conscious behaviour to the unconscious sequences in physical events. Thus, for the vulgar and the theologians, he remained what would now be called a materialist.
Kant
The English philosophy of experience and the Continental philosophy of à priori spiritualism, after their brief convergence in the metaphysics of Berkeley, parted company once more, the empirical tradition being henceforth represented, not only by Hume, but in a more or less anti-Christian and much more superficial form by Voltaire, Rousseau, and the French Encyclopædists; while the Leibnizian philosophy was systematised and taught in Germany by Wolf, and a dull but useful sort of modernised Aristotelianism was set up under the name of "common sense" by Thomas Reid (1710-1796) and his school in the Scottish Universities.
The extraordinary genius who was to re-combine the parted currents in a speculative movement of unexampled volume, velocity, and depth showed nothing of the precocity that had distinguished Berkeley and Hume. Immanuel Kant (1724-1804), the son of a saddler of Scottish extraction, was born at Königsberg in Prussia, where he spent his whole life, holding a chair at the University from 1770 to 1797. It is related that on the day of his death a small bright cloud was seen sailing alone across the clear blue sky, of such a remarkable appearance that a crowd assembled on the bridge to watch it. One of them, a common soldier, exclaimed, "That is Kant's soul going to heaven!" – a touching and beautiful tribute to the illustrious German, whose lofty, pure, and luminous spirit it was uniquely fitted to characterise.
Kant grew up among the Pietists, a school which played much the same part in Germany that the Methodists and the Evangelicals played in England; indeed, it was from them that John Wesley received his final inspiration. The Königsberg student came in time to discard their theology while retaining the stern Puritan morality with which it was wedded, and even, Rationalist as he became, some of their mystical religiosity. What drew him away to philosophy seems to have been first the study of classical philology and then physical science, especially as presented to him in Newton's works. And so the young man's first ambition, after settling down as a University teacher at Königsberg, was to extend the Newtonian method still further by explaining, on mechanical principles, the origin and constitution of that celestial system whose movements Newton had reduced to law, but whose beginning he had left unaccounted for except by – what was not science – the direct fiat of omnipotence.
Kant offered a brilliant solution of the problem in his Natural History of the Heavens (1755), a work embodying the celebrated nebular hypothesis rediscovered forty years later by Laplace. It has been well observed that great philosophers are mostly, if not always, what at Oxford and Cambridge would be called "double-firsts" – that is, apart from their philosophy, they have done first-class work in some special line of investigation, as Descartes by creating analytical geometry, Spinoza by applying Biblical criticisms to theology, Leibniz by discovering the differential calculus, Locke by his theory of constitutional government, Berkeley by his theory of vision, Hume by his contributions to history and political economy. Kant's cosmogony may have been premature and mistaken in its details; but his idea of the heavenly bodies as having originated from the condensation of diffused gaseous matter still holds its ground; and although the more general idea of natural evolution as opposed to supernatural creation is not modern but Greek, to have revived and reapplied it on so great a scale is a service of extraordinary merit.
The next great event in Kant's intellectual career is his rejection of Continental apriorism in metaphysics for the empiricism of the English school, especially as regards the idea of causation. For a few years (1762-1765) Kant accepts Hume's theory that there is nothing in any succession of events or in change generally to prove on grounds of pure reason that there must be more in it than a customary sequence. To believe that anything may happen without a cause does not involve a logical contradiction; and at that time he believed nothing to be known à priori except that the denial of which involves such a contradiction. But on reconsidering the basis of mathematical truth it seemed to him to be something other than the logical laws of Identity and Contradiction. When we say that seven and five are twelve we put something into the predicate that was not affirmed in the subject, and also when we say that a straight line is the shortest distance between two points. Yet the second proposition is as certain as the first, and both are certain in the highest degree, more certain than anything learned from experience, and needing no experience to confirm them.
So much being admitted, we have to recognise a fundamental division of judgments into two classes, analytic and synthetic. Judgments in which the predicate adds nothing to the subject are analytic. When we affirm all matter to be extended, that is an instance of the former, for here we are only making more explicit what was already contained in the notion of matter. On the other hand, when we affirm that all matter is heavy, that is an instance of the latter or synthetic class, for we can think of matter without thinking that it has weight. Furthermore, this is not only a synthetic judgment, but it is a synthetic judgment à posteriori; for the law of universal gravitation is known only by experience. But there are also synthetic judgments à priori; for, as we have just seen, the fundamental truths of arithmetic and geometry belong to this class, as do also by consequence all the propositions logically deduced from these – that is to say, the whole of mathematical science.
