David Hume (1711–76) is one of the most important among philosophers, because he developed to its logical conclusion the empirical philosophy of Locke and Berkeley, and by making it self-consistent made it incredible. He represents, in a certain sense, a dead end: in his direction, it is impossible to go further. To refute him has been, ever since he wrote, a favourite pastime among metaphysicians. For my part, I find none of their refutations convincing; nevertheless, I cannot but hope that something less sceptical than Hume's system may be discoverable.
His chief philosophical work, the Treatise of Human Nature, was written while he was living in France during the years 1734 to 1737. The first two volumes were published in 1739, the third in 1740. He was a very young man, not yet in his thirties; he was not well known, and his conclusions were such as almost all schools would find unwelcome. He hoped for vehement attacks, which he would meet with brilliant retorts. Instead, no one noticed the book; as he says himself, 'it fell dead-born from the press'. 'But,' he adds, 'being naturally of a cheerful and sanguine temper, I very soon recovered from the blow.' He devoted himself to the writing of essays, of which he produced the first volume in 1741. In 1744 he made an unsuccessful attempt to obtain a professorship at Edinburgh; having failed in this, he became first tutor to a lunatic and then secretary to a general. Fortified by these credentials, he ventured again into philosophy. He shortened the Treatise by leaving out the best parts and most of the reasons for his conclusions; the result was the Inquiry into Human Understanding, for a long time much better known than the Treatise. It was this book that awakened Kant from his 'dogmatic slumbers'; he does not appear to have known the Treatise.
He wrote also Dialogues Concerning Natural Religion, which he kept unpublished during his lifetime. By his direction, they were published posthumously in 1779. His Essay on Miracles, which became famous, maintains that there can never be adequate historical evidence for such events.
His History of England, published in 1755 and following years, devoted itself to proving the superiority of Tories to Whigs and of Scotchmen to Englishmen; he did not consider history worthy of philosophic detachment. He visited Paris in 1763, and was made much of by the philosophes. Unfortunately, he formed a friendship with Rousseau, and had a famous quarrel with him. Hume behaved with admirable forbearance, but Rousseau, who suffered from persecution mania, insisted upon a violent breach.
Hume has described his own character in a self-obituary, or 'funeral oration', as he calls it: 'I was a man of mild dispositions, of command of temper, of an open, social and cheerful humour, capable of attachment, but little susceptible of enmity, and of great moderation in all my passions. Even my love of literary fame, my ruling passion, never soured my temper, notwithstanding my frequent disappointments.' All this is borne out by everything that is known of him.
Hume's Treatise of Human Nature is divided into three books, dealing respectively with the understanding, the passions, and morals. What is important and novel in his doctrines is in the first book, to which I shall confine myself.
He begins with the distinction between 'impressions' and 'ideas'. These are two kinds of perceptions, of which impressions are those that have more force and violence. 'By ideas I mean the faint images of these in thinking and reasoning.' Ideas, at least when simple, are like impressions, but fainter. 'Every simple idea has a simple impression, which resembles it; and every simple impression a correspondent idea.' 'All our simple ideas in their first appearance are derived from simple impressions, which are correspondent to them, and which they exactly represent.' Complex ideas, on the other hand, need not resemble impressions. We can imagine a winged horse without having ever seen one, but the constituents of this complex idea are all derived from impressions. The proof that impressions come first is derived from experience; for example, a man born blind has no ideas of colours. Among ideas, those that retain a considerable degree of the vivacity of the original impressions belong to memory, the others to imagination.
There is a section (Book I, part i, sec. vii) 'Of Abstract Ideas', which opens with a paragraph of emphatic agreement with Berkeley's doctrine that 'all general ideas are nothing but particular ones, annexed to a certain term, which gives them a more extensive significance, and makes them recall upon occasion other individuals, which are similar to them.' He contends that, when we have an idea of a man, it has all the particularity that the impression of a man has. 'The mind cannot form any notion of quantity or quality without forming a precise notion of degrees of each.' 'Abstract ideas are in themselves individual, however they may become general in their representation.' This theory, which is a modern form of nominalism, has two defects, one logical, the other psychological. To begin with the logical objection: 'When we have found a resemblance among several objects,' Hume says, 'we apply the same name to all of them.' Every nominalist would agree. But in fact a common name, such as 'cat', is just as unreal as the universal CAT is. The nominalist solution of the problem of universals thus fails through being insufficiently drastic in the application of its own principles; it mistakenly applies these principles only to 'things', and not also to words.
