A Short History of Modern Philosophy: From Descartes to Wittgenstein, Second Edition (19 page)

Hume here expresses three fundamental views which, in one form or another, have reappeared as definitive of empiricism from his day to ours. Conclusions established by pure reasoning are certain and necessary only because they are, if true, empty. Even mathematics expresses nothing but the relations among ideas, so that its propositions are true only by virtue of the ideas expressed in them, or, what amounts to the same thing, ‘true by virtue of the meanings of terms’. Secondly, the only alternative mode of knowledge, that of matters of fact, does not generate necessary truth, but simply summarises what happens to be true and what might have been otherwise. Thirdly (as Hume goes on to make clear), the only source of any knowledge of matters of fact is experience. The ideas expressed in factual propositions will all ultimately derive their content from the impressions that served to generate them. There can thus be no
a priori
proof of any matter of fact. For example, we could not demonstrate
a priori
that the world either does or does not originate from a God; that either we do or do not survive death; that either there are or are not ‘substances’ which constitute the reality behind the veil of appearance. In this way Hume raises what is

the justest and most plausible objection against a considerable part of metaphysics; that they are not properly a science, but arise either from the fruitless efforts of human vanity, which would penetrate into subjects utterly inaccessible to the understanding; or from the craft of popular superstitions, which, being unable to defend themselves on fair ground, raise these entangling brambles to cover and protect their weakness.

From this standpoint Hume is able to adopt and turn to his own sceptical ends the criticisms that Berkeley had offered of Locke’s supposed theories of ‘material substance’ and of ‘abstract ideas’. The first, together with the associated distinction between primary and secondary qualities, Hume rejected immediately as superstition, scarcely bothering to examine either Locke’s real intention or Berkeley’s scant but vivid reasoning against it. (As Hume put it, in the terms of his theory of meaning—a theory to which he held dogmatically despite its intended status as a conclusion of scientific observation—there can be no impression of material substance; it follows therefore that there can be no idea of it, so the very term ‘material substance’ is meaningless.) The theory of abstract ideas Hume regarded as incompatible with a fundamental premise of his philosophy, referring to Berkeley’s ‘great and valuable discovery’ that, since everything that exists is both individual and determinate in all its properties, the very idea of an existent with the attribute of ‘generality’ involves an absurdity. In the place of this absurd supposition Hume argued that individual ideas might ‘agglomerate’ so as to introduce into our thinking the necessary element of generality. This theory—the theory of the ‘association of ideas’—he took in essentials from Locke (who had taken it from Harvey). The theory retains in Hume its original status of a refutable empirical hypothesis. This fact eventually caused it to fall in confusion before the onslaughts of Kant’s theory of knowledge. Nevertheless from these unpromising beginnings Hume was able to formulate a philosophy that presented a powerful challenge to metaphysics. The first subject of his sceptical attack was the concept of causality, fundamental to every scientific enterprise, including that enterprise in which Hume supposed himself to be engaged.

Causality and induction

The idea of cause is one of ‘necessary connection’, according to Hume. His argument points in two directions: first, towards the demolition of the view that there are necessary connections in reality; secondly, towards an explanation of the fact that we nevertheless have the idea of necessary connection. The argument undergoes significant alterations in the first
Enquiry
and abounds in subtleties and complexities which cannot here detain us. In essence it is this.

The idea of necessary connection cannot be derived from an impression of necessary connection—for there is no such impression. If
A
causes
B,
we can observe nothing in the relation between the individual events
A
and
B
besides their continguity in space and time, and the fact that
A
precedes
B.
We say that
A
causes
B
only when the conjunction between
A
and
B
is constant—that is, when there is a regular connection of
A
-type and
B
-type events, leading us to expect
B
whenever we have observed a case of
A
Apart from this constant conjunction, there is nothing that we observe, and nothing that we could observe, in the relation between
A
and
B,
that would constitute a bond of ‘necessary connection’. In which case, given the premise that every idea derives from an impression, it may seem as though there were no such idea as that of necessary connection, and that those who speak of such a thing are uttering empty and meaningless phrases.

Why is Hume so confident that ‘necessary connections’ between events cannot be observed? His reasoning seems to be this: causal relations exist only between
distinct
events. If
A
causes
B,
then
A
is a distinct event from
B.
Hence it must be possible to identify
A
without identifying
B.
But if
A
and
B
are identifiable apart from each other, we cannot deduce the existence of
B
from that of
A
: the relation between the two can only be a matter of fact. Propositions expressing matters of fact are always contingent; it is only those conveying relations of ideas that are necessary. If there were a relation of ideas between
A
and
B,
then there might also be a necessary connection—as there is a necessary connection between
2 + 3
and
5.
But in that case
A
and
B
would not be distinct, any more than
2 + 3
is distinct from
5.
The very nature of causality, as a relation between distinct existences, rules out the possibility of a necessary connection.

We say that
A
causes
B,
then, because of a constant conjunction between
A
and
B.
This constant conjunction causes us to associate the idea of
B
with the impression of
A,
and so to expect
B
whenever we encounter
A.
Such is the force of habit, that the experience of
A
compels this idea of
B,
which therefore arises in us with the kind of involuntariness and vivacity which, according to Hume, are the distinguishing marks of belief. Hence we are compelled to believe that
B
will follow
A,
and this impression of determination gives rise to the idea of necessary connection. The impression is not an impression of a causal relation—or an impression of anything else in the external world. It is simply a feeling that arises spontaneously within us, whenever we encounter the constant conjunction of events. Nevertheless, we misread the resulting idea, as though it had been derived from an impression of necessary connection between
A
and
B.
Thence comes the idea of cause as necessary connection. This is an instance of the mind’s tendency to ‘spread itself upon objects’—to see the world as decked out in qualities and relations which have their origin in us and which correspond to no external reality.

