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 conjoined together. . . . 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
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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
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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 induc-
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tion 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.
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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
