Causation and Liability in Tort Law

DOI: http://dx.doi.org/10.5235/20403313.5.2.217 (2014) 5(2) Jurisprudence 217–243

ARTICLES

Causation and Liability in Tort Law

Desmond M Clarke*

For the !rst time in our legal history, persons are made liable for damage even though they may not have caused it at all, simply because they have materially contributed to the risk of causing that damage.1

The theory of tortious liability has long assumed as a general principle that, in the words of Lord Bridge, ‘the law, which only Parliament can change, requires proof of fault causing damage as the basis of liability in tort’.2 Evidently, the mere fact that someone’s behaviour affects another causally is not enough to imply liability; there are further questions about the kind of behaviour involved, the de!nition of ‘harm’ or its legal equivalents, and the agent’s prior knowledge of the likely con-sequences of their behaviour.3 Assuming satisfactory resolutions of those issues, it was widely accepted as a necessary condition for a successful action that an alleged tortfeasor’s behaviour caused the injury or damage that constituted a tort. That implies that, when a court is asked to decide if a litigant has satis!ed that condition, it must assume answers to two interrelated questions: (1) what is meant by causation or a causal relation? (2) what evidence is suf!cient to conclude that a causal relation obtains between a defendant and the injury for which they are held legally liable? An answer to the second question depends on what is said or assumed in reply to the !rst, and any dif!culty encountered in explaining what is meant by a cause

* Emeritus Professor of Philosophy, University College Cork and Member of the Royal Irish Academy. I am grateful to the Irish Jurisprudence Society, which provided an opportunity to present an early version of this paper.

1 Barker v Corus (UK) plc [2006] 2 AC 572 (HL), 616 per Baroness Hale.2 Wilsher v Essex Health Authority [1988] 1 AC 1074 (HL), 1092 (emphasis added). 3 Tony Honoré, Responsibility and Fault (Hart Publishing, 2002) 101: ‘In tort law, the issue is not …

“what caused this harm?” but “Did the fact that the defendant behaved in a certain unlawful or undue risk-creating way cause it?”’; Peter Cane, Responsibility in Law and Morality (Hart Publishing, 2002) 133: ‘Causes link legally proscribed conduct with legally proscribed outcomes.’

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will be re"ected in specifying the kind of evidence that supports a judgement of causation.4

One might try to avoid both questions by claiming that the law does not assume any concept or theory of causation, and that the guiding concept in tort cases is that of legal responsibility—which, in turn, may be treated as if it were a primitive concept. According to this account, which is a form of legal ascriptivism, human agents are held legally liable for certain kinds of exemplary actions (including neg-ligence), and their liability for damages in other cases is decided by comparison or contrast with paradigm cases. In other words, the law does not borrow or mimic some independent concept of causation, from ordinary language or elsewhere; instead, it deploys a distinctive concept of legal responsibility in such a way that the concept of causation could be translated out completely without loss from the language of torts.

There is a good reason, however, which applies generally to all judgements of value or law, for not adopting that suggestion. If differential moral or legal judge-ments are to avoid being arbitrary or sliding into an in!nite regress, they must correspond to the presence or absence of some speci!able natural (ie non-moral or non-legal) properties in the realities to which they are applied. For example, if one action is said to be morally good or lawful while another is not, it must be because the former possesses some property or feature that is absent in the latter. Otherwise, the distinction is based on a further moral or legal distinction, and one embarks on a regress—to escape from which one would have to identify eventually the natural properties on which the normative judgements rest. This necessary dependence of moral/legal valuations on independent natural properties is usually called ‘super-venience’. When applied to tort law, it means that it would be arbitrary to attribute legal responsibility to an agent in one case and not in another unless there is some relevant natural difference between the two cases which supports the differential judgements, and the involvement or otherwise of agents in causing or producing an injury is one of the relevant factors on which a legal judgment is supervenient.5 We hold agents responsible for the effects of their conduct because, in some sense to be speci!ed, the effects result from their behaviour. If causation were not inde-pendent of legal or moral liability, it would be a mere tautology to say that someone should be held liable for an injury because they caused it.

Once words such as ‘causing’, ‘producing’ or their synonyms are used, it follows that courts will be required to decide if the relevant relation (whatever it is) obtained between a given agent and a speci!c injury. Unfortunately, there are deep, widespread, and apparently irreconcilable disagreements among philosophers about the meaning of ‘causation’ or ‘causal relation’ or, for those who are less metaphysically reticent, about the ontology of the reality to which those

4 James Woodward, Making Things Happen: A Theory of Causal Explanation (Oxford University Press, 2003) 23 also addresses this issue of the interdependence between the meaning of a causal explana-tion and the evidence that could support it.

5 Michael S Moore summarises this argument in Causation and Responsibility: An Essay in Law, Morals, and Metaphysics (Oxford University Press, 2009) 4–5, 448.

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words apply.6 Courts, however, especially in the United Kingdom, have sometimes adopted an attitude of philosophical agnosticism by claiming that, in order to decide tort cases, they need only an ordinary-language concept of causation with which all competent users of the English language are equipped. Accordingly Lord Hoffmann, in Empress Car Co (Abertillery) Ltd v National Rivers Authority, quoted with approval Alphacell Ltd v Woodward: ‘what or who has caused a certain event to occur is essentially a practical question of fact which can best be answered by ordinary common sense rather than by abstract metaphysical theory.’7

This re"ects the account of legal causation adopted in Hart and Honoré’s well-known Causation and the Law: ‘the assertion often made by the courts, especially in England, that it is the plain man’s notions of causation (and not the philosopher’s or the scientist’s) with which the law is concerned, seems to us to be true.’8 Hart and Honoré claimed that ‘causal judgments … are not speci!cally legal. They appeal to a notion which is part of everyday life and which ordinary people, including jury-men, can handle with a minimum of guidance.’9 According to this account, the law shares a concept of causation with descriptions made by non-professionals and it is expressed adequately in non-technical language—a view that was characteristic of the ‘ordinary language’ philosophy that was prevalent in Oxford at that time.10 This theory of language assumed that one could deploy a concept successfully with-out being able to de!ne its usage (depending on how ‘de!ne’ is understood). Hart and Honoré argued accordingly that those who practise the law of torts enjoy an extensive degree of linguistic freedom to use the term ‘cause’ without having to explain what it means.

Despite such apparently comforting defences against intrusions from philos-ophy or science, however, the choice presented by many tort cases today is not between common sense and metaphysics or science, but between an explicit meta-physics and one that is implicit and unexamined, where the latter is disguised as common sense. It is especially clear that the philosophical agnosticism of the courts is merely simulated when they apply a unique criterion for testing the presence of causation—namely the ‘but for’ test—and then claim to have avoided metaphysics (as if it were a disreputable discipline, like magic or astrology) by describing an implicit philosophical choice as common sense.

6 The extent of the disagreement is illustrated in collections such as H Beebee, C Hitchcock and P Menzies (eds), The Oxford Handbook of Causation (Oxford University Press, 2009) and E Sosa and M Tooley (eds), Causation (Oxford University Press, 1993), and in a wide range of monographs which support alternative interpretations of causation.

7 [1992] 2 AC 22, [1972] AC 824, 827. It is not clear what ‘essentially’ and ‘ordinary’ mean in this context. Lord MacFadyen summarised the alternatives available to the courts as ‘a common sense rather than a philosophical or scienti!c approach to causation’ in McDonald or Cross v Highlands and Islands Enterprise [2000] Scot CS 307, para 117. The "ight from philosophy to common sense and pragmatism was endorsed in L Khoury, Uncertain Causation in Medical Liability (Hart Publishing, 2006) 14–15.

8 Causation and the Law (Clarendon Press, 2nd edn 1985) 1. See also xxiv: ‘As we pointed out [in the !rst edition], courts have continually claimed that it is the ordinary man’s conception of cause that is used by the law and enters into various forms of legal responsibility.’

9 Hart and Honoré (n 8) lv.10 Tony Honoré has con!rmed his intuitions about ordinary language concepts of causation in Respon-

sibility and Fault (Hart Publishing, 2002) 5. For a critique, see Cane (n 3) 129.

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The more plausible alternative is to acknowledge the incontrovertible implications of the conceptual history of causation, namely that this concept is theory-laden, as Norwood Hansen argued many years ago, and that the meaning of the term varies with the conceptual frameworks in which it is used.11 When the Greek words that are often translated as ‘cause’ (aitia or aitios) were popularised by Aristotle, they ‘originally ascribed responsibility with a view to fault-!nding or giving credit’.12 Nonetheless, even Aristotle conceded that the Greek term that cor-responds approximately to ‘cause’ could be used in at least four different senses, each of which functioned in alternative kinds of explanation of the same event.13 The subsequent conceptual history of the term ‘cause’ has signi!cantly extended the complexity of its disparate uses, to such an extent that it is more plausible today to acknowledge that it means many different things than to assume that it has a unique meaning about which almost no one agrees.14

The signi!cance of appealing to the conceptual history of terms can be illus-trated by comparing the words ‘cause’ and ‘mind’. They have similarly long and complex histories, both in their Greek origins and in their subsequent varied uses. It is presumably obvious that we cannot understand psychiatric illnesses today, or attempt to cure them, simply by referring to the way in which the term ‘mind’ is used in ordinary English. Likewise, we cannot understand adequately the in"uence of an agent’s behaviour on the occurrence or otherwise of phenomena that are classi!ed as injuries by using a concept of causation that is borrowed from ordinary usage.