Up to this point Kant would have carried the whole Cartesian school, and, more generally, all the modern Platonists, along with him; while he would have given the English empiricists and their French disciples a rather hard nut to crack. For they would have had to choose between admitting that mathematics was a mass of identical propositions or explaining, in the face of Hume's criticism, what claims to absolute certainty its truths, any more than the Law of Causation, possess. Now, the great philosophical genius of Kant is shown by nothing more than by this, that he did not stop here. Recognising to the same extent as Locke and Hume that all knowledge comes from experience – at any rate, in the sense of not coming by supernatural communication, as Malebranche and Berkeley thought – he puts the famous question, How are synthetic judgments à priori possible? Or, as it might be paradoxically expressed, How come we to know with the most certainty the things that we have not been taught by experience? The answer is, that we know them by the most intimate experience of all – the underlying consciousness that we have made them what they are. Our minds are no mere passive recipients, in which a mass of sensations, poured in from some external source, are then arranged after an order equally originated from without; there is a principle of spontaneity in our own subjectivity by which the objective order of nature is created. What Kant calls the Matter of knowledge is given from without, the Form from within. And this process begins with the imposition of the two great fundamental Forms, Space and Time, on the raw material of sensation by our minds.
By space and time Kant does not mean the abstract ideas of coexistence and succession; nor does he call them, as some critics used incorrectly to suppose, forms of thought, but forms of intuition. We do not build them up with the help of muscular or other feelings, but are conscious of them in a way not admitting of any further analysis. The parts of space, no doubt, are coexistent, but they are also connected and continuous; more than this, positions in space do not admit of mutual substitution; the right hand and left hand glove are perfectly symmetrical, but the one cannot be superimposed on the other. Besides, all particular spaces are contained in universal space, not as particular conceptions are contained in a general conception, but as parts of that which extends to infinity, and where each has an individual place of its own, repeating all the characters of space in general except its illimitable extension. And the same is true of time, with this further distinction from abstract succession, that succession may be reversed; whereas the order of past, present, and future is irreversibly maintained.
The contemporary school of Reid in Scotland, and the subsequent Eclectic school of Victor Cousin in France, would agree with Kant in maintaining that sensuous experience will not account for our knowledge of space and time. But they would protest, in the name of common sense, against the reduction of these apparently fundamental elements to purely subjective forms. They would ask, with the German critic Trendelenburg, Why cannot space and time be known intuitively and yet really exist? Kant furnishes no direct answer to the question, but he has suggested one in another connection. Mathematical truth is concerned with spatial and temporal relations, and for that truth to be above suspicion and exception we must assume that the objects with which it deals are wholly within our grasp – that our knowledge of them is exhaustive. But there could be no such assurance on the supposition that, besides the space and time of our sensuous experience, another space and time existed independently of our consciousness as attributes of things in themselves – possibly differing in important respects from ours – as, for example, a finite, or a non-continuous, or a four-dimensional space, and a time with a circular instead of a progressive movement.
This easy assumption that reality accommodates itself to our intellectual convenience, instead of our being obliged to accommodate our theories of knowledge to reality, runs through and vitiates the whole of Kant's philosophy. But, taking the narrower ground of logical consistency, one hardly sees how his principles can hold together. We are told that the subjectivity of space and time is not presented as a plausible hypothesis, but as a certain and indubitable truth, for in no other way can mathematical certainty be explained. The claim is questionable, but let it be granted. Immediately a fresh difficulty starts up. What is the source of our certainty that space and time are subjective forms of intuition? If the answer is, because that assumption guarantees the certainty of mathematics, then Kant is reasoning in a circle. If he appeals – as in consistency he ought – to another order of subjectivity as the sanction of his first transcendental argument, such reasoning involves the regress to infinity.
Again, on Kant's theory, time is the form of intuition for the inner sense. So when we become conscious of mental events we know them only as phenomena; we remain ignorant of what mind is in itself. But before the publication in 1770 of Kant's inaugural dissertation on The Sensible and the Intelligible World every one, plain men and philosophers alike, believed that the consciousness of our successive thoughts and feelings was the very type of reality itself; and they held this belief with a higher degree of assurance than that given to the axioms of geometry. By what right, then, are we asked to give up the greater for the less, to surrender our self-assurance as a ransom for Euclid's Elements or even for Newton's Principia?
