Causal Inference in Hume

Causal Inference in Hume (DRAFT)

1 Causal Inference in Hume

Abstract: The part of Hume's Treatise that we look to when we're interested in his views about causation turns out to be not so much about causation per se, but a discussion of causal reasoning of one form or another. Traditionally, Hume is interpreted as holding that the idea of necessary connection does not play a role in causal inference. Part of what motivates this view is the assumption that custom based inferences are immediate and automatic. But Hume in fact thought that many causal inferences are reflective and deliberate. For reasoners to undertake such inferences, they must possess and deploy the concept of a non-demonstrative argument or reason. I suggest that Hume took the idea of necessary connection to play this role in causal inference.

2 DRAFT 11/1/04

Causal inference in Hume

1. The part of Hume's Treatise that we look to when we're interested in his views about causation turns out to be not so much about causation per se, but a discussion of causal reasoning of one form or another. Hume seems to think that the concept or idea of causation figures in this sort of reasoning. What exactly is the idea, and what role, if any, does it play in this reasoning? Hume observes that in order for two objects to be related as cause and effect, they must be successive and contiguous. He goes on to ask,

Shall we then rest contented with these two relations of contiguity and succession, as affording a compleat idea of causation? By no means. An object may be contiguous and prior to another, without being consider'd as its cause. There is a NECESSARY CONNEXION to be taken into consideration; and that relation is of much greater importance, than any of the other two above-mention'd. (T77, 1.3.2.11)1

But what is this necessary connection? Hume's usual empiricist procedure for getting clear on some idea is to examine the impression from which it derives. (Mention copy principle; page references to it.) The sense impressions we have of causally related objects, however, reveal nothing of causal significance beyond contiguity and succession (refs). But Hume does not conclude, as he does in his discussion of the philosophical notion of substance (T222-3), that there is no impression and, in fact, no corresponding idea. Instead, Hume starts “beating about the neighboring fields” (T78) with the aim of finding the missing impression. What we get is a very lengthy discussion of “probable reasoning”.

1 See also T87; 1.3.6.3, where Hume speaks of necessary connection as an essential part of the causal relation. Hume is trying to articulate the concept of causation as it figures in causal reasoning or inference. This isn't necessarily to give an analysis of the meaning of the word causation, nor even to specify the content of the corresponding idea. Contrast e.g. Fogelin, 41-2. That's because the word causation, and the idea that according to Hume gives meaning to the word, needn't be the concept that figures in the reasoning. The articulation of the concept as it figures in causal inference is not as beholden to the vagaries of ordinary language.

3 Probable reasoning or inference is so called by Hume to contrast with the certainty of demonstrative reasoning found for example in mathematical deduction. Two features of probable reasoning are important for our purposes (T73-4, T89). First, it is the basis for all of our beliefs in unobserved matters of fact, i.e. those that go beyond what is given in current experience or memory. For example, I see footprints, and infer that some creature has walked by recently. Or I notice an egg rolling toward the edge of the table and then predict that it will fall and break on the floor. The second notable feature of probable reasoning is that it involves relations of cause and effect: the footprints are the effect of the animal’s having walked by, the egg breaks because it strikes the floor. No inferences can be made unless the objects in question are taken to be causally related. Without a causal connection between them, a footprint would yield no evidence, justification, or basis for concluding that an animal had passed through. For Hume, probable reasoning is causal reasoning. Hume emphasizes necessary connection as a part of the idea of causation because, I contend, of the justificatory role he sees for it in causal or probable inference. He says,

“…the only [relation], that can be trac’d beyond our senses, and informs us of existences and objects, which we do not see or feel, is causation. This relation, therefore, we shall endeavour to explain fully before we leave the subject of the understanding” (T74b).

Notice that Hume doesn't think that reasoning simply causes or leads us to forming certain beliefs. Rather, it is (or is supposed to) inform us of matters of fact beyond what we have perceived. This suggests that the reasoning potentially amount to justification for the beliefs that it yields. This interpretation receives some corroboration from another passage where Hume says that the causal relation is the only relation that gives "assurance from the existence of action of one object, that 'twas follow'd or preceded by any other existence or action" (T73-4, my emphasis). If causation figures centrally in reasoning that informs and assures us of some conclusions, then it's hard not to think that causation has a role in justifying these conclusions. Shortly thereafter, at T77 quoted above, Hume asserts that necessary connection is the most important element of the idea of causation – presumably, I would add, for its role in probable reasoning. This becomes more explicit later on, when at T165 Hume says that necessary connection is the foundation of our inference from cause to effect. So Hume seems to be

4 advocating the following semantic claim regarding our concept or idea of causation:

(S) Causation includes necessary connection; part of what it means for objects to be related as cause and effect is that a necessary connection holds between them.