The assumed transparency or theory-independence of the common-sense con-cept of causation also !ts poorly with the fact that the concept is deployed in tort law to express both normative and factual judgements. As in Aristotle’s original usage, we not only link the behaviour of individuals with certain consequences of their acts or omissions, but we also hold the agents responsible for such conse-quences. This involves applying to a discrete slice of behavioural history two distinct kinds of interpretation, one factual and the other normative, so that discussions of legal causation may oscillate confusingly between them. These interpretative frame-works have been distinguished by the American Law Institute under the rubrics (a) ‘factual causation’ and (b) ‘scope of liability for consequences of breach’.15 While the former pertains to the claim that some damage or injury is an objective conse-quence of the agent’s behaviour, the latter applies a theory of legal responsibility to that consequential relation by holding agents worthy of blame or credit for conse-quences that are speci!ed by the legal norms applied.

11 See NR Hanson, Patterns of Discovery (Cambridge University Press, 1961) 50–69. This is a familiar thesis in the history of science, in which the meaning of words such as ‘force’ (vis in Latin) varies from one theory to another while occupying similar roles in each.

12 Sarah Broadie, ‘The Ancient Greeks’ in The Oxford Handbook of Causation (n 6) 21.13 That it is possible to have more than one cause or explanation for a single event had been acknowl-

edged earlier in Plato’s Phaedo (96–99); the fact that Socrates was in jail could be explained by at least two causes.

14 See Nancy Cartwright, ‘Causation: One Word, Many Things’ (2004) 71 Philosophy of Science 805. 15 For the rationale, see Jane Stapleton, ‘Choosing What We Mean by “Causation” in the Law’ (2008)

73 Missouri Law Review 433, 458.


In the following discussion, I concentrate almost exclusively on (a), the so-called factual causation by which injuries or damage are linked to an agent’s behaviour in a way that is suf!ciently objective to support judgments of legal liability. ‘Factual’ in this context contrasts with ‘normative’ rather than with ‘theoretical’ for, as I argue below, there is no sense in which causal relations are observable, and any claim that something is causally related to something else is a theoretical claim about a factual reality.16 Different causal claims may involve varying degrees of theoreticity, but even the most spontaneous and intuitively uncontentious causal judgements involve an implicit reference to the explanatory patterns in nature to which we appeal when we say that some event or phenomenon is part of the causal history of another.

One of the preliminary questions, then, in any tort case is whether some agent is or is not part of the causal history of an event or condition that is classi!ed (by a rel-evant legal norm) as an injury. This can be answered, in what I call common-sense examples, on the basis of inductive generalisations. If A kills B by shooting him, there is enough experiential evidence to support a causal connection between the shooting and the subsequent death without further ado. However, when courts use scienti!c or medical evidence to decide a tort case, this is no longer the case. They must then depend on the account of explanation that is adopted by those theories, and it is a fundamental mistake to attempt to impose on scienti!c explanations a conceptually incompatible and confused concept of causation that is borrowed from common-sense causal judgements. The conclusion for which I argue is that courts cannot use scienti!c evidence in tort cases without depending on the theo-ries within which that evidence is understood and con!rmed. Since such theories long ago abandoned the conceptual framework of common-sense causation, it is a fundamental mistake in tort decisions that rely on scienti!c or medical evidence to apply the ‘but for’ test as an appropriate criterion of factual causation. The two are conceptually incommensurable.

The discussion is divided into three sections: (1) common-sense judgements of causation, which analyses how such judgements are made and the evidence required to support them; (2) causation and scienti!c theories, in which the mod-ern scienti!c equivalent of causal relations in outlined; and (3) causation and legal responsibility, which reviews the implications for tortious liability of the law’s use of scienti!c evidence.

1. COMMON-SENSE JUDGEMENTS OF CAUSATION

When commentators analyse what is meant by ‘causation’ or related terms, they invariably re"ect on familiar examples of causation that are assumed to be indis-putable. This apparently innocent starting point, however, becomes a fatal mistake

16 Hart and Honoré (n 8) 92 argue that ‘cause’ is similar to ‘table’ insofar as there are undisputed cases where each term applies, and there is a penumbra of other cases in which the reference is less clear. They conclude mistakenly that questions about what caused something are independent of any theory and are mere ‘questions of fact’.

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if one then assumes that the examples chosen may be used as models for all genu-ine cases of causation. As already indicated, the conceptual history of the term ‘cause’ shows that it has been understood in many different ways, and there is no reason to assume a priori that an analysis of the language used to describe causal interactions between macroscopic bodies can provide an understanding of causa-tion in all other cases. One might anticipate, instead, that Skyrms’ comment—that our common-sense concept of cause is ‘an amiably confused jumble of theories’—applies not only to causation in quantum mechanics but much more widely.17 The mistake of assuming that all causes must resemble one’s chosen paradigms cannot be avoided by describing as ‘metaphysics’ a linguistic analysis based on a limited number of familiar examples—as if metaphysics did not depend on experience or escaped from experiential control by the level of abstractness to which it aspires. With that important proviso about the scope of their subsequent application, there are some useful lessons for tort law in the ways in which we makes inferences about causation from evidence available to any competent observer of familiar natural events.

The literature on causation often contrasts sequences of events that are accepted as genuine examples of causation with others that are not accepted as such. If one person shoots another and the latter dies, it is accepted that the shooter (or their action) caused the death of the victim. In contrast, if independent, accurate clocks show the same time, we do not conclude that they are causally related. Genuine causal connections, in contrast with mere constant conjunctions, are said to involve a counterfactual dependence between a putative cause and its effect such that, in Hume’s words, ‘if the !rst object had not been, the second never had existed’.18 In the example of contiguous clocks, if one clock stops or if someone modi!es it, the other usually continues to function as before. This counterfactual feature is expressed in the familiar ‘but for’ test. Supporters of this test disagree about whether it de!nes what is meant by causation or, as Stapleton argues, whether it merely provides an ‘algorithm’ for recognising genuine causal connections.19 In either case, it is neces-sary to identify the evidence that supports the corresponding counterfactual claim, and to explain why that evidence supports a conclusion about a causal connection.

The popularity and apparent theoretical neutrality of the ‘but for’ test result from one very famous attempt to explain how we acquire the concept of causation and what is implied by its deployment to describe sequences of events—that of David Hume in the eighteenth century. While many of Hume’s detailed arguments (and the empiricist epistemology that inspired them) may be ignored here, one element of his analysis captured the intuition that there is something apparently necessary about the connection between causes and their effects that is absent in

17 Quoted by Richard Healey, ‘Causation in Quantum Mechanics’ in The Oxford Handbook of Causation (n 6) 684.

18 David Hume, An Enquiry Concerning Human Understanding and Other Writings, S Buckle (ed) (Cam-bridge University Press, 2007) 70 (emphasis in original).

19 Stapleton (n 15) 471 ff. In Fairchild v Glenhaven, Lord Bingham opined that if the traditional ‘but for’ test were set aside, ‘an employer may be held liable for damage he has not caused’: Fairchild v Glenhaven [2003] 1 AC 32, 67. That suggests a very close but unspeci!ed connection between the test and the concept of causation.


other sequences of events. There are connotations of necessity in what are usually called (a) the suf!cient or (b) the necessary conditions for the occurrence of a given event or phenomenon.20

First, there are connotations of necessity in the assumption that, once a cause is in place, the related effect necessarily follows. For that reason, a cause has often been understood as the set of conditions that are suf!cient for the occurrence of the associated effect; it is impossible for a cause, as such, not to give rise to its effect.21 Secondly, there are connotations of necessity in counterfactual claims about the conditions that are necessary for the occurrence of an event, when one says: had C not occurred, E would not have occurred, or: but for C, E would not have occurred.22 Both types of modal claim draw attention to one of the questions addressed by Hume: how could we know that one (or more) events or factors are necessarily linked with the occurrence of another event, either as necessary or suf-!cient conditions, when the necessity of the connection cannot be observed?

In reply to this query, Hume argued that the only evidence available to the most acute observers is the constant conjunction, within our experience, of types of event. In simple and familiar examples, we notice that every time events of a certain type occur (which are then labelled c-events), they are followed by events of type e (so-called effects), and we never experience any exception (when a c occurs, but not an e). Of course, this latter comment is not always true. We sometimes observe exceptions, but when they occur we either provide an explanation that protects our belief in an underlying constant conjunction or we assume that it is possible to provide such. We then conclude, from the experience of the nearly universal co-occurrence of c and e and from the temporal order in which they occur, that one is causally related to the other. For Hume, constant conjunction and the temporal ordering of events provide the (only) evidence for their causal relationship.23

It is not clear how this could support a counterfactual claim that a particular C was a necessary condition for the occurrence of a given E. One cannot argue validly that, since all c-events result in e-events, and there was an e-event, therefore there

20 There is no suggestion in the choice of words used here that the kind of entity that may be a cause or effect is limited to events. One could accept Hugh Mellor’s classi!cation of causes and effects into two general types: (1) individuals (which includes people, animals, and any entity that functions as an individual); and (2) facts, or states of affairs that are described by true statements. See DH Mellor, The Facts of Causation (Routledge, 1995) 7–10. ‘Event or phenomenon’ in the sentence above is a place-holder for anything that may function as a cause or effect.

