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
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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 what we know empirically without introducing any unperceived things or occurrences. As regards our present problem, all psychological knowledge can be stated with-
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out 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.
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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. (Spatiotemporal 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 object 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
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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 it 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