Once more, surely mathematics is concerned not with space and time as such, but with their artificial delimitations as points, lines, figures, numbers, moments, etc. And it may be granted that these are purely subjective in the sense of being imposed by our imagination (with the aid of sensible signs) on the external world. What if this subjectivity were the true source of that peculiar certainty belonging to synthetic judgments à priori? True, Kant counts in our judgments about the infinity and eternity of space and time with other accepted characteristics of theirs as intuitive certainties. But there are thinkers who find the negation of such properties not inconceivable, so that they cannot be adduced as evidence of a priority, still less of subjectivity.
Eleven years after the inaugural dissertation Kant published his most important contribution to philosophy, The Critique of Pure Reason (1781). Pure Reason means the faculty by which ideas are obtained independently of all experience, and the critic's object is to ascertain how far such ideas are valid. As a preliminary to that inquiry the question is also mooted, How is experience possible? It is answered by a critique of the understanding or faculty of conception; and as conception implies perception, this again is prefaced by a section in which Kant's theory of space and time is repeated and reinforced.
It will be remembered that what started the whole of the new criticism was Hume's sceptical analysis of Causation; and the central interest of The Critique of Pure Reason lies in the effort to reconstitute the causal law in the light of the new theory of knowledge; but so enormous is the mass of technicalities piled up for this purpose as largely to conceal it from view, and, on its disclosure, to give the idea of a gigantic machine set in motion to crack a nut. And the nut after all is not cracked; the shell slips from between the grappling surfaces long before they meet.
We have seen how Kant interpreted every judgment as a synthesis of subject and predicate. Now, whether the synthesis be à priori or à posteriori, a study of the forms of judgment as enumerated in the common logic shows that there are four, and only four, ways in which it can be effected. All judgments fall under the following classes: Quantity, Quality, Relation, and Modality – terms whose meaning will be presently explained. And each of these again is tripartite. We may say (i.) that one A is B, or that some A's are B, or that all A's are B; (ii.) that A is B, that A is not B, that not all A's are B; (iii.) that A is B, that A is B if C is D, that A is either B, C, or D; or (iv.) that A may be B, that A is B, or that A must be B. The reason why there are four and only four classes is that judgment has to do with the subject in reference to the predicate, which gives Quantity; with the predicate in reference to the subject, which gives Quality; with the connection between the two, which gives Relation; and with the synthesis between them in reference to our knowledge of it, which gives Modality.
Now, according to Kant, that there should be so many kinds of judgment and no more implies that our understanding contributes a formal element to the constitution of all knowledge, consisting of four combining principles, without which experience would be impossible. He calls these Categories, and they are enumerated in the following table: —
(i.) Quantity.
Unity, Plurality, Totality.
(ii.) Quality.
Reality, Negation, Limitation.
(iii.) Relation.
Substance and Accident; Cause and Effect; Action and Reaction (Reciprocity).
(iv.) Modality.
Possibility and Impossibility; Existence and Non-Existence; Necessity and Contingency.
A study of the Categories suggests some rather obvious criticisms on the Critical Philosophy itself. (i.) The first two terms in each triad evidently form an antithetical couple, of which the third term is the synthesis. Here we have the first germ of a disease by which the systems of Kant's successors were much more seriously infected. In the table it is shown by the intrusion of Limitation, a wholly superfluous adjunct to Reality and Negation; in the conversion of Reciprocity into a wholly fictitious synthesis of Substantiality with Causation; and in the complete absurdity of making Necessity a combination of Possibility with Existence. (ii.) Innate ideas, after they had been exploded by Locke, are reintroduced into philosophy by a sufficiently transparent piece of legerdemain. For assuming that the human intelligence possesses a power of organising and drilling the sensuous appearances which without its control would appear only as a disorderly mob, it by no means follows that they must thereby be referred to an extraphenomenal principle. But such a principle is plainly implied by the category of Substance. Used in a scholastic sense, it does not mean the sensuous attributes of a thing taken altogether, but something that underlies and supports them. And Kant himself seems to take his category in that significance. For he claims to deduce from it the law of the indestructibility of matter; as if I could not say snow is white without committing myself to the assertion that the ultimate particles of snow have existed and will exist for ever. (iii.) The substitution of Causation for logical sequence, as implicated in the hypothetical judgment of Relation, is perfectly scandalous; and still more scandalous is substitution of Reciprocity or Action and Reaction for Disjunction. The last points require to be examined a little more in detail.