[Formulating this is a somewhat delicate matter, since Hume seems to think that it is literally unintelligible to think that a necessary connection holds between objects. I’ll need to make use of an extended notion of necessary connection that holds between objects in virtue of their surrogates being necessarily connected in a narrower sense.]

And Hume seems to hold (S) because of:

(J) The causal relation plays a justificatory role in probable reasoning, and can only do so if it is understood as involving necessary connection.

2. It is surprising, then, that when we turn to perhaps the most important part of the discussion of probable or causal inference – namely, section 1.3.6 (Of the inference from the impression to the idea) – Hume defends positive and negative theses about this inference, each of which appears to be at odds with the justificatory role accorded to necessary connection. The negative thesis seems to reveal a deep skepticism about probable reasoning. Hume argues that causal inference and all the resulting beliefs are not based on any reason, that it is "impossible for us to satisfy ourselves by our reason, why we shou'd extend that experience beyond those particular instances, which have fallen under our observation" (T91). Such a skepticism is at odds with the rationale for having necessary connection as part of our concept of causation. Necessary connection was to play a justificatory role in probable inference. But now we have no need for such an element in this reasoning – because nothing will legitimize (in any epistemic sense) the causal inferences that constitute this reasoning. Thus, even if our prior understanding of causation is one that includes necessary connection, it now seems possible to replace it with a conception of causation that would dispense with necessary connection. (Hume also expresses skepticism about probable reasoning later on, in 1.4. But the tension between the alleged skepticism of 1.3.6 and the

5 necessity of necessity thesis is especially problematic, since 1.3.6 falls within the ambit of the discussion the express purpose of which is to articulate the notion of necessary connection presented as essential for causation.) The positive thesis of 1.3.6 is that causal inference is immediate, and the result of the constant conjunction of objects in experience, and natural custom or habit (T87, T93, T102-4). [Elaborate.] Like the negative thesis, this thesis also appears to be in tension with (J). For if the inferential transition is as immediate and as unreflective as the positive thesis appears to suggest, there seems not to be any role for the idea of necessary connection to play, let alone the justificatory role (J) would accord to it.

3. So the negative and positive theses of 1.3.6 are each seemingly at odds with Hume’s earlier remarks about the importance of necessary connection for causation. The traditional strategy for resolving this tension is simply not to take those earlier remarks at face value. (J) is, on this view, a provisional claim , temporarily taken up for rhetorical purposes. It is presented as a part of tradition or common prejudice that can be discarded once Hume’s skeptical and psychological insights are appreciated The traditional understanding can be challenged on two grounds, corresponding to how it interprets the negative and positive parts of Hume’s discussion here at 1.3.6 and elsewhere. The negative thesis that probable inference is not based on reason is traditionally and (from what I gather) popularly understood to be a defense of skepticism about probable reasoning. But Treatise 1.3.6 has more recently been the subject of an extensive and rich literature that seeks to show that Hume is not the skeptic that tradition has made him out to be. Second, the traditional view interprets Hume’s positive thesis to be that all causal inference is a result of automatic habit, so that when presented with one object the inference to the other is always immediate. There is no room, in this picture, for a role to be played by causation or necessary connection. But it seems, on the contrary, that Hume recognizes causal inferences that are much less immediate than the traditional view would allow. And these more deliberate probable inferences suggest a role for necessary connection in causal inference. Both of these challenges to the traditional understanding need to be investigated. I turn first to Hume’s positive picture of probable inference: custom/habit.

6 [I do not have the space here to address the interpretations of the negative thesis that seek to show that Hume is not presenting a skeptical argument in 1.3.6. I will focus on Garrett’s interpretation, and show how it can be improved/supplemented so as to withstand Millican’s critique of it. (refs). My defense of Garrett here will make use of my interpretation below of role that necessary connection is supposed to play in causal inference.]

4. It is undeniable that Hume often describes causal inference as the automatic and unreflective exercise of habit built up through experience of constant conjunction (immediate and automatic: 87, 93m&b, especially section 7 note on 97; custom: 97, 102-4,170; habit 179). (See also 1.3.15 on the reason of animals, esp. the remark at 178 that ‘beasts never discern a necessary connection’.) Such remarks prompt some interpreters to conclude that necessary connection (or the idea thereof) plays no role in probable reasoning. For example, Noonan says that

…it is a brute fact about human beings that they are so constituted that experience of a constant conjunction of A’s and B’s will create in them a disposition to form an idea of an A when presented with an idea of a B and conversely…The brute fact of the constant conjunction of A’s and B’s in experience (that is, the bare fact of the occurrence of that pattern in experience), independently of its being known or reflected on, will suffice to create the disposition… (Noonan, 132)

Noonan continues:

Thus it now appears that our idea of necessary connection, on Hume’s account, is a redundant addition to our stock of mental ideas, something which need have no reflection in the inferences we make, the beliefs we hold and so on. Its status is that of a mere epiphenomenon, a side-product of what goes on…(Noonan 153) (Others?)