The psychological objection is more serious, at least in connection with Hume. The whole theory of ideas as copies of impressions, as he sets it forth, suffers from ignoring vagueness. When, for example, I have seen a flower of a certain colour, and I afterwards call up an image of it, the image is lacking in precision, in this sense, that there are several closely similar shades of colour of which it might be an image, or 'idea', in Hume's terminology. It is not true that 'the mind cannot form any notion of quantity or quality without forming a precise notion of degrees of each.' Suppose you have seen a man whose height is six feet one inch. You retain an image of him, but it probably would fit a man half an inch taller or shorter. Vagueness is different from generality, but has some of the same characteristics. By not noticing it, Hume runs into unnecessary difficulties, for instance, as to the possibility of imagining a shade of colour you have never seen, which is intermediate between two closely similar shades that you have seen. If these two are sufficiently similar, any image you can form will be equally applicable to both of them and to the intermediate shade. When Hume says that ideas are derived from impressions which they exactly represent he goes beyond what is psychologically true.
Hume banished the conception of substance from psychology, as Berkeley had banished it from physics. There is, he says, no impression of self, and therefore no idea of self (Book I, part iv, sec. vi). 'For my part, when I enter most intimately into what I call myself, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I never catch myself at any time without a perception, and never can observe anything but the perception.' There may, he ironically concedes, be some philosophers who can perceive their selves; 'but setting aside some metaphysicians of this kind, I may venture to affirm of the rest of mankind, that they are nothing but a bundle or collection of different perceptions, which succeed each other with inconceivable rapidity, and are in a perpetual flux and movement'.
This repudiation of the idea of the Self is of great importance. Let us see exactly what it maintains, and how far it is valid. To begin with, the Self, if there is such a thing, is never perceived, and therefore we can have no idea of it. If this argument is to be accepted, it must be carefully stated. No man perceives his own brain, yet, in an important sense, he has an 'idea' of it. Such 'ideas', which are inferences from perceptions, are not among the logically basic stock of ideas; they are complex and descriptive—this must be the case if Hume is right in his principle that all simple ideas are derived from impressions, and if this principle is rejected, we are forced back on 'innate' ideas. Using modern terminology, we may say: Ideas of unperceived things or occurrences can always be defined in terms of perceived things or occurrences, and therefore, by substituting the definition for the term defined, we can always state that we know empirically without introducing any unperceived things or occurrences. As regards our present problem, all psychological knowledge can be stated without introducing the 'Self'. Further, the 'Self', as defined can be nothing but a bundle of perceptions, not a new simple 'thing'. In this I think that any thoroughgoing empiricist must agree with Hume.
It does not follow that there is no simple Self; it only follows that we cannot know whether there is or not, and that the Self, except as a 'bundle' of perceptions, cannot enter into any part of our knowledge. This conclusion is important in metaphysics, as getting rid of the last surviving use of 'substance'. It is important in theology, as abolishing all supposed knowledge of the 'soul'. It is important in the analysis of knowledge, since it shows that the category of subject and object is not fundamental. In this matter of the ego Hume made an important advance on Berkeley.
The most important part of the whole Treatise is the section called 'Of Knowledge and Probability'. Hume does not mean by 'probability' the sort of knowledge contained in the mathematical theory of probability, such as that the chance of throwing double sixes with two dice is one thirty-sixth. This knowledge is not itself probable in any special sense; it has as much certainty as knowledge can have. What Hume is concerned with is uncertain knowledge, such as is obtained from empirical data by inferences that are not demonstrative. This includes all our knowledge as to the future, and as to unobserved portions of the past and present. In fact, it includes everything except, on the one hand, direct observation, and, on the other, logic and mathematics. The analysis of such 'probable' knowledge led Hume to certain sceptical conclusions, which are equally difficult to refute and to accept. The result was a challenge to philosophers, which, in my opinion, has still not been adequately met.