21 The necessity with which the relevant effect follows from the presence of suf!cient conditions also inspired the occasionalist theory of causality in the 17th century. Its proponents argued that, since no natural event ever follows necessarily from any combination of prior natural events, the only genuine cause is God, and all apparent natural causes are merely the occasions on which God exer-cises uniquely ef!cacious causal powers. See S Nadler, Occasionalism (Oxford University Press, 2011); W Ott, Causation and Laws of Nature in Early Modern Philosophy (Oxford University Press, 2009).

22 Throughout I use capital letters to denote speci!c events or phenomena, and I use lower-case italics, such as c, for variables. In some cases, I write ‘c-events’ to emphasise that we are talking of types of event.

23 Alternatively, in the analysis favoured by some interpreters of Hume, we de!ne an observed constant conjunction of c and e as a ‘causal relationship’ and thereby compensate for the unobservability of necessity by replacing it with something that is observable. See Helen Beebee, Hume on Causation (Routledge, 2006) and, for a realist interpretation, Galen Strawson, The Secret Connexion: Causation, Realism and David Hume (Clarendon Press, 1992).

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must have been a c-event. That would involve the well-known fallacy of af!rming the consequent. To claim that C was a necessary condition for the occurrence of E involves imagining various possible worlds that are different from the one that is actually observed, and in which C does not occur. We then ask: would E have occurred in any of those possible worlds? Unless we also exclude from those imag-ined worlds anything else that could cause E, it would be invalid to conclude that C is necessary for the occurrence of E; indeed, to make that exclusion is to assume the necessity of C for the occurrence of E—which is precisely what one is trying to establish! For example, even if we observe that discarded cigarettes are involved in the occurrence of many forest !res, we cannot conclude validly that a !re that would be otherwise indistinguishable from a given !re could not have been caused by a lighted match or any of an inde!nitely large number of other possible sources of !re.24 That suggests that any claim that C is a necessary condition for E must include at least the implicit condition: in relevantly similar circumstances.25 If the cir-cumstances were suf!ciently different, however, it would be entirely possible for E to occur without C and the constant conjunction of c and e in the past would be irrelevant to deciding whether something entirely different, for example C1, caused E.

There is therefore an inference involved—rather than the observance of a mat-ter of fact—in describing something as a necessary condition for the occurrence of something else when events or phenomena of the relevant types have been observed to occur in a constant conjunction, even in cases where both are observed. In many tort cases, however, we have evidence only of some injury or damage, E, and there is an evidential question of identifying some factors, C1, C2, etc, that are not observ-able now and are to be hypothesised as possible causal factors. For example, we see a house that has burned to the ground, and we try to !nd necessary conditions that are relevant to a claim for damages. The inferences required here involve at least the following steps:

(i) we are familiar with a number of scenarios, each of which is supported by a Humean constant conjunction; for example, house !res often occur following a spark from a !re, an electrical short-circuit, a bolt of lightning, etc;

(ii) we examine which (if any) scenario best matches the case before us;

(iii) the evidence in a particular case excludes all of these possible scenarios except an electrical short-circuit;

24 The word ‘indistinguishable’ camou"ages very intractable problems associated with describing effects for which a causal explanation is sought. If enough detail is included in the description of a particular effect, eg a !re that occurred at a speci!c time and place, and had the property of having caused fear in a given observer, no two !res would ever be suf!ciently similar to support a Humean generalisation. However, if we allow enough latitude in our descriptions to generate a Humean induction, we thereby camou"age the dif!culties involved in making inferences about necessary conditions. See eg Bas C van Fraassen, The Scienti!c Image (Clarendon Press, 1980) 116–26.

25 Any phrase such as ‘relevantly similar’ merely defers rather than solves the problems raised here. See David Lewis’s attempts to provide a semantics for counterfactual propositions, by reference to possible worlds that are suf!ciently similar to our world, in J Collins et al (eds), Causation and Coun-terfactuals (MIT Press, 2004) 20–30.


(iv) it was therefore probably the case that an electrical short-circuit occurred, and we know from experience that, when many other conditions are satis!ed, short-circuits are constantly conjoined with !res. Hence, a short-circuit was probably one of the conditions necessary for this !re.

The probabilistic character of the inference does not compromise its validity, since it is the only evidence available for identifying a cause of the !re.

Thus the counterfactual claim that, for example, a particular !re would not have occurred but for a short-circuit rests, as Hume argued, on a comparison between the effect that was observed in this case and the relevant circumstances in which it occurred, and what has been observed to occur in other relevantly similar cases of which we have experience. Although we need not know what would have happened in different circumstances, we can say that E would not have occurred in these conditions had C not occurred, only because we have observed c and e occur-ring together in the past in similar circumstances. It follows from this analysis that every instance of identifying a necessary condition for some phenomenon or event involves an inference of some kind, even if the inference is so spontaneous that we are unaware of making it.

One of the most signi!cant conclusions of this analysis is that the ‘but for’ test fails to satisfy an essential requirement for any test, namely that it provide an independent method of verifying or con!rming what one wishes to test.26 In judge-ments of causation, we want to know if some constant conjunction between types of event or phenomenon is or is not evidence of a genuine causal connection between them. However, when one asks if something would not have occurred ‘but for’ the occurrence of something else, this question does not introduce or uncover any new or independent evidence to help answer that question. There is no other evidence available, in any given situation in which we wish to identify a necessary condition for some event’s occurrence, apart from what has already been mentioned above, namely: observed patterns of the concurrence of types of event, and evidence to the effect that the instant case matches one of those patterns. The so-called ‘but for’ test, therefore, is not a test at all, but merely a fanciful way of stating what we may infer when we compare the instant case with what we know from experience. It is not a criterion that may be applied independently of that evidence and, by asking the question in this way, no new con!rming evidence is uncovered.

Finally, it follows from Hume’s analysis that we do not have to understand the precise mechanism by which C is related to E in order to conclude that one is a cause of the other. In fact, the history of science suggests that we often notice the conjunction of two types of event or phenomenon, and that we construct a theory to explain the apparent connection between them only subsequently (eg the tides and the motion of the moon). Nonetheless, unless one de!nes causation merely in terms of a constant conjunction of c-events and e-events, something further is

26 For example, although a litmus test for the relative acidity of a solution depends on the chemistry of the dye used to make litmus papers, the test functions as such because the change in colour alone provides evidence for the conclusion. We do not infer the change of colour from some other evi-dence; the change of colour is directly observable and implies the conclusion about acidity.

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required to convert a constant conjunction between events into a genuinely causal connection.27

Unfortunately, one necessary condition is never enough for the occurrence of any event E; E occurs only when all the conditions are satis!ed that together are suf!cient for its occurrence. Therefore, if one asks what was necessary for the occur-rence of a given event, there is no non-arbitrary way to restrict the list of necessary conditions to what happened in the recent past, and one slides imperceptibly into what David Lewis called ‘countless distinct, converging causal chains’.28 As long as causes are understood as necessary conditions, therefore, there are inde!nitely many events that are genuinely causes of any given phenomenon because they are all equally necessary for its occurrence.29

Despite this conclusion, it is often assumed that it is possible to distinguish between a cause of some event and a mere condition for its occurrence. Thus, in one of the examples used by Hart and Honoré, we believe (on scienti!c evi-dence) that a hay barn would not have gone on !re had there not been suf!cient oxygen present in its immediate environment. An adequate supply of oxygen, therefore, is a necessary condition for the event described by the phrase ‘the hay barn burned’.30 Likewise, in cases of workplace bullying, it is obvious that had a particular employee not been employed by their employer, the former could not have been bullied by the latter; being employed is a logically necessary condition for the event described as workplace bullying. Even more strikingly, in the example used by David Lewis, someone’s birth is a necessary condition of their death. Those who defend a distinction between genuine causes and mere necessary conditions wish to reject oxygen as a relevant cause of a particular !re and, a fortiori, to reject the employment contract between an employee and their employer as a cause of his/her being bullied by their employer. It sounds even less in keeping with com-mon usage to say that someone’s death was caused by their birth.

Nonetheless, all these necessary conditions are as relevant to explaining the events in question as any other necessary condition, although they are not the ones that we consider when assigning legal responsibility for !res, deaths, or workplace injuries to employees. That suggests that normative considerations are deployed to !lter, among all the conditions that are necessary for the occurrence of some event, those that are relevant to legal responsibility, and that the word ‘cause’ is then used

27 Hume’s answer was that the extra ingredient was a psychological factor, namely, our expectation that an e-event would follow the occurrence of a c-event. While Hume was correct not to look in the natural world for the extra feature, I argue below that the application of the concept ‘cause’ to a situation involves applying to it an explanatory scheme, and that the necessity implicit in the concept of causation results from the explanation that it implies.

28 D Lewis, ‘Causal Explanation’ in D-H Ruben (ed), Explanation (Oxford University Press, 1993) 182.29 C Wells and O Quick raise the same objection in Reconstructing Criminal Law (Cambridge University

Press, 4th edn 2010) 101: ‘The straightforward “but for” test of causation … does not offer a satisfac-tory basis for selecting the legally relevant cause, for it will identify the very multiplicity of causes which is the problem.’

30 It is worth noting here that no one could observe this necessary condition by simply looking at burning barns. Lavoisier’s theoretical and experimental work was required even to have the concept of oxygen, and to recognise its role in anything that burns. Hart and Honoré’s example does not illustrate a commonsense perception, but a scienti!c conclusion.


with both normative and factual connotations. Without such a normative !lter, however, there is no objective fact of the matter that distinguishes mere conditions from what are usually described as causes.31

The ‘but for’ test (or, as I argue, the evidence about constant conjunctions on which it is parasitic) is much too inclusive because all conditions that are necessary for the occurrence of any event satisfy it equally easily. The test is also too exclusive, because it fails in the case of familiar counterexamples of simultaneous over-deter-mination, where two or more sets of conditions are separately suf!cient (when combined with other conditions) to result in some effect, so that neither one alone can be said to be necessary for its occurrence.32 For example, two independent brush !res approach a house, and each is suf!cient (together with other condi-tions) to result in its destruction. In that case, it cannot be said of either !re: had that not occurred, the house would not have been destroyed. Therefore, if the ‘but for’ test were applied strictly as a criterion of causation, neither brush !re could be said to cause the destruction of a house that has actually burned to the ground.

Since there are often completely different sets of conditions that are inde-pendently suf!cient for the occurrence of any event E, what initially appears as a necessary condition for E’s occurrence is such only in some of the possible sets of conditions in which it occurs. This suggested, to some of those who tried to de!ne causation, that a cause is one necessary condition among a number of fac-tors which, together, are suf!cient but not necessary for the occurrence of the effect in question.33 However, this combination is subject to the kinds of objec-tion raised against both necessary and suf!cient conditions. No matter what fac-tor one chooses as a necessary condition, it may not in fact be necessary because a suf!ciently similar effect could have resulted from a different set of conditions. Secondly, the conditions that would be suf!cient for the occurrence of any event are inde!nitely numerous and usually unspeci!able. While it is often said (as a kind of shorthand) that a brush !re was a suf!cient condition for the destruction of a house, it is obvious that many other conditions must be satis!ed before a house can be destroyed, and they are all included implicitly (or are covered by a vague phrase such as ceteris paribus) in the claim that some speci!c conditions are suf!cient for the occurrence of a particular event. Thirdly, in some probabilistic explanations in modern science, no set of conditions is ever suf!cient to guarantee a particular effect.

31 P Lipton provided a persuasive critique of the attempt by Hart and Honoré to identify non-normative criteria by which mere necessary conditions may be distinguished from genuine causes. See ‘Cau-sation Outside the Law’ in H Gross and R Harrison (eds), Jurisprudence: Cambridge Essays (Oxford University Press, 1992) 127–48. See also Wells and Quick (n 29) 101.

32 In Fairchild v Glenhaven [2003] 1 AC 32 at 44, Lord Bingham quoted with approval the acknowledge-ment by Mason CJ in the High Court of Australia, in March v E & MH Stramare Pty Ltd [1991] 171 CLR 506, 508, of the counter-intuitive result of applying the ‘but for’ test as an exclusive test of causa-tion.

33 J Mackie, ‘Causes and Conditions’ in Sosa and Tooley (n 6) 34: ‘the so-called cause is, and is known to be, an insuf!cient but necessary part of a condition which is itself unnecessary but suf!cient for the result.’ This is summarised as an INUS condition. Honoré endorses a slightly different account ((n 10) 98–120), the so-called NESS analysis (a necessary element of a suf!cient set).

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The objections to the ‘but for’ test of causation are so persuasive that it is dif-!cult to understand why the common law tradition continues to invoke it, apart from the authority it allegedly acquired by the frequency of its use. Lord Hoff-mann commented, in Gregg v Scott, that the traditional test was con!rmed by ‘a good deal of authority’.34 That suggestion, however, seems to be fundamentally mistaken, because no test or concept of factual causation can acquire legal author-ity, no more than any other factual or theoretical assumption that is borrowed by the law from other disciplines. To assume otherwise is similar to the theological faux pas of assuming biblical authority for the ordinary-language description of the sun moving around the earth. Just as the Bible may assume astronomical descrip-tions without endorsing them, legal practitioners may assume current theories of causation without endorsing them legally. There are many alternative theories of causation available, and it falls outside the competence of judges to decide matters that belong to other disciplines. Indeed, the acknowledged limits of judges’ com-petence may partly explain the reluctance of law lords in some earlier judgments to engage with so-called metaphysical or scienti!c disputes.

For the same reason, it cannot be a conclusion of this discussion that there is some correct interpretation of factual causation favoured by this author that the courts should adopt. The opposite is the case. Courts need expert testimony from witnesses to decide if some act or omission ‘caused’ an injury, to which they then apply speci!cally legal theories of responsibility and liability attribution that fall within the disciplinary competence of lawyers. If the experts’ evidence misleads the courts or if it turns out, in retrospect, to have been mistaken, the courts can do no better than accept the best evidence presented to them.

Of course courts do not need expert evidence in many common-sense torts because the causal link between the act or omission of the tortfeasor and the injury for which they are held liable is undisputed. Even in those cases, however, the ‘but for’ test is nothing more than a legal !ction, because its application uncovers no independent evidence of genuine causal connections. As long as the phrase ‘close enough’ remains unde!ned, any reference to what would happen in other possible worlds that are close enough to our world to provide a reliable comparator merely disguises the logic of our inferences. It seems as if the courts have come close to recognising the ineliminable defects of the ‘but for’ test on some occasions, but they seem to have baulked at the implications of that realisation and defended their continued use of the test by substituting a legal concept of causation that is de!ned in terms of liability as the decisive factor in their judgment (in that case, is the ‘but for’ test free-wheeling?).35

If the ‘but for’ test were abandoned either as a genuine test or as a universally applicable de!nition of factual causation, that would not mean that we should look for some other agreed, unique analysis of what is meant by the ordinary language

34 Gregg v Scott [2005] 2 AC 176 (HL), 197. Daniel Kahneman, Thinking, Fast and Slow (Allen Lane, 2011) 217 suggests another reason (‘a powerful professional culture’) for the reluctance of profes-sional groups to acknowledge and correct illusions of skill and validity, when ‘they are sustained by a community of like-minded believers’.

35 This was Lord Hoffmann’s way out in Fairchild, which is cited below (n 64).


concept of a cause. While some commentators continue to believe that it must be possible to complete this analytical task satisfactorily, others accept that the enter-prise is doomed to failure. I shall assume that a unique and agreed analysis of ‘cause’ is both unnecessary and impossible, and I mention two of the reasons why.36

One reason is that ‘causation’ is not a name for a speci!c natural relation that obtains in the real world (such as a spatial relation or an electrical relation); instead, it names a wide variety of relations that obtain only within the propositional world or in our language about the world.37 We apply an explanatory framework to various realities that we observe—admittedly, a very primitive framework of induc-tive generalisations in Hume’s examples—and, within that framework, there are necessary relations between our descriptions of various events or phenomena. Once an explanatory scheme is applied to the occurrence of some phenomenon, some features of the natural world are called causes because of the explanatory role that the corresponding concepts or descriptions occupy in our language.

For that reason, contra Moore, there is no reason why omissions or non-events cannot be called causes.38 By naming them thus, we do not claim that non-real events exercise a mysterious agency in the actual world; rather, the descriptions of omissions exercise an explanatory role in our discourse about the world. Helen Beebee has also argued that absences or omissions cannot be causes, because she adopts the theory that causation is a relation between events.39 She accordingly shares Moore’s misgivings about the metaphysical status of negative events and fur-thermore argues that, if events of that type were accepted, a counterfactual analysis of causation would imply an inde!nitely large number of causes for any effect that is said to be caused by a negative event. For example, it might be said that the failure of a gardener to water a plant caused it to die. However, it is also true of eve-ryone for whom providing water was possible that, had they not failed to do so, the plant would have survived. That implies that if the failure of a speci!c gardener to water a plant could be the cause of its dying, then the discrete failures of everyone else in the universe (including President Obama) to provide water would be its cause for the same reason. According to Beebee, that would multiply unreasonably the real causes of the plant’s death.

This objection assumes that causation is a relation between discrete events. However, our scienti!c theory of why plants die does not discriminate between inde!nitely many omissions as discrete events that could explain the death of a plant. Our theory of dying plants suggests only that an absence of water (among other factors) may explain the death of plants, rather than discrete events associ-

36 Cf P Lipton, Inference to the Best Explanation (Routledge, 2nd edn 2004) 31.37 Hume also transfers the necessity of causal connections to our thoughts in the Enquiry (n 18) 70, but

for a different reason: ‘When we say, therefore, that one object is connected with another, we mean only that they have acquired a connexion in our thought, and give rise to this inference, by which they become proofs of each other’s existence.’

38 Moore (n 5) 54–55. Stephen Mumford endorses Moore’s denial of causation to non-events in ‘Causes for Laws’ (2013) 4 Jurisprudence 109, at 110: ‘Moore is perfectly right to say that omissions are not causes.’

39 ‘Causing and Nothingness’ in Collins et al (n 25) 291.

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ated with many agents who could have irrigated them. Likewise, if an evacuated sealed container collapses, one explanatory factor is the absence of air (or any other material) in the chamber rather than innumerable agents who failed to !ll it. Missing air did not do anything (and, if air particles could be individuated, no identi!able volume of air was actively missing), nor do we explain the container’s collapse by reference to an inde!nitely long list of omissions by those who could have supplied air. Our theory of air pressure, the strength of the container’s struc-ture, etc explain readily why the absence of air is one factor in the container’s collapse. The proliferation of causes in Beebee’s objection results from the unre-strained multiplication of values of the single variable, water.

According to this interpretation, therefore, there are as many kinds of cause as there are different kinds of explanation, and any factor that features in a viable explanation (including a factor with a zero value) may be called a cause. That is one reason why it is impossible in principle to provide a unique de!nition or crite-rion for recognising causes.

Another reason for abandoning efforts to de!ne uniquely the ordinary lan-guage concept of cause is that we almost never look for what David Lewis calls a ‘maximal true answer’ when we ask a ‘why question’; that is, we never expect an answer that includes all relevant correct information.40 We invariably seek only part of that maximal set, namely the part that !lls in some gap in our current informa-tion or understanding. Another way of saying the same thing is that many questions involve an implicit or explicit comparison or foil.41 If we ask why leaves turn yellow in autumn, the implicit alternatives may include changing colour at some other time of the year or turning some other colour, rather than exploding or turning into !re"ies. If the question is ambiguous, it can be clari!ed by rephrasing: eg why do leaves turn yellow in autumn rather than in summer? The scope of the original question is thereby narrowed, and the range of possible explanations is conse-quently limited. We never ask for a complete causal history of any particular event, going all the way back to the Big Bang. Rather, we assume a complex, lengthy, and unspeci!ed causal history, and we request information about just some features of that history that would resolve our immediate quandary.

The interim conclusion from this discussion is that so-called common-sense judgements of causation are made by applying to our experience an explanatory framework that is known by induction from observations of (relatively) constant conjunctions between observable events. We can never observe causes, as such, nor can we observe relations of causation. Rather, when asked to identify what caused some phenomenon, we infer from our experience of constant conjunctions, and from the evidence of what happened in a given case, that (i) one inductively con-!rmed pattern of events rather than another is likely to be relevant to this case; (ii) within that pattern, the presence or absence of some condition(s) rather than others correlates with the results observed. Secondly, the inde!nitely large number of conditions that are relevant in principle to a causal account of any event or phe-

40 Lewis (n 28) 196.41 P Lipton, ‘Contrastive Explanation’ in Ruben (n 28) 207–27.


nomenon is usually narrowed by the interests of the questioner or by the context in which one asks why something happened. Most importantly, no new information or relevant evidence to support judgements of causation is uncovered by applying the ‘but for’ test. We may imagine anything we wish about other possible worlds, but all the available evidence for factual causation is available only in this world of our limited, actual experience.

2. CAUSATION AND SCIENTIFIC THEORIES

Bertrand Russell adverted, almost a century ago, to a profound conceptual and methodological change that occurred during the Scienti!c Revolution.

The law of causality, I believe, like much that passes muster among philosophers, is a relic of a bygone age, surviving, like the monarchy, only because it is erroneously sup-posed to do no harm.42

One of the de!ning features of the new sciences was a fundamental change in our understanding of explanation, as a result of which the concept of a cause became redundant and was replaced by scienti!c laws and theories. Accordingly, the term ‘cause’ disappeared completely from the languages in which such laws and theo-ries are expressed. That may seem like a paradoxical statement, or even one that is obviously false. However, it can be con!rmed easily by consulting any scienti!c discipline and by checking if the concept of a cause appears as part of its laws or theories. Even in cases with which non-scientists are familiar, such as Boyle’s Law, the concepts that are used within that law are those of pressure, volume and abso-lute temperature.43 The same total abandonment of causal terms is apparent in the social sciences. For example, the quantity equation of exchange in economics expresses a relation of equality between (a) the quantity of money in a given econ-omy multiplied by the average number of times it changes hands in a year, and (b) the total quantity of goods and services created and sold in that year multiplied by the average price of those goods.44

The laws or theories that assume the explanatory role in modern science that was previously attributed to causes are characterised by four features that are rel-evant to their use as evidence in tort cases:

(i) Scienti!c theories typically identify a number of variables (ie linguistic items) that denote theoretical entities (ie the corresponding features of the real

42 Mysticism and Logic (Longmans, Green, 1918) 180. The absence of causes from modern scienti!c theories was also discussed by Hanson (n 11) 51–52.

43 Boyle’s Law is: PV = KT, where K is a constant. It is also true that there is no term to denote causes in the theory of gases that provides a theoretical explanation of Boyle’s Law.

44 This is often expressed as MV = PQ, in which M is the quantity of money, V the average number of times it changed hands, Q the quantity of goods/services and P their average value or price. On the interpretation of this ‘law’ see I Fisher, Elementary Principles of Economics (Macmillan, 1919). For the use of theoretical terms in the language of psychology, see G Mandler and W Kessen, The Language of Psychology (Wiley, 1962).

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world), and they specify (usually in a mathematical formula) the relationship(s) between those variables or between changes in their values. The relations between the variables are rarely expressed by a simple equality sign.

(ii) The second feature of scienti!c theories, which is equally relevant to evidence invoked in some tort cases, is the theoretical (in contrast with observational) character of many of their key concepts. The semantic status of such theoreti-cal terms, their relationship to observation terms, and the extent to which they acquire some connection with reality through the experimental application of a theory, have all been familiar issues in the philosophy of science for many decades.45

(iii) Thirdly, scienti!c theories are irredeemably hypothetical. This results from the indirect manner in which theories are con!rmed by the evidence that sup-ports them. It is universally acknowledged that any scienti!c theory is accepted conditionally, and that it remains permanently open to amendment or, at the limit, to refutation or replacement.46

(iv) Finally, the laws that relate the values of variables in many scienti!c theories are probabilistic, and the inferences that they support provide nothing more than calculations of an increase or decrease in the probability of some event occurring.47

Thus scientists construct models of relationships that obtain between the values of parameters that are characteristic of the theories used in their own sub-disciplines. These parameters are identi!ed, for the most part, by theoretical terms, and the relations between changes in the values of parameters are unavoidably hypotheti-cal and probabilistic. In brief, a massive conceptual and epistemic shift occurred during the Scienti!c Revolution in the seventeenth century, as a result of which the explanatory functions that were attributed to causes in Greek philosophy and in the tradition that resulted from it have been assumed by scienti!c laws and theo-ries.48 The fundamental logic of theory construction was transformed because it was no longer limited to observation and induction, which characterised the work of earlier natural scientists. In the language proposed by the American pragmatist

45 Although the theoretical character of the concepts used in modern science is almost universally agreed, there is less agreement about whether such terms should be understood realistically or instrumentally. Van Fraassen (n 24) argues for the latter, while realism is defended by W Sellars, Science, Perception and Reality (Routledge, 1963), and Lipton (n 36). The choice between realism and various forms of instrumentalism is irrelevant to the argument here, since tort law could operate equally well even if scienti!c theories were understood instrumentally as mere predicting devices.

46 This is summarised in D Clarke, ‘Hypotheses’ in D Clarke and C Wilson (eds), The Oxford Handbook of Philosophy in Early Modern Europe (Oxford University Press, 2nd edn 2013) 249–71.

47 For an early discussion of probabilistic explanations, see CG Hempel, Aspects of Scienti!c Explanation (Free Press, 1970); for a recent analysis of causation in probabilistic contexts, see Judea Pearl, Causal-ity: Models, Reasoning, and Inference (Cambridge University Press, 2nd edn 2009).

48 J Milton, ‘Laws of Nature’ in D Garber and M Ayers (eds), The Cambridge History of Seventeenth-Century Philosophy (Cambridge University Press, 1998) I, 680–701; T Schmaltz, ‘From Causes to Laws’ in Clarke and Wilson (n 46) 32–50.


Charles Sanders Peirce, the new sciences relied on epistemically risky retroductive inferences that alone made possible the postulation of the theoretical entities on which their success depended.49

Consequently, when tort decisions rely on scienti!c evidence, it makes no sense to request a categorical answer to a question framed as follows: had C not occurred, would E not have occurred? There is no reliable procedure for mapping a ‘but for’ question onto a scienti!c explanation of some event, partly because of the number of variables involved, and partly because their values do not vary independently of each other. In scienti!c explanations, in contrast with imagined and simpli!ed pos-sible worlds, we cannot freeze the values of all the variables involved, assume their reciprocal independence, and then infer a categorical answer to a question such as: had condition C not been satis!ed (or had its value been otherwise), would E not have occurred?

In summary, all the relations that we can discover between various factors that are denoted by the terms of a scienti!c theory are mediated by the theory itself. Does that mean that we cannot identify any causes by using modern science? That depends on what is meant by a ‘cause’. If one seeks a scienti!c explanation of some event or phenomenon, it comes at the price of surrendering the simplifying expectations about causation that result from failing to acknowledge its conceptual history. Scienti!c theories are concerned with idealised event-types or models rather than with speci!c historical events. Accordingly, if one requests a scienti!c expla-nation of a particular event—such as an injury to or the death of a person—any plausible answer will involve applying one or more scienti!c theories to the event in question. In contrast with the kind of common-sense explanations discussed in Section 1, where we !t some observed event into a pattern of constant conjunctions between observed phenomena, in scienti!c explanations we map a theoretical model onto the phenomenon to be explained. The explanation is likely to have the form: assuming that various conditions were satis!ed in this case, and assuming the validity of the theories on which we rely (including the theories on which the scienti!c instruments used are based), one can correlate the values (including zero value or complete absence) of speci!ed variables with the observed result, with a given degree of probability.50 This is the best we can do—a hypothetical, probabilis-tic inference couched in all the quali!cations that follow logically from the manner in which the theories that we apply were discovered and con!rmed.

49 The magnitude and signi!cance of this change is explored in E McMullin, ‘The Impact of Newton’s Principia on the Philosophy of Science’ (2001) 68 Philosophy of Science 279. Hume failed to recognise that the explanatory terms used in scienti!c theories are not derived from sensations, and that the con!rmation of theories is not limited to induction over observable objects and events.

50 This is illustrated by the way in which uncontested evidence was expressed in Hanrahan v Merck Sharp and Dohme [1988] ILRM 629, 642, in which the Irish Supreme Court, on appeal, accepted that there was a causal connection between the plaintiff’s ill-health and emissions from the defendant’s facility: ‘I would say that if it is shown that fumes, dusts, vapours, chemicals are present in the botanical life or animal life in the area, and if appropriate environmental meteorology in the evidence is compat-ible, and if it is shown that materials, acids, vapours are emanating from a source contiguous to the farm, then the balance of probabilities very much favours (the conclusion) that his lung disease can be attributed to a toxic substance’ (emphasis added).

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Thus, when scienti!c evidence and its associated theories are required to explain how some injury occurred, there is no sense in asking whether someone or some factor was ‘the cause’ of the injury, unless ‘the cause’ means ‘a factor in the most plausible explanation’.51 The issue remains, of course, of determining how to assign legal or moral responsibility for events or phenomena that are thus explained scienti!cally. That involves applying another theory—a normative theory about legal or moral responsibility.

3. CAUSATION AND LEGAL RESPONSIBILITY

As acknowledged above, decisions in tort law involve both factual and normative judgements, which presuppose two signi!cantly distinct kinds of theory. One type of theory is invoked (implicitly or otherwise) to decide whether an appropriate explanatory relation obtains between the descriptions of two or more events or phe-nomena in the natural world (typically, the action or inaction of a human agent and an injury to someone else). Secondly, decisions in tort law presuppose a legal theory of responsibility according to which an injury is attributed to a given agent. Human agents (or legal entities) are expected to anticipate the likely consequences of their own actions/inactions within limits, and society then holds them legally responsi-ble for those consequences (if various conditions are satis!ed).52 In other words, a theory of legal responsibility, with deep roots in the history of ethics, is applied to human actions as they are implemented in the physical world.

Assuming the intentionality (in the relevant sense) of some agent’s behaviour, it is relatively easy to map a theory of legal responsibility onto simple cases of ‘fac-tual causation’ that depend on inductive generalisations of observed events. If an agent sets !re to a house with a "amethrower or kills another with a blow of an axe, a familiar Humean conjunction (between a type of action and a type of sub-sequent injury or damage) supports a causal inference to which a theory of legal responsibility is then applied. Whoever wields the "amethrower or the axe is held responsible.

This model is less useful, however, in torts based on negligence. Even in familiar examples where an injury occurs on a building site—for example, when a falling hammer hits a worker and injures him/her physically—the causal connection between the impact of the hammer and the worker’s injury may be uncontrover-sial, but the connection between various safety procedures that the employer failed to observe and the worker’s injury is much less so. In such cases, it is impossible to conclude validly—simply by applying the ‘but for’ test—that the injury to the

51 Kahneman (n 34) distinguishes the psychology and epistemic validity of two ways of thinking, with-out denying that each has a distinctive use. In a similar way, the argument here is that spontaneous commonsense judgements of causation are valid within the limited scope of what is observable and known pre-theoretically, while explanatory judgements that rely on scienti!c or medical evidence depend on the theories within which such explanations are provided.

52 RA Duff, Answering for Crime: Responsibility and Liability in the Criminal Law (Hart Publishing, 2nd edn 2009) 19–36 offers an analysis of responsibility as a triadic relationship of answerability of someone, to someone else, in virtue of a normative rule.


worker would not have occurred had the employer complied with the law or, in other words, that a negligent omission by the employer was a necessary condition for the occurrence of the injury. Nonetheless, it may be possible to show that there is a statistical correlation between (a) a failure to observe legally required safety reg-ulations and (b) the frequency with which similar injuries to construction workers occur, and thus (c) that there is an inductive basis for linking employer negligence and employee injuries. The employer, if judged to be legally liable, is held respon-sible for failing to observe safety regulations which, had they been observed, would have reduced the probability of injuries to employees. Even if all the safety regula-tions had been observed, however, there is no evidence to show that their strict observance would have prevented all injuries; therefore, their non-observance (or the employer’s negligence in implementing them) cannot be a necessary condition for a speci!c instance of workplace injury.

This issue of mapping a theory of legal responsibility onto an alleged connec-tion between a speci!c instance of human behaviour and a given injury becomes much more problematic if a decision about that connection relies on scienti!c evidence. In that case, the only evidence that could conceivably link an employer’s omissions and an employee’s injury is a theory (or, usually, more than one theory), according to which a wide range of interdependent variables, with speci!c values, are likely to explain an injury of a certain kind. It would therefore be impossible in such cases to prove that, had an employer not failed to act as legally required, the employee would not have suffered that type of injury. Since it is impossible, it would be unreasonable for the law to require it.

When scienti!c or medical evidence is involved, therefore, our best relevant theories suggest, not a simple Humean conjunction between omissions on the part of employers and the occurrence of certain injuries, but that the behaviour of employers is one factor that is relevant to explaining the occurrence of the injury, if many other conditions are also satis!ed. Agents are held legally responsible for knowingly changing, or negligently failing to guarantee, the value of at least one variable that is (a) subject to their reasonable control and (b) involved in the scien-ti!c explanation of an injury. This shows how misleading it is to think of ‘the cause’ of some injury as a set of suf!cient conditions for its occurrence (or a necessary condition within that set) or to look for some ‘mechanism’ (often with connota-tions of being picturable or observable in principle) that links a ‘cause’ with a spe-ci!c event or phenomenon. Such are not provided by scienti!c theories.

The link between a breach of duty and a subsequent injury was examined in McGhee v National Coal Board, in which employees claimed that the failure of their employer to provide facilities for washing off kiln dust ‘caused’ their subse-quent injury.53 In that case, Lord Reid rejected the distinction between ‘materially increasing the risk that the disease will occur and making a material contribution to its occurrence’, while relying for support on what he called ‘a broad and practical viewpoint’ and a concept of causation that ‘is based on the practical way in which the ordinary man’s mind works in the everyday affairs of life’.54 This amounted to

53 [1973] 1 WLR 1.54 Ibid, 5.

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exploiting the "exibility of the ordinary-language concept of a cause to accommo-date intuitions about what justice required in this case. Lord Salmon came closer to the understanding of causation that is defended above: ‘A factor, by itself, may not be suf!cient to cause injury but if, with other factors, it materially contributes to causing injury, it is clearly a cause of injury.’55 It would have helped subsequent tort decisions if the law lords had acknowledged that one factor is never a suf!cient con-dition for the occurrence of any event or injury; that all factors that may constitute legal causes are no more than contributing factors; that, apart from judgements of factual causation that are supported by observed constant conjunctions, the only evidence that could possibly link any factor (or its omission) with the occurrence of an injury is the best medical or scienti!c theory that is currently available; and that any evidence which relies on the application of medical or scienti!c theories cannot provide anything more than an estimate of the probability of an injury’s occurrence.

The evidential task of linking the actions or omissions of one agent with an injury to another appears even more challenging when the injury involved is psy-chiatric or mental. Walker v Northumberland County Council accepted that the duty of employers is not limited to protecting employees from physical injury:

Whereas the law on the extent of this duty [ie that of an employer] has developed almost exclusively in cases involving physical injury to the employee as distinct from injury to his mental health, there is no logical reason why risk of psychiatric damage should be excluded from the scope of an employer’s duty of care or from the co-extensive implied term in the contract of employment.56

Since the decision in Walker was concerned primarily with the foreseeability of the injury rather than its psychiatric character, there is independent authority for clas-sifying psychiatric or psychological illnesses as an ‘injury’ in McDonald or Cross v Highlands & Islands Enterprise: ‘foreseeable psychiatric harm and foreseeable physi-cal harm can be seen as two sub-categories of the general category of foreseeable harm … and it seems to me to be reasonable to ask if there is any logical reason for treating the sub-categories differently from each other.’57 In fact, there is no rea-son to exclude an injury from the scope of tort law merely because it is classi!ed as psychiatric or psychological. It would amount to a discredited kind of metaphysical dualism—the kind of poor metaphysics that is often described as common sense—to assume that physical and mental injuries are radically different in kind, and it would imply an unjusti!able and narrow materialism to limit genuine injuries to observable physical injuries, such as broken limbs.58

55 Ibid, 11.56 [1995] I All ER 737, [1994] EWHC QB 2, 9.57 [2000] Scot CS 307, para 60. See also Page v Smith [1996] AC 155 (HL), 181, where Lord Browne-

Wilkinson acknowledged that ‘medical science has now advanced so far that … in certain circumstances, a defendant can be liable for illness or injury, whether psychiatric or physical … without any direct physical impact on, or injury to, the limbs or organs of the plaintiff’.

