A History of Western Philosophy

by Bertrand Russell

  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 a 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 cause except 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.* 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.”†

  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 no
t 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.