This criticism of the common concept of causation was not entirely new,
¹
but it was pursued by Hume at great length and with considerable rigour, and the dispute to which it gave rise remains one of the enduring problems of metaphysics. In addition, Hume presented a further problem to the advocates of scientific investigation. This problem has come to be known as the problem of induction. Since the relationship between distinct objects and events is always contingent, there can be no necessary inference from past to future. It is therefore perfectly conceivable that an event which has always occurred with apparent regularity and in obedience to what we call the laws of nature, should not occur. The sun may not rise tomorrow, and this would be entirely consistent with our past experience. What then justifies us in asserting on the basis of past experience either that the sun will rise tomorrow, or that it is even probable that it will do so? This problem can be seen to be general. Since scientific laws state universal truths, applicable at all times and in all places, then necessarily no finite amount of evidence can exhaust their content. Hence no evidence available to finite creatures such as we are can guarantee their truth. What therefore justifies us in asserting them?

The external world

While Hume’s most original contribution to metaphysics is to be found in that systematic attack on the Cartesian idea of an
a priori
science, he also added a new dimension to scepticism of a more traditional form. This is the scepticism which arises from reflections on the disparity that exists between our knowledge of ourselves as subjects and our knowledge of an objective world. Hume begins from the idea that things which exist at different times must be both distinct and in principle distinguishable. A fundamental ingredient in our conception of a physical object is that of ‘identity through time’. Without identity through time the idea of objectivity is imperilled. In a world of instantaneous things, it would seem impossible to distinguish our fleeting experiences from the objects which occasion them. There would be exactly the same evidence for our judgements about both, in which case the distinction between them (between appearance and reality) would break down. Hume argued that we cannot rely on the concept of identity over time in order to make this distinction. If we could rely on this concept, then we could come to the conclusion that objects endure from one moment to another, and hence that they may exist, in principle, when unobserved.

But how could we have the idea of existence unobserved, when there can be no corresponding impression? Such an idea cannot be referred to the ‘outer’ world, but only (as Hume diagnoses it) to the workings of our imagination. The imagination constantly constructs from the fragmentary deliverances of sense-perception the images of enduring things. The resulting idea—of ‘identity’—is, like that of necessary connection, a product of custom and association. Hume contrasts the idea of ‘identity’ with that of ‘unity’. Whenever we are presented with an impression we are simultaneously presented with an impression of unity. This unity of a thing with itself is indistinguishable from the impression, and therefore from the idea, of an ‘object’.

When presented with two impressions at different times, we are presented with an impression not of unity but of duality, and no effort of the imagination can justify, even if it may in some way produce, the thought of ‘identity’ as a distinct and discriminable experience. Lacking the impression of identity, we lack also the idea, from which it would seem to follow that the whole notion of an external world is thrown in doubt. All that we can legitimately signify by referring to such a world is some element of ‘constancy’ and ‘coherence’ among our impressions.

It should be noted that Hume—while he also relied on and to some extent reiterated Berkeley’s attack on Locke—has, in this argument, focused on a wholly new aspect of the problem of the external world. In submitting the concept of ‘identity through time’ to sceptical examination, Hume brought to the attention of later philosophers the fundamental pattern of thought on which all our ideas of objectivity finally rest. The principle of his scepticism—that of the contingent connection between distinct existences—shows the extent to which the concept of causality and that of objectivity are vulnerable to the same doubts and might (as Kant was to argue) be protected by the same anti-sceptical strategies. It was to become increasingly apparent that there are not two problems— one concerning causality and induction, the other concerning the external world—but one, the problem of objective knowledge as such. This problem could be manifest in many ways, but it remained solved or unsolved in accordance with the ability of a philosopher to argue for real connections between separately identifiable objects.

The self

Perhaps the most surprising aspect of Hume’s scepticism is to be found in his theory of the self. It might be thought that a philosopher so determined to emphasise the inadequacy of our claims to knowledge when set beside the secure basis of experience would at least be content with the Cartesian position that, being certain of my own experience, I know that I exist.

But what, asks Hume,
is
this ‘I’ whose existence is so audaciously asserted in all thinking? When he looks into his own mind, he finds many separate particulars: impressions, ideas and the activities exemplified in their relations. But he finds no particular, whether impression or idea, which corresponds to the ‘I’ of which we so confidently assert existence. If I ask myself what I am, then the only satisfactory answer is that I consist, not in this or that impression or idea, but in the totality of my impressions and ideas. This theory, sometimes referred to as the ‘bundle’ theory of the self, arises by extending into the mental realm the familiar objections to the concept of individual substance. These objections Berkeley had already levelled against Locke’s theory of the physical world, and Hume largely approved of them. In the absence of any mental ‘substance’, there is nothing for me to be identical with, save either an impression, or an idea, or some bundle of the same. With the same spirit that had unearthed what were to become the standard epistemological problems of metaphysics, Hume proceeded to disclose parallel difficulties for ethics. Two in particular serve to cast doubt on the possibility of an objective moral system. The first is introduced thus: