Conclusiones

Introduction: causal realism

A white billiard ball collides with a stationary black billiard ball at time t and a fraction of a second later, at time t+, the black ball moves off towards one of the pockets on the billiard table. At t+, a spectator sneezes. Call the collision of the white ball with the black ball at t event e₁, the black ball moving off at t+ event e₂ and the spectator sneezing at t+ event e₃. Intuitively, we would judge that e₁ caused e₂ but that e₁ did not cause e₃. What are we doing when we make causal judgements such as these? One answer is that we are expressing beliefs: when we judge that e₁ caused e₂ but not e₃ we are expressing the belief that e₁ stands in a relation to e₂ that it does not stand in to e₃. What relation? One answer is the relation of necessary connection: the occurrence of e₁ made necessary the occurrence of e₂. On the other hand, although e₁ was followed by e₃, the occurrence of e₁ did not make necessary the occurrence of e₃. Given e₁, in some sense e₂ (unlike e₃) had to happen. Causal realism, as understood here, holds that causal judgements express beliefs about necessary connections between events, that at least some of these beliefs are true (and justifi ed), and that they are true in virtue of the obtaining of mind-independent states of affairs. (We can also think of the causal relation as obtaining between objects or facts, but throughout this entry we will think of it as obtaining between events). So causal realism holds that the judgement that e₁ caused e₂ expresses a belief that there is a relation of necessary connection between them, that this belief is true (and justifi ed), and that the obtaining of this relation in no way depends upon the thoughts, feelings or mental activity of humans.

Causal realism seems like a piece of common sense. Historically, however, it is challenged by the writings of the Scottish philosopher David Hume (1711–76), especially part 3 of book 1 of his A Treatise of Human Nature (1739) and §4–7 of his An Enquiry Concerning Human Understanding (1748). In the second and third sections, we outline the ingredients of Hume’s case against causal realism, and in the fourth section we outline some possible alternatives to causal realism that might be attributed to

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Hume. (Note that it is logically possible for e₁ to occur without e₂ following: in Humean terminology e₁ and e₂ are “distinct existences.” The central issue about causal realism raised by Hume is thus whether there are mind-independent “necessary connections”

between “distinct existences”).

Hume’s naturalism and empiricism

Hume’s project, announced in the Treatise, is to provide a “solid foundation” for the

“science of man” by explaining the “principles of human nature” (1978 [1739]: xvi): in effect, a naturalistic account of the workings of the human mind that views it as suscep-tible to a broadly scientifi c treatment. Hume writes: “the only solid foundation we can give to this science itself must be laid on experience and observation” (1978 [1739]:

xvi). The challenge to causal realism emerges from Hume’s attempt to apply this naturalistic–empiricist approach to causal judgement.

Hume uses a generic term – “perceptions” – to refer to states of mind, and distin-guishes between impressions and ideas. Impressions are, roughly, experiences: sense-experiences, “outward sentiments” or “impressions of sensation,” such as visual or tactual experiences, and introspectable experiences, “inward sentiments” or “impres-sions of refl ection,” such as joy, sadness, anger and desire. The cornerstone of Hume’s empiricism is his claim that all concepts – or ideas – are copies of resembling impres-sions: “By ideas I mean the faint images of [impressions] in thinking and reasoning”

(1978 [1739]: 1). However, since we have the concept of a golden mountain, yet no corresponding impression (we’ve never experienced one), Hume refi nes his empiricist claim by distinguishing between simple and complex ideas and impressions: “Simple perceptions or impressions and ideas are such as admit of no distinction or separation.

The complex are the contrary to these, and may be distinguished into parts” (1978 [1739]: 2). The empiricist claim is then enshrined in Hume’s Copy Principle: all our ideas are either (a) simple ideas copied from some resembling impression or (b) complex ideas ultimately composed of simple ideas. Since the idea of a golden mountain is a complex idea, the fact that we have it despite never having experienced one is consistent with Hume’s empiricism, since it is composed of ideas (such as that of gold) that do correspond to resembling impressions.

The Copy Principle can be viewed as a semantic principle, according to which the content of our ideas ultimately derives from experience, or as a genetic claim, according to which experience is the ultimate causal source of our ideas, or both. Either way, it imposes constraints upon accounts of causal judgement. If the judgement that e₁ caused e₂ expresses the belief that e₁ and e₂ stand in a relation of necessary connection, then we either have to show that the idea of necessary connection implicated in this belief is a complex idea composed ultimately of simple ideas copied from resembling impressions, or, if the idea of necessary connection is held to be simple, fi nd an impression from which the idea of necessary connection itself is copied. If neither of these is possible, then the notion that we are so much as capable of making causal judgements is threatened, either because there is no idea of necessary connection (semantic) or there are no grounds for attributing such an idea to us (genetic). Either way, causal realism would be threatened.

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Hume’s search for the impression of necessary connection

Hume can thus be viewed as attempting to clarify the idea of necessary connection (or

“power”) by looking for the impression or impressions from which it is ultimately derived:

To be fully acquainted … with the idea of power or necessary connection, let us examine its impression; and in order to fi nd the impression with greater certainty, let us search for it in all the sources, from which it may possibly be derived. (1975 [1748]: 63)

Consider again our example involving the billiard balls. When we look outward at the goings-on on the table, we see a sequence of events – including e₂ following e₁ – but, Hume argues, we receive no impression of a relation of necessary connection between them:

When we look about us towards external objects, and consider the operation of causes, we are never able, in a single instance, to discover any power or necessary connection; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. We only fi nd, that the one does actually, in fact, follow the other. The impulse of one billiard-ball is attended with motion in the second. This is the whole that appears to the outward senses. The mind feels no sentiment or inward impression from this succession of objects: Consequently, there is not, in any single, particular instance of cause and effect, any thing which can suggest the idea of power or necessary connection. (1975 [1748]: 63)

In favour of this claim, Hume argues that given all of the information that our senses yield about the white billiard ball up to and including the point at which it strikes the black billiard ball, we could not predict what will happen to the latter the instant immediately after it is struck. If we did receive an impression of necessary connection from observing the billiard table up to and including the point at which the collision takes place, we would be in a position to make such a prediction. So Hume concludes that we do not get an impression of necessary connection from observing a single causal transaction between external events.

Hume next considers whether we might get an impression of necessary connection from introspecting on a single causal transaction involving our own minds, e.g. the event of my willing my arm to move causing my arm to move upwards. He rejects this suggestion. First, if we had an impression of necessary connection between the mental act of volition and the bodily event of my arm’s moving upwards, we would understand the mind–body relationship. But far from understanding “the secret union of soul and body,” there is no relationship “in all nature more mysterious” (1975 [1748]: 65) than this. Second, if we had such an impression we would understand why it is that I can move my arm but not my liver. Since we don’t understand this, it again follows that we have no such impression. (To the rejoinder that we do in fact understand – courtesy of

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modern physiology – why I can move my arm but not my liver, Hume would reply that this understanding is not based on the experience of a single causal transaction). Third, there is a complex sequence of events between the mental act of willing and the movement of my arm, events involving “certain muscles, and nerves, and animal parts”

(1975 [1748]: 66). Since all of these events are part of a sequential causal process, one event in the sequence causes the next, which causes the next, and so on. So all of these events are necessarily connected to their neighbours. If we had impressions of necessary connection between the events and their neighbours we would understand, independ-ently of further experience, how one link in the causal chain causes the next. We don’t, so again Hume’s conclusion is that we have no such impressions.

Could we get an impression of necessary connection from refl ecting on a single instance of a causal transaction involving an act of will and another mental event, e.g. the event of my willing myself to think of a glass of Laphroaig leading to an idea of such a glass appearing in my mind? Hume argues against this suggestion. First, if we had such an impression we would understand the mind’s ability to produce ideas at will. But according to Hume we don’t, since “This is a real creation; a production of something out of nothing”

and this is “entirely beyond our comprehension” (1975 [1748]: 68). Second, if we had such an impression, we’d be able to understand, on the basis of a single experience, why we can conjure up at will an idea of Tony Blair but not a sentiment of approbation towards him. Since we’re not able to understand this on the basis of a single experience, we have no such impression. Likewise, we’d be able to explain on the basis of a single experience why “we are more master of our thoughts … fasting, than after a full meal” (1975 [1748]:

65). We can’t, again showing that we have no such impression.

Indeed, the fact that the impression of necessary connection is so hard to track down, that “even in the most familiar events, the energy of the cause is as unintelligible as in the most unusual,” leads philosophers such as Malebranche (1638–1715) to the doctrine of occasionalism, according to which, when one billiard ball collides with another “it is the Deity himself … who by a particular volition, moves the second ball” (1975 [1748]:

70). Hume rejects this theory as taking us “into fairy land”: it does not help us trace the impression of necessary connection, since we are equally ignorant “of the manner or force by which a mind, even the supreme mind, operates either on itself or on body”

(1975 [1748]: 72).

Does it follow from the arguments above that we have no idea of necessary connection and that our “causal judgements” are in fact meaningless? No: according to Hume we can locate the impression from which the idea of necessary connection is copied, but only by looking beyond single instances of causal transactions.

On all previous occasions on which a billiard ball has struck another stationary ball, the collision has been followed by the stationary ball’s moving off. Also, when we saw the white billiard ball approaching the stationary black ball, we inferred that the black ball would likewise move off. When this happens, we say that the white ball’s colliding with the black ball is the cause of the black ball’s movement:

[W]hen one particular species of event has always, in all instances, been conjoined with another, we make no longer any scruple of foretelling one upon

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the appearance of the other, and of employing that reasoning, which can alone assure us of any matter of fact or existence. We then call the one object, Cause;

the other, Effect. (1975 [1748]: 74)

How can exposure to many more instances of billiard ball collisions help us in the search for the impression of necessary connection? Is it that such exposure reveals something in the sequence of events (or “in the objects”) that yields the impression of a necessary connection between them?

Hume suggests not. When we view the 1,000th instance of a billiard ball being struck by another and then moving off, as far as the external events are concerned there is nothing experienced in that transaction that wasn’t also experienced in the fi rst:

’Tis evident … that the repetition of like objects in like relations of succession and contiguity discovers nothing new in any one of them; since we can draw no inference from it, nor make it a subject either of our demonstrative or probable reasonings. (1978 [1739]: 163)

Causal claims cannot be established by demonstrative (a priori) reasoning: “I shall venture to affi rm, as a general proposition, which admits of no exception, that the knowledge [of the relation between cause and effect] is not, in any instance attained by reasonings a priori” (1975 [1748]: 27). For any given pair of causally related events we can always conceive of the cause happening without the effect:

When I see, for instance, a Billiard-ball moving in a straight line towards another; even suppose motion in the second ball should by accident be suggested to me, as the result of their contact or impulse; may I not conceive, that a hundred different events might as well follow from that cause? (1975 [1748]: 29).

We can conceive of e₁ being followed, not by e₂, but by e₄, in which both balls instan-taneously become stationary. So no a priori reasoning can allow us to infer the occur-rence of e₂ from the occurrence of e₁.

Moreover, even after exposure to the prior constant conjunction of e₁-type events with e₂-type events we are unable to rely on a posteriori or “probable” reasoning to infer the occurrence of e₂ from the occurrence of e₁. To infer the occurrence of e₂ from the occurrence of e₁, and the fact that all previous e₁-type events have been followed by e₂-type events, we would need to rely on the supposition “that instances, of which we have had no experience, must resemble those, of which we have had experience, and that the course of nature continues uniformly the same” (1978 [1739]: 89). But this proposition cannot be established either by a priori or a posteriori reasoning. It cannot be established by a priori or “demonstrative” reasoning, since we can conceive of a situation in which the black ball doesn’t move off after being struck by the white ball, despite the fact that in the past, events of the latter type have always been followed by the stationary ball’s moving off. Nor can it be established by a posteriori or “probable” reasoning.

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Arguing that since in the past, instances of which we have had no experience have resembled those of which we have had experience, in the future instances of which we have had no experience will resemble those of which we have had experience, would be to argue in a circle. (This argument – in Book 1, pt 3, §6 of Hume’s Treatise and §4 of his Enquiry – is clearly related to the traditional “problem of induction,” but various commentators have questioned whether it is in fact a genuine concern of Hume’s [see e.g. Beebee 2006: 7]).

Hence, experience of the constant conjunction of e₁-type events with e₂-type events reveals no impression of necessary connection obtaining between them. So how does the experience of the constant conjunction yield the sought-after impression of necessary connection? Hume answers:

[T]here is nothing in a number of instances, different from every single instance, which is supposed to be exactly similar; except only, that after a repetition of similar instances, the mind is carried by habit, upon the appearance of one event, to expect its usual attendant, and to believe that it will exist. This connection, therefore, which we feel in the mind, this customary transition of the imagination from one object to its usual attendant, is the sentiment or impression from which we form the idea of power or necessary connection.

(1975 [1748]: 75)

Thus, the impression of necessary connection is an impression of refl ection that arises in the following way. When we experience a constant conjunction of two types of events, say e₁-type events and e₂-type events, they “acquire a union in the imagi-nation” (1978 [1739]: 93): we become disposed, in virtue of “Custom” or “Habit” (1975 [1748]: 43), to infer an e₂-type idea from an e₁-type impression. This “customary transition of the mind” is accompanied by a feeling of irresistibility or compulsion. Given an impression of a white ball striking a black ball we feel compelled to form the idea of the black ball’s moving off. This feeling of compulsion is the impression from which the idea of necessary connection is copied.

Causal anti-realism

What are the implications for causal realism of Hume’s search for the impression of necessary connection? What, according to Hume, are we doing when we judge that e₁ caused e₂?

Hume claims that in making causal judgements we exhibit a tendency to project aspects of our psychology on to the world:

[T]he mind has a great propensity to spread itself on external objects, and to conjoin with them any internal impressions, which they occasion, and which always make their appearance at the same time that these objects discover themselves to the senses. Thus as certain sounds and smells are always found to attend certain visible objects, we naturally imagine a conjunction, even in

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place, betwixt the objects and qualities, tho’ the qualities be of such a nature as to admit of no such conjunction, and really exist nowhere … [T]he same propensity is the reason, why we suppose necessity and power to lie in the objects we consider, not in our mind, that considers them; notwithstanding it is not possible for us to form the most distant idea of that quality, when it is not taken for the determination of the mind, to pass from the idea of an object to that of its usual attendant. (1978 [1739]: 167; see also 1975 [1748]: 78)

How can we best make sense of Hume’s talk of “the mind spreading itself on external objects” and “transferring the feeling of customary connection to objects”?

Projectivism

Causal realism holds that the judgement that e₁ caused e₂ expresses a belief, a psycho-logical state that can be assessed in terms of truth and falsity. In general, an account of a type of judgement that views them as expressing beliefs is called a cognitivist account of judgements of that type. The projectivist interpretation of Hume sees him as rejecting causal realism by giving a non-cognitivist account of causal judgement, according to

Causal realism holds that the judgement that e₁ caused e₂ expresses a belief, a psycho-logical state that can be assessed in terms of truth and falsity. In general, an account of a type of judgement that views them as expressing beliefs is called a cognitivist account of judgements of that type. The projectivist interpretation of Hume sees him as rejecting causal realism by giving a non-cognitivist account of causal judgement, according to