58 McDonald also rejected the claim that employers are responsible only for psychiatric injury that ‘takes the form of nervous shock, ie a sudden assault on the nervous system’. Lord MacFadyen explained at para 63: ‘If that submission were sound, it would mean that an employer who knew without doubt


Once psychiatric injuries are accepted as genuine and subject to tort law, novel evidential issues arise about con!rming (a) the fact that an injury has occurred, and (b) the causal connection between the injury and the behaviour of an alleged tortfeasor. It should be both obvious and acceptable that any evidence about the occurrence of a psychiatric condition depends on applying one or more theories to the behaviour (including the linguistic behaviour) of someone who claims to have been injured. There is no sound epistemological reason for general scepticism about accepting such evidence. Most of what we know about the world, especially in contexts that are central to our security, is mediated by theories: for example, theories about how nuclear generating stations operate, how planes arrive at their destination, and how the drugs that are prescribed daily to millions of people are unlikely to harm most of them. The apparent stumbling block for those involved in tort law, therefore, is not the epistemological status of evidence for the occurrence of a psychological injury, but (b) above, the evidence that supports a causal con-nection between the behaviour of an alleged tortfeasor and a speci!c psychiatric or psychological illness.

This issue was addressed in Hatton v Sutherland, in which a number of employ-ers appealed against a !nding of liability for employees’ psychiatric illnesses that were allegedly ‘caused’ by stress at work.59 In her judgment, Lady Hale clari!ed the relevant law in England and Wales in sixteen ‘practical propositions’. One proposi-tion was that, even if an employer’s breach of duty is established and the employee subsequently suffers from a ‘mental illness’, the ‘claimant must show that the breach of duty has caused or materially contributed to the harm suffered. It is not enough to show that occupational stress has caused the harm.’60 Proposition (15) of the summary addressed situations in which there may have been more than one ‘cause’ involved in explaining an injury: ‘Where the harm suffered has more than one cause, the employer should only pay for that proportion of the harm suffered which is attributable to his wrongdoing unless the harm is truly indivisible.’61 These propositions were guided by a simple picture, in which the connection between a cause and its effect is almost mechanical and observable—so that two causes, like two hunters, could act independently and jointly—and that picture is then applied to an alleged case of negligence. As already indicated, such a picture fails to !t any possible connection between omissions by employers and injuries to employees.

The requirement that a claimant identify a single cause of an injury among numerous instances of employer negligence (understood as competing possible ‘causes’) reappeared in Fairchild v Glenhaven.62 Whereas in McGhee the two pos-sible ‘causes’ of injury in dispute were individuated temporally—the employer’s

that the working conditions in which he required an employee to operate were so stressful that it was objectively likely that, over time, the employee would succumb to psychiatric illness, and who nevertheless continued to subject his employee to those conditions despite growing signs that he was developing such psychiatric illness, would incur no liability for the loss and damage suffered by the employee as a result of developing the psychiatric illness.’

59 [2002] 2 All ER 1.60 Ibid, 20.61 Ibid, 20.62 Fairchild v Glenhaven [2002] 1 WLR 1052 (CA).

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breach of duty in respect of kiln dust before and after departure from the same workplace—the alternative potential ‘causes’ in Fairchild were the negligence of different employers in respect of a single employee. Fairchild thus exempli!ed a number of the de!ciencies identi!ed in the ‘but for’ test. One was the over-deter-mination of possible ‘causes’ discussed above—that there were in place a number of distinct instances of employer negligence, each of which was equally suf!cient (if other conditions had been satis!ed) to have resulted in the injury. Fairchild also exempli!ed the epistemological challenge of attempting to show that an injury would not have occurred if a given instance of negligence had not occurred. Even if the claimants had worked for only one employer, it would have been impossi-ble to show that they would not have contracted mesothelioma had that employer not been negligent. As in the example of a hammer falling on a building site, the connection between negligence and subsequent injury can be established only by probabilistic theories about the factors relevant to the occurrence of mesothelioma.

Nonetheless, the Court of Appeal applied the ‘but for’ test to the available sci-enti!c evidence. It accepted that exposure to asbestos dust is usually one relevant factor in the development of mesothelioma, and that the initiation of the disease probably requires only a very small number of asbestos !bres that it is possible to ingest in a short period (in other words, a long period of ingesting asbestos !bres is unnecessary). The appellants had worked for a number of employers who had exposed them to similar risks from asbestos dust, and there was no evidence to show that their illness commenced during one speci!c instance of exposure rather than another. The traditional ‘but for’ test required the appellants to establish, as a condition of !nding any speci!c employer liable, that the negligent exposure to asbestos dust by that particular employer was a necessary condition of the injury. Since this could not be established, the court concluded that it had not been shown of any speci!c employer ‘that the particular tortfeasor caused the disease’.63

On appeal, however, the House of Lords unanimously reversed this judgment because it seemed unfair that, although the injury to each appellant probably developed during a period when only one of their employers was negligent, it was impossible to identify the relevant negligent employer from the evidence available, partly because there was a long interval between the initial ingestion of !bres and the appearance of symptoms of the disease. Lord Hoffmann concluded that the relevant issue in the appeal was the law’s understanding of the causal connection required to attribute liability to one of the employers, even though ‘medical sci-ence cannot prove whose asbestos is more likely than not to have produced the cell mutation which causes the disease’.64 That assumed that the ‘but for’ test was still the appropriate way to identify the factual cause of the illness, but in certain cases (of which this was one) the legal threshold for holding someone responsible may be satis!ed even when the evidence available makes it impossible to apply that test successfully. Baroness Hale subsequently concluded, in Barker, that the lords had severed the traditional connection between liability and causation (but only if the latter is understood exclusively in terms of the ‘but for’ test): ‘For the !rst time in

63 Ibid, 1081.64 [2003] 1 AC 32, 74 (emphasis added).


our legal history, persons are made liable for damage even though they may not have caused it at all, simply because they have materially contributed to the risk of causing that damage.’65 However, the lords did not even agree on what they had decided in Fairchild. Lord Rodger commented in Barker that the analysis offered by his fellow law lords involved ‘not so much reinterpreting as rewriting the key deci-sions in McGhee … and Fairchild’.66 The reluctance of the lords to abandon the ‘but for’ test and to generalise as a principle that ‘damages be awarded in all cases in which the defendant may have caused an injury and has increased the likelihood of the injury being suffered’ was underlined by Lord Hoffmann in Gregg.67

Barker v Corus compounded the confusions already implicit in earlier judg-ments. Lord Hoffmann accepted that a successful claimant must prove

on a balance of probability that the defendant’s conduct did cause the damage in the sense that it would not otherwise have happened [ie must satisfy the ‘but for’ test] … the state of scienti!c knowledge about the mechanism by which asbestos !bres cause mesothelioma did not enable any claimant who had been exposed to more than one signi!cant source of asbestos to satisfy this test.68

If the lords had understood correctly what kind of relation it is possible to establish, by using a scienti!c theory, between exposure to asbestos !bres and the onset of mesothelioma, they would not have identi!ed the ‘state of scienti!c knowledge’ as the reason why the claimants’ case failed the test for factual causation. It failed because the lords applied the wrong test, and did not understand that scienti!c theories explain events without using the ‘but for’ test to identify a cause.

This failure was con!rmed by Lord Scott’s re"ection in Barker that, in Fairch-ild, ‘liability was not imposed on any of the defendant employers on the ground that the employer’s breach of duty had caused the mesothelioma … That causative link had not been proved against any of them.’69 He concluded that liability was imposed because each employer, ‘by his breach of duty, had materially increased the risk that the employee would contract mesothelioma’. However, on any plau-sible interpretation of modern scienti!c theories, the most one can ever conclude about any employer’s negligence is that it increases the probability of the injuries that the prohibition of such negligence is designed to reduce. Lord Rodger came close to acknowledging this in Barker (depending on how one understands the word ‘equate’): ‘exposing the claimant … to the risk of injury is in this situation equated with causing his injury.’70

None of these critical comments about how appeal courts decide cases that rely on scienti!c evidence implies that their decisions could be as quantitative as the scienti!c theories on which they rely. As acknowledged above, tort decisions also

65 Barker v Corus (UK) Ltd [2006] 2 AC 572 (HL), 616.66 Ibid, 601.67 [2005] 2 AC 176 (HL), 197. The liability of those who tortiously exposed others to asbestos !bres in

the disputed circumstances that applied in Fairchild has since been settled by s 3 of the Compensa-tion Act 2006.

68 [2006] 2 AC 572 (HL), 579–80.69 Ibid, 595–6.70 Ibid, 611.

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rely necessarily on a theory of legal liability, which remains as unscienti!c as it has always been.71 Nonetheless, the common-sense character of our intuitions about justice is no excuse for rejecting the explanatory resources of modern science where they are relevant, and an explicit acknowledgement of the latter may help clarify the former. Indeed, the factors that usually determine whether someone is liable for the consequences of their conduct include their knowledge at the time of acting and the reasonableness of a society’s expectations that they possess the type or degree of knowledge that would be required to foresee those consequences. In cases where the explanatory relation between an action and its consequences pre-supposes knowledge of a scienti!c theory, therefore, it seems less consistent with traditional theories of liability to hold someone liable in retrospect for an injury that could not have been foreseen by reasonably well-informed agents at the time of their alleged tortious conduct, either because the required theory was not avail-able at the time of acting or because it would have been unreasonable to demand that degree of expertise of the agents involved.