Hume begins by distinguishing seven kinds of philosophical relation: resemblance, identity, relations of time and place, proportion in quantity or number, degrees in any quality, contrariety, and causation. These, he says, may be divided into two kinds: those that depend only on the ideas, and those that can be changed without any change in the ideas. Of the first kind are resemblance, contrariety, degrees in quality, and proportions in quantity or number. But spatio-temporal and causal relations are of the second kind. Only relations of the first kind give certain knowledge; our knowledge concerning the others is only probable. Algebra and arithmetic are the only sciences in which we can carry on a long chain of reasoning without losing certainty. Geometry is not so certain as algebra and arithmetic, because we cannot be sure of the truth of its axioms. It is a mistake to suppose, as many philosophers do, that the ideas of mathematics 'must be comprehended by a pure and intellectual view, of which the superior faculties of the soul are alone capable'. The falsehood of this view is evident, says Hume, as soon as we remember that 'all our ideas are copied from our impressions'.
The three relations that depend not only on ideas are identity, spatio-temporal relations, and causation. In the first two, the mind does not go beyond what is immediately present to the senses. (Spatio-temporal relations, Hume holds, can be perceived, and can form parts of impressions.) Causation alone enables us to infer some thing or occurrence from some other thing or occurrence: "'Tis only causation, which produces such a connexion, as to give us assurance from the existence or action of one object, that 'twas followed or preceded by any other existence or action.'
A difficulty arises from Hume's contention that there is no such thing as an impression of a causal relation. We can perceive, by mere observation of A and B, that A is above B, or to the right of B, but not that A causes B. In the past, the relation of causation had been more or less assimilated to that of ground and consequent in logic, but this, Hume rightly perceived, was a mistake.
In the Cartesian philosophy, as in that of the Scholastics, the connection of cause and effect was supposed to be necessary, as logical connections are necessary. The first really serious challenge to this view came from Hume, with whom the modern philosophy of causation begins. He, in common with almost all philosophers down to and including Bergson, supposes the law to state that there are propositions of the form 'A causes B', where A and B are classes of events; the fact that such laws do not occur in any well-developed science appears to be unknown to philosophers. But much of what they have said can be translated so as to be applicable to causal laws such as do occur; we may, therefore, ignore this point for the present.
Hume begins by observing that the power by which one subject produces another is not discoverable from the ideas of the two objects, and that we can therefore only know cause and effect from experience, not from reasoning or reflection. The statement 'what begins must have a cause', he says, is not one that has intuitive certainty, like the statements of logic. As he puts it: 'There is no object, which implies the existence of any other if we consider these objects in themselves, and never look beyond the ideas which we form of them.' Hume argues from this that it must be experience that gives knowledge of cause and effect, but that it cannot be merely the experience of the two events A and B which are in a causal relation to each other. It must be experience, because the connection is not logical; and it cannot be merely the experience of the particular events A and B, since we can discover nothing in A by itself which should lead to produce B. The experience required, he says, is that of the constant conjunction of events of the kind A with events of the kind B. He points out that when, in experience, two objects are constantly conjoined, we do in fact infer one from the other. (When he says 'infer', he means that perceiving the one makes us expect the other; he does not mean a formal or explicit inference.) 'Perhaps, the necessary connection depends on the inference,' not vice versa. That is to say, the sight of A causes the expectation of B, and so leads us to believe that there is a necessary connection between A and B. The inference is not determined by reason, since that would require us to assume the uniformity of nature, which itself is not necessary, but only inferred from experience.
Hume is thus led to the view that, when we say 'A causes B', we mean only that A and B are constantly conjoined in fact, not that there is some necessary connection between them. 'We have no other notion of cause and effect, but that of certain objects, which have been always conjoinedtogether…. We cannot penetrate into the reason of the conjunction.'