Other commentators suggest for necessary connection an anemic role at best. Garrett points out that since the impression of necessary connection is a feeling of determination that we have when presented with one or the other of constantly conjoined objects, what prompts or causes (in the regularity sense) the inference (in a properly habituated individual) is not merely the presented object, but the object along with the feeling of

7 determination. Thus, the feeling or impression of determination would also count as a determination of the mind (in the regularity sense). I describe this as an anemic role since it is not really required for the drawing of the inference. It seems that we would draw the inferences even if we never had the impression. (See Garrett (forthcoming), p. 9). On such interpretations, the early remarks suggestive of (J) (the thesis that necessary connection plays a role in causal inference) as Hume’s considered view are either rejected outright or overly domesticated. Only the later remarks, presented subsequent to and in light of the thesis about the role of custom and habit, are indicative of what Hume really thinks should be a part of our idea of causation. The traditional interpretation is right in holding that Hume does think that there are causal inferences that are immediate and unreflective, and that these inferences offer little opportunity for necessary connection to play a substantive role. But the traditional view goes wrong in thinking that Hume described all causal inferences in this way. Some inferences are not immediate, and Hume seems to have seen a role for the idea of necessary connection in at least some of these inferences. There are several reasons for thinking so. First, even late in his discussion, Hume asserts that necessity is the basis of the inference (T165). More worrying for this traditional view that all inferences are immediate, is the following puzzle. If Hume thinks that our causal inferences are just the product of custom mechanically conceived, then it’s not clear why he’s concerned (as he seems to be in 1.3.6 and elsewhere) to find a connection between objects we take to be related as cause and effect. Why would we need to discern or be shown a connection between the objects if we’re just automatically making a transition from one to the other, if it’s just a basic or primitive psychological fact that such a transition occurs? Hume's picture, rather, seems to be that without some sort of connection, causal inferences, or at least some significant class of them, would not be undertaken. Although the ideas are not related in such a way as to be connected by the understanding, we can draw the inferences because of their connection in the imagination. Thus,

Had ideas no more union in the fancy than objects seem to have to the understanding, we cou'd never draw any inference from causes to effects, nor repose belief in any matter of fact. (T93; more relevant

8 text before and after this sentence. Note that in this context H seems to be using connection and union interchangeably.)

It might be objected that when Hume speaks in this context of a connection or union of ideas, he means nothing more than the fact that there is an association or transition between the ideas in the imagination. But the problem with identifying connection with association or transition is that Hume seems to want to explain at least some instances of transition or inference between ideas in terms of the connection between them. Hume says,

When the mind, therefore passes from the idea or impression of one object to the idea or belief of another, it is not determin'd by reason, but by certain principles, which associate together the ideas of these objects, and unite them in the imagination…[T]he principles of union among ideas, I have reduc'd to three general ones, and have asserted, that the idea or impression of any object naturally introduces the idea of any other object, that is resembling, contiguous to, or connected with it. (92, my emphasis)

For the sort of association of ideas we have in causal inference, the relevant relation is connection, which Hume elsewhere refers to as cause and effect. And this connection will hardly explain the transition if it simply is the transition. So we cannot defend the traditional interpretation of Hume on causal inference by identifying connection with transition or association. We still need an explanation of why Hume would be interested in a connection, and the traditional interpretation that the inference is automatic and immediate has nothing to say about this. Don't get me wrong. Some causal inferences can be described, more or less correctly, as automatic and immediate. The passages noted above suggest that Hume thought so. For these inferences, the association in the imagination is all there is to the inference; connection plays no explanatory role in these cases. [This ought to be qualified, given what I say in another paper.] But the interest in a connection suggests that Hume thought that there were other causal inferences (associations or unions of ideas in the imagination) that cannot be described as automatic and immediate, and for which the idea of a connection is significant. A final consideration against the traditional reading of Hume on causal inference is the ample evidence that Hume indeed thinks that there are instances of probable reasoning/causal inference that fail to fit the model

9 of unreflective automatic habit. Hume describes several such cases. First, in the case of inferences involving judgments of causality (rules for which he gives in 1.3.15), Hume says