For similar reasons, judges are not expected or required to be informed about all the scienti!c theories on which they rely in their judgments nor, a fortiori, to choose from among competing theories when scienti!c experts disagree. The bur-den of establishing a causal connection between an injury and a speci!c agent rests squarely on the plaintiff; if there are alternative theories that link or fail to link allegedly tortious behaviour with an injury, the courts are in the same epistemic situation as they encounter with all cases of con"icting expert evidence. An appeal to the ‘but for’ rule in those circumstances is neither appropriate nor helpful. They have no option but to follow the expert evidence and, if that is inconclusive, the plaintiff’s case fails.

CONCLUSION

The concept of causation that is embedded in the law of torts has been extrapolated to breaking point from simple pictures of common-sense examples of causation, such as a hunter shooting an arrow into the body of an injured plaintiff. Some court decisions and legal theorists combine those simple pictures with a belief in deter-minism, which re"ects presuppositions of classical physics rather than the degree of uncertainty that characterises modern scienti!c or medical theories. However, there is no scienti!c basis or legal justi!cation for such a belief. Stapleton writes, as if Heisenberg and twentieth-century science had never existed: ‘though the world is deterministic, proof of its phenomena may have to resort to probabilistic evi-dence,’ and Lord Hoffmann opined in Gregg v Scott (in a more metaphysical style) that ‘everything has a determinate cause, even if we do not know what it is’.72 Such

71 I am grateful to the anonymous referees for raising the question explicitly of how to apply a relatively imprecise or commonsense concept of responsibility to an inference about explanatory factors that depend on a scienti!c theory, although an adequate discussion would require another, even longer article.

72 Stapleton, ‘Causation in the Law’ in The Oxford Handbook (n 6) 750; Gregg v Scott [2005] 2 AC 176 (HL), 196, which was repeated by Lord Scott in Barker v Corus [2006] 2 AC 572 (HL) at 597. Lord


conceptually limited starting points lead one to expect that there is a ‘determi-nate cause’ of every effect, that identifying who or what caused something is simply ‘a practical question of fact’, and that there must be some kind of mechanism involved in the relation between cause and effect, even if we often fail to discover it.

None of these expectations is remotely adequate to the characteristic explana-tions provided by scienti!c theories. Even in the simple cases often used to illustrate Hume’s analysis—such as one billiard ball striking another—although the motion of billiard balls and their impact on each other is observable, the causal connec-tion by which we link events before and after the impact cannot be observed. To describe the interaction of billiard balls as ‘causal’ is to !t a particular event into a pattern that is inductively con!rmed, and thus to explain the motions of the billiard balls as exemplifying what we conclude is a pattern in nature.

This kind of causal or explanatory interpretation of observable macro-events is transformed almost beyond recognition when events or phenomena are explained by reference to modern medical/scienti!c theories, in which the concept of a cause is replaced by laws of nature and theoretical models. The explanatory connections that are provided by such theories fail to exemplify the intuitions that are implicit in the common-sense concept of a cause. From the point of view of explaining why some event occurred, therefore, it makes no sense in a scienti!c theory to identify one factor as ‘the cause’ of some other event or condition or as the only operative explanatory factor.

It follows that legal phrases such as ‘caused or materially contributed to’ are seriously misleading, because of the ambiguity of the disjunctive ‘or’. The law lords used ‘or’ as an exclusive disjunction, thereby implying two alternatives (the second of which is wider in scope), whereas the two disjuncts should be understood as synonyms. Any plausible understanding of what is involved in scienti!c explana-tions implies that every factor that is included in the relevant theory plays a role that had been described in pre-seventeenth-century language as causal. The same applies to the distinction discussed by Lord Reid in McGhee, ‘between materially increasing the risk that the disease will occur and making a material contribution to its occurrence’.73 Lord Reid accepted that it might be possible to make such a distinction ‘where our knowledge of all the material factors is complete’ or that such a distinction might be defended in ‘logic or philosophy’. However, it is clear that, apart from very simple (idealised) cases in which the explanatory factors avail-able are both few in number and independent of each other, there is no distinction between materially increasing the risk that some event will occur and causing it to occur. They are conceptually indistinguishable. Likewise, in the epigraph quoted

Hoffmann also expressed the following equally unsupported metaphysical view in Gregg v Scott [2005] 2 AC 176 (HL) at 196: ‘There is no inherent uncertainty about what caused something to happen in the past or about whether something which happened in the past will cause something to happen in the future. Everything is determined by causality.’ Stapleton implies a similar conviction when she writes, in ‘Causation in the Law’ at 745: ‘the world is out there, seamless and rolling along manifesting what we call the physical laws of nature in complex con"uence and combinations’, and she refers to the ‘!xed laws of nature’ (750). In this case, an assumption of pre-modern physics is converted into metaphysics.

73 [1973] 1 WLR 1 (HL), 5.

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above from Baroness Hale, when scienti!c evidence is used to decide a tort case, there is no distinction between causing an injury and materially contributing to the risk of its occurrence.74

Consequently, the issue to be addressed by a court in tort cases that rely on scienti!c evidence should not be whether a plaintiff can prove a counterfactual dependence of an injury on the behaviour of the defendant (including their neg-ligent failure to act). The issue should be whether, given the relationship between variables that are identi!ed by an appropriate and well-con!rmed scienti!c theory, a particular agent is held legally liable when: (1) an injury has actually occurred;75 and (2) the agent has changed the values of any of those variables in a manner that is likely—given our current theories—to result in injury to the plaintiff; and (3) the likelihood of such injuries could be foreseen by reasonably informed agents.

This changes the focus of decisions about legal liability from an insoluble search for a causal nexus to society’s expectations of acceptable behaviour on the part of reasonably informed agents. For example, in the kind of injury discussed in Hatton, the law could set a standard of care that is legally required of employ-ers, while current psychiatric and medical theory would provide the probabilistic connections between various kinds of employer behaviour (including omissions) and speci!c types of injury to employees, including psychiatric injuries. Once the evidence establishes that an employee has been injured and that, for example, the employer has been negligent (as judged by current legislation), it should be suf!cient to establish, with a speci!c degree of probability, that our current best theories include negligence of that type in the explanation of that kind of injury.76 It cannot be further demanded that a plaintiff prove that a particular injury would not have occurred if the employer had not been negligent. The reason why is that it is impossible to do so, and the ‘law does not require a pursuer or plaintiff to prove the impossible’.77

There are many examples in the history of ideas where an apparently sensi-ble question—the answer to which seemed elusive—was recognised eventually as unanswerable.78 The law of torts should consider whether it sets a standard that

74 Cf JG Fleming, ‘Probabilistic Causation in Tort Law’ (1989) 68 Canadian Bar Review 661, which links probabilistic evidence with the causal indeterminacy of defendants and plaintiffs and with ‘modern scienti!c epistemology’ (662).

75 Rothwell v Chemical & Insulating Co Ltd [2008] 1 AC 281 (HL) offered different views about what constitutes an injury. While Lord Hoffmann opined (at 293) that the occurrence of pleural plaques did not constitute ‘damage’, Lord Hope (at 297) accepted that it was not an abuse of language ‘to describe pleural plaques as an injury’. Lord Scott (at 305) seemed to use the terms ‘damage’, ‘harm’ and ‘injury’ as synonyms. However, the decision rested not on ‘how medical experts may classify the condition in question’, but on whether the law recognised pleural plaques as actionable.

76 The phrase ‘on the balance of probabilities’ is inadequate to resolve this issue; one needs a more exact speci!cation of the degree of probability required to identify what were traditionally called causal factors.

77 Lord Simon in McGhee [1973] 1 WLR 1, 8.78 For example, the history of modern dynamics made a signi!cant leap forward when scientists

stopped trying to answer the question ‘why does an arrow in "ight continue to move in space?’. No explanation is required for its continued motion, although a change in its velocity or direction would require an explanation.


is inadequately understood and impossible to satisfy, especially in cases that rely on scienti!c evidence, if it requires plaintiffs to satisfy the ‘but for’ test in order to prove that the behaviour of a defendant ‘caused’ their injuries. It should be suf!cient to show that there is a theory-based explanatory relation between the behaviour and the injury, since this relation is the modern equivalent of what Aris-totle called causation.

There is no suggestion here that the courts should adopt a particular philo-sophical theory from among those favoured by disputing philosophers, since courts acknowledge that they lack jurisdiction or competence to resolve philosophical or scienti!c disputes. The point is rather that courts cannot rely on scienti!c or medical evidence without accepting, if only conditionally, the theories on which that evidence is based. If ‘causal’ is understood as ‘explanatory’, then courts may accept as many kinds of causal relation as there are plausible scienti!c theories; and where experts disagree about the scienti!c explanation of some event or phe-nomenon, the courts cannot resolve the disagreement by substituting, in place of the evidence available, their own speculations about what would have happened in other possible worlds. Therefore, apart from what I have called common-sense cases, the question whether some factor was ‘causally’ involved in (ie was part of the explanation of) the occurrence of some phenomenon is a scienti!c question that is answered, if at all, by the relevant experts.

It follows from this conclusion that, once scienti!c or medical evidence is rel-evant to explain the occurrence of an injury, decisions about factual causation must be based on expert evidence rather than on any ‘algorithm’ or test that had been applied traditionally by courts with limited success to common-sense cases. And since the burden of proof is on the plaintiff, if the experts disagree the case will be lost, but not because the plaintiff fails to satisfy the ‘but for’ test.

Documents

Causation and Liability in Tort Law