He backs up his theory with a definition of 'belief', which is, he maintains, 'a lively idea related to or associated with a present impression'. Through association, if A and B have been constantly conjoined in past experience, the impression of A produces that lively idea of B which constitutes belief in B. This explains why we believe A and B to be connected: the percept of A is connected with the idea of B, and so we come to think that A is connected with B, though this opinion is really groundless. 'Objects have no discoverable connexion together; nor is it from any other principle but custom operating upon the imagination, that we can draw any inference from the appearance of one to the experience of another.' He repeats many times the contention that what appears to us as necessary connection among objects is really only connection among the ideas of those objects: the mind is determined by custom, and ''tis this impression, or determination, which affords me the idea of necessity'. The repetition of instances, which leads us to the belief that A causes B, gives nothing new in the object, but in the mind leads to an association of ideas; thus 'necessity is something that exists in the mind, not in objects'.
Let us now ask ourselves what we are to think of Hume's doctrine. It has two parts, one objective, the other subjective. The objective part says: When we judge that A causes B, what has in fact happened, so far as A and B are concerned, is that they have been frequently observed to be conjoined, i.e. A has been immediately, or very quickly, followed by B; we have no right to say that A must be followed by B, or will be followed by B on future occasions. Nor have we any ground for supposing that, however often A is followed by B, any relation beyond sequence is involved. In fact, causation is definable in terms of sequence, and is not an independent notion.
The subjective part of the doctrine says: The frequently observed conjunction of A and B causes the impression of A to cause the idea of B. But if we are to define 'cause' as is suggested in the objective part of the doctrine, we must reword the above. Substituting the definition of 'cause', the above becomes:
'It has been frequently observed that the frequently observed conjunction of two objects A and B has been frequently followed by occasions on which the impression of A was followed by the idea of B.'
This statement, we may admit, is true, but it has hardly the scope that Hume attributes to the subjective part of his doctrine. He contends, over and over again, that the frequent conjunction of A and B gives no reason for expecting them to be conjoined in the future, but is merely a cause of this expectation. That is to say: Experience of frequent conjunction is frequently conjoined with a habit of association. But, if the objective part of Hume's doctrine is accepted, the fact that, in the past, associations have been frequently formed in such circumstances, is no reason for supposing that they will continue, or that new ones will be formed in similar circumstances. The fact is that, where psychology is concerned, Hume allows himself to believe in causation in a sense, which, in general, he condemns. Let us take an illustration. I see an apple, and expect that, if I eat it, I shall experience a certain kind of taste. According to Hume, there is no reason why I should experience this kind of taste: the law of habit explains the existence of my expectation, but does not justify it. But the law of habit is itself a causal law. Therefore if we take Hume seriously we must say: Although in the past the sight of an apple has been conjoined with expectation of a certain kind of taste, there is no reason why it should continue to be so conjoined: perhaps the next time I see an apple I shall expect it to taste like roast beef. You may, at the moment, think this unlikely; but that is no reason for expecting that you will think it unlikely five minutes hence. If Hume's objective doctrine is right, we have no better reason for expectations in psychology than in the physical world. Hume's theory might be caricatured as follows: 'The proposition "A causes B" means "the impression of A causes the idea of B".' As a definition, this is not a happy effort.
We must therefore examine Hume's objective doctrine more closely. This doctrine has two parts: (1) When we say 'A causes B', all that we have a right to say is that, in past experience, A and B have frequently appeared together or in rapid succession, and no instance has been observed of A not followed or accompanied by B. (2) However many instances we may have observed of the conjunction of A and B, that gives no reason for expecting them to be conjoined on a future occasion, though it is a cause of this expectation, i.e. it has been frequently observed to be conjoined with such an expectation. These two parts of the doctrine may be stated as follows: (1) in causation there is no indefinable relation except conjunction or succession; (2) induction by simple enumeration is not a valid form of argument. Empiricists in general have accepted the first of these theses and rejected the second. When I say they have rejected the second, I mean that they have believed that, given a sufficiently vast accumulation of instances of a conjunction, the likelihood of the conjunction being found in the next instance will exceed a half; or, if they have not held exactly this, they have maintained some doctrine having similar consequences.