…even experimental philosophy, which seems the most natural and simple of any, requires the utmost stretch of human judgment. There is no phaenomenon in nature, but what is compounded and modify’d by so many different circumstances, that in order to arrive at the decisive point, we must carefully separate whatever is superfluous, and enquire by new experiments, if every particular circumstance of the first experiment was essential to it…the utmost constancy is requir’d to make us persevere in our enquiry, and the utmost sagacity to choose the right way…If this be the case even in natural philosophy, how much more in moral, where there is a much greater complication of circumstances. (T175; 1.3.15.11)

It is even more evident that Hume countenances reflective and deliberate inference in his discussion of reasoning about probabilities of causes. For this sort of reasoning, Hume explicitly describes a model of immediate and unreflective inference based on custom, something he calls “imperfect habit”. While some inferences proceed this way, Hume says

this method of proceeding we have but few instances of in our probable reasonings; and even fewer than in those, which are deriv’d from the uninterrupted conjunction of objects. (1.3.12.7)

Rather, we usually proceed in a way that is more reflective:

we commonly take knowingly into consideration the contrariety of past events; we compare the different sides of the contrariety, and carefully weigh the experiments, which we have on each side; Whence we may conclude, that our reasonings of this kind arise not directly from the habit, but in an oblique manner; which we must now endeavour to explain. (1.3.12.7)

(Further evidence: the model of immediate inference also fails to fit Hume’s discussion of causal inference based on a single case. See T105.)

4. So Hume doesn’t think that causal inference is always the automatic and unreflective upshot of habit. The cases of more reflective and sophisticated

10 reasoning that Hume recognizes suggest an alternative to traditional understanding, one that does accord to the idea of necessary connection a role in causal inference. What is distinctive about the reflective and conscious causal inferences, and which sets them apart from the immediate and automatic inferences is that in the former the subject seems to be concerned with the evidence or reason for whatever conclusion she draws. Care, resourcefulness, and persistence is generally required to discern the evidence, reasons, or justification needed for drawing what the subject takes to be a legitimate conclusion. It seems that one could not engage in this sort of endeavor without having some notion of argument or inference that connects the evidence or reasons with the conclusion. Conscious, reflective causal inference requires “discerning” a connection between objects related as cause and effect so as to be able to draw an inference from one to the other, or to construct an argument with a premise concerning one, and a conclusion concerning the other. In contrast to an automatic and immediate transition, a conscious or reflective inference requires some sort of connection between ideas. The connection needn’t actually entail that the inference is justified. I could be mistaken in thinking that one idea is evidence or reason for the other, or that there is anything like a valid or otherwise legitimate argument from one to the other. So the connection is not so much a relation between the ideas themselves, but that of my taking there to be some sort of evidential, justificatory relation between them. Without my taking there to be some such rational or evidential connection between cause and effect, I will not be able to draw the inference, at least not reflectively. (And if I happen to draw the inference in a more immediate fashion but upon reflection do not take there to be a connection, I would then withdraw the inference, or suspend belief in the conclusion. These are, after all, cases of reflective, deliberate inferences. Whether or not they are correct, and whether or not there are systematic skeptical doubts that undermine the entire inferential practice, the psychological fact that there are such reflective inferences and inferential practices requires attributing to the subject some notion of evidence, argument, intelligible connection.) In short, the connection Hume has in mind is that of an intelligible connection – a non-demonstrative argument or inference, where inference is here understood as a normative notion and not merely a mechanical transition between ideas or beliefs. If a reasoner is not equipped with the idea of a non-demonstrative argument or justification, then he or she will be unable to undertake the sort of reflective and deliberate causal inference that

11 Hume thinks constitutes much of the probable reasoning that we, legitimately or illegitimately, undertake.

5. I am arguing that certain conceptual resources are required for a reasoner to undertake probable reasoning that is reflective and deliberate. In particular, the reasoner will need the concept or idea of non-demonstrative argument, justification, or reason. My suggestion is that the idea of necessary connection is just this idea, but after the projective error has been committed and it has been applied to objects. There are a number of important issues that I would like to address, but which will have to wait for another occasion. I’ll mention one in closing. The suggestion that necessary connection does in fact play a role in causal inference imposes constraints on how we are to interpret Hume’s discussion in 1.3.14. There, Hume identifies the source of our idea of necessary connection as an impression of reflection, and he further characterizes it as a determination of mind. In explaining what Hume could have meant by this, we need to keep in mind that the resulting idea of necessary connection must play an important role in much of the probable reasoning we undertake.