History of Western Philosophy - Bertrand Russell [379]
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 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