I do not wish, at the moment, to discuss induction, which is a large and difficult subject; for the moment, I am content to observe that, if the first half of Hume's doctrine is admitted, the rejection of induction makes all expectation as to the future irrational, even the expectation that we shall continue to feel expectations. I do not mean merely that our expectations may be mistaken; that, in any case, must be admitted. I mean that, taking even our firmest expectations, such as that the sun will rise tomorrow, there is not a shadow of reason, for supposing them more likely to be verified than not. With this proviso, I return to the meaning of 'cause'.
Those who disagree with Hume maintain that 'cause' is a specific relation, which entails invariable sequence, but is not entailed by it. To revert to the clocks of the Cartesians: two perfectly accurate chronometers might strike the hours one after the other invariably, without either being the cause of the other's striking. In general, those who take this view maintain that we can sometimes perceive causal relations, though in most cases we are obliged to infer them, more or less precariously, from constant conjunction. Let us see what arguments there are for and against Hume on this point.
Hume summarizes his argument as follows:
'I am sensible, that of all the paradoxes, which I have had, or shall hereafter have occasion to advance in the course of this treatise, the present one is the most violent, and that 'tis merely by dint of solid proof and reasoning I can ever hope it will have admission, and overcome the inveterate prejudices of mankind. Before we are reconcil'd to this doctrine, how often must we repeat to ourselves, that the simple view of any two objects or actions, however related, can never give us any idea of power, or of a connexion betwixt them: that this idea arises from a repetition of their union: that the repetition neither discovers nor causes anything in the objects, but has an influence only on the mind, by that customary transition it produces: that this customary transition is, therefore, the same with the power and necessity, which are consequently felt by the soul, and not perceiv'd externally in bodies?'
Hume is commonly accused of having too atomic a view of perception, but he allows that certain relations can be perceived. 'We ought not,' he says, 'to receive as reasoning any of the observations we make concerning identity, and the relations of time and place; since in none of them the mind can go beyond what is immediately present to the senses.' Causation, he says, is different in that it takes us beyond the impressions of our senses, and informs us of unperceived existences. As an argument, this seems invalid. We believe in many relations of time and place which we cannot perceive: we think that time extends backwards and forwards, and space beyond the walls of our room. Hume's real argument is that, while we sometimes perceive relations of time and place, we never perceive causal relations, which must therefore, if admitted, be inferred from relations that can be perceived. The controversy is thus reduced to one of empirical fact: Do we, or do we not, sometimes perceive a relation which can be called causal? Hume says no, his adversaries say yes, and it is not easy to see how evidence can be produced by either side.
I think perhaps the strongest argument on Hume's side is to be derived from the character of causal laws in physics. It appears that simple rules of the form 'A causes B' are never to be admitted in science, except as crude suggestions in early stages. The causal laws by which such simple rules are replaced in well-developed sciences are so complex that no one can suppose them given in perception; they are all, obviously, elaborate inferences from the observed course of nature. I am leaving out of account modern quantum theory, which reinforces the above conclusion. So far as the physical sciences are concerned, Hume is wholly in the right; such propositions as 'A causes B' are never to be accepted, and our inclination to accept them is to be explained by the laws of habit and association. These laws themselves, in their accurate form, will be elaborate statements as to nervous tissue—primarily its physiology, then its chemistry, and ultimately its physics.
The opponent of Hume, however, even if he admits the whole of what has just been said about the physical sciences, may not yet admit himself decisively defeated. He may say that in psychology we have cases where a causal relation can be perceived. The whole conception of cause is probably derived from volition, and it may be said that we can perceive a relation, between a volition and the consequent act, which is something more than invariable sequence. The same might be said of the relation between a sudden pain and a cry. Such views, however, are rendered very difficult by physiology. Between the will to move my arm and the consequent movement there is a long chain of causal intermediaries consisting of processes in the nerves and muscles. We perceive only the end terms of this process, the volition and the movement, and if we think we see a direct causal connection between these we are mistaken. This argument is not conclusive on the general question, but it shows that it is rash to suppose that we perceive causal relations when we think we do. The balance, therefore, is in favour of Hume's view that there is nothing in causeexcept invariable succession. The evidence, however, is not so conclusive as Hume supposed.
Hume is not content with reducing the evidence of a causal connection to experience of frequent conjunction; he proceeds to argue that such experience does not justify the expectation of similar conjunctions in the future. For example: when (to repeat a former illustration) I see an apple, past experience makes me expect that it will taste like an apple, and not like roast beef; but there is no rational justification for this expectation. If there were such a justification, it would have to proceed from the principle 'that those instances, of which we have had no experience, resemble those of which we have had experience'. This principle is not logically necessary, since we can at least conceive a change in the course of nature. It should therefore be a principle of probability. But all probable arguments assume this principle, and therefore it cannot itself be proved by any probable argument, or even rendered probable by any such argument. 'The supposition, that the future resembles the past, is not founded on arguments of any kind, but is derived entirely from habit.'1 The conclusion is one of complete scepticism:
'All probable reasoning is nothing but a species of sensation. 'Tis not solely in poetry and music, we must follow our taste and sentiment, but likewise in philosophy. When I am convinced of any principle, 'tis only an idea, which strikes more strongly upon me. When I give the preference to one set of arguments above another, I do nothing but decide from my feeling concerning the superiority of their influence. Objects have no discoverable connexion together; nor is it from any other principle but custom operating upon the imagination, that we can draw any inference from the appearance of one to the existence of another.'2
The ultimate outcome of Hume's investigation of what passes for knowledge is not what we must suppose him to have desired. The sub-title of his book is: 'An attempt to introduce the experimental method of reasoning into moral subjects.' It is evident that he started out with a belief that scientific method yields the truth, the whole truth, and nothing but the truth; he ended, however, with the conviction that belief is never rational, since we know nothing. After setting forth the arguments for scepticism (Book I, part iv, sec. i), he goes on, not to refute the arguments, but to fall back on natural credulity.
'Nature, by an absolute and uncontrollable necessity, has determined us to judge as well as to breathe and feel; nor can we any more forbear viewing certain objects in a stronger and fuller light, upon account of their customary
connexion with a present impression, than we can hinder ourselves from thinking as long as we are awake, or seeing the surrounding bodies, when we turn our eyes towards them in broad sunshine. Whoever has taken the pains to refute this total scepticism, has really disputed without an antagonist, and endeavoured by arguments to establish a faculty, which nature has antecedently implanted in the mind, and rendered unavoidable. My intention then in displaying so carefully the arguments of that fantastic sect, is only to make the reader sensible of the truth of my hypothesis, that all our reasonings concerning causes and effects are derived from nothing but custom; and that belief is more properly an act of the sensitive, than of the cogitative part of our natures.'
'The sceptic,' he continues (Book I, part iv, sec. ii), 'still continues to reason and believe, even though he asserts that he cannot defend his reason by reason; and by the same rule he must assent to the principle concerning the existence of body, tho' he cannot pretend by any arguments of philosophy to maintain its veracity … We may well ask, what causes us to believe in the existence of body? But 'tis vain to ask, whether there be body or not? That is a point, which we must take for granted in all our reasonings.'
The above is the beginning of a section 'Of scepticism with regard to the senses.' After a long discussion, this section ends with the following conclusion:
'This sceptical doubt, both with respect to reason and the senses, is a malady, which can never be radically cured, but must return upon us every moment, however we may chase it away, and sometimes may seem entirely free from it…. Carelessness and inattention alone can afford us any remedy. For this reason I rely entirely upon them; and take it for granted, whatever may be the reader's opinion at this present moment, that an hour hence he will be persuaded there is both an external and internal world.'
There is no reason for studying philosophy—so Hume maintains—except that, to certain temperaments, this is an agreeable way of passing the time. 'In all the incidents of life we ought still to preserve our scepticism. If we believe, that fire warms, or water refreshes, 'tis only because it costs us too much pains to think otherwise. Nay, if we are philosophers, it ought only to be upon sceptical principles, and from an inclination which we feel to be employing ourselves after that manner.' If he abandoned speculation, 'I feel I should be a loser in point of pleasure; and this is the origin of my philosophy.'
Hume's philosophy, whether true or false, represents the bankruptcy of eighteenth-century reasonableness. He starts out, like Locke, with the intention of being sensible and empirical, taking nothing on trust, but seeking whatever instruction is to be obtained from experience and observation. But having a better intellect than Locke's, a greater acuteness in analysis, and a smaller capacity for accepting comfortable inconsistencies, he arrives at the disastrous conclusion that from experience and observation nothing is to be learnt. There is no such thing as a rational belief: 'If we believe that fire warms, or water refreshes, 'tis only because it costs us too much pains to think otherwise.' We cannot help believing, but no belief can be grounded in reason. Nor can one line of action be more rational than another, since all alike are based upon irrational convictions. This last conclusion, however, Hume seems not to have drawn. Even in his most sceptical chapter, in which he sums up the conclusions of Book I, he says: 'Generally speaking, the errors in religion are dangerous; those in philosophy only ridiculous.' He has no right to say this. 'Dangerous' is a causal word, and a sceptic as to causation cannot know that anything is 'dangerous'.
In fact, in the later portions of the Treatise, Hume forgets all about his fundamental doubts, and writes much as any other enlightened moralist of his time might have written; he applies to his doubts the remedy that he recommends, namely 'carelessness and inattention'. In a sense, his scepticism is insincere, since he cannot maintain it in practice. It has, however, this awkward consequence, that it paralyses every effort to prove one line of action better than another.
It was inevitable that such a self-refutation of rationality should be followed by a great outburst of irrational faith. The quarrel between Hume and Rousseau is symbolic: Rousseau was mad but influential, Hume was sane but had no followers. Subsequent British empiricists rejected his scepticism without refuting it; Rousseau and his followers agreed with Hume that no belief is based on reason, but thought the heart superior to reason, and allowed it to lead them to convictions very different from those that Hume retained in practice. German philosophers, from Kant to Hegel, had not assimilated Hume's arguments. I say this deliberately, in spite of the belief which many philosophers share with Kant, that his Critique of Pure Reason answered Hume. In fact, these philosophers—at least Kant and Hegel—represent a pre-Humian type of rationalism, and can be refuted by Humian arguments. The philosophers who cannot be refuted in this way are those who do not pretend to be rational, such as Rousseau, Schopenhauer, and Nietzsche. The growth of unreason throughout the nineteenth century and what has passed of the twentieth is a natural sequel to Hume's destruction of empiricism.
It is therefore important to discover whether there is any answer to Hume within the framework of a philosophy that is wholly or mainly empirical. If not, there is no intellectual difference between sanity and insanity. The lunatic who believes that he is a poached egg is to be condemned solely on the ground that he is in a minority, or rather—since we must not assume democracy—on the ground that the government does not agree with him. This is a desperate point of view, and it must be hoped that there is some way of escaping from it.
Hume's scepticism rests entirely upon his rejection of the principle of induction. The principle of induction, as applied to causation, says that, if A has been found very often accompanied or followed by B, and no instance is known of A not being accompanied or followed by B, then it is probable that on the next occasion on which A is observed it will be accompanied or followed by B. If the principle is to be adequate, a sufficient number of instances must make the probability not far short of certainty. If this principle, or any other from which it can be deduced, is true, then the causal inferences which Hume rejects are valid, not indeed as giving certainty, but as giving a sufficient probability for practical purposes. If this principle is not true, every attempt to arrive at general scientific laws from particular observations is fallacious, and Hume's scepticism is inescapable for an empiricist. The principle itself cannot, of course, without circularity, be inferred from observed uniformities, since it is required to justify any such inference. It must therefore be, or be deduced from, an independent principle not based upon experience. To this extent, Hume has proved that pure empiricism is not a sufficient basis for science. But if this one principle is admitted, everything else can proceed in accordance with the theory that all our knowledge is based on experience. It must be granted that this is a serious departure from pure empiricism, and that those who are not empiricists may ask why, if one departure is allowed, others are to be forbidden. These, however, are questions not directly raised by Hume's arguments. What these arguments prove—and I do not think the proof can be controverted—is, that induction is an independent logical principle, incapable of being inferred either from experience or from other logical principles, and that without this principle science is impossible.