The Possibility of "Inference Causation": Inferring Cause-in-Fact and the Nature of Legal Fact-Finding

McGill Law Journal ~ Revue de droit de McGill

THE POSSIBILITY OF INFERENCE CAUSATION:
INFERRING CAUSE-IN-FACT AND THE NATURE OF

LEGAL FACT-FINDING

Russell Brown*

This article defends what it refers to as
inference causation: a fact-finders drawing of
a causal link between a defendants actions and
a plaintiffs suffering in tort claims in the ab-
sence of expert scientific evidence.

This type of reasoning, affirmed in 1990 by
Justice Sopinka in the Supreme Court of Can-
ada decision, Snell v. Farrell, has encountered
significant academic criticism. The author de-
fends inference causation by considering evi-
dence theory. First, he shows that inference
causation forms a part of laws veritismits
commitment to the truthsince legal fact-
findings aim is always to seek out the best ob-
tainable truth, rather than the absolute truth.
Second, he critiques the primacy of scientific
evidence by showing that both its reasoning
process and the nature of its conclusions are dif-
ferent from those of legal fact-finding. Last, the
author shows that all fact-findingparticularly
all legal fact-findingis already inferential.
Scientific evidence forms but one of many dif-
ferent elements that are analyzed by fact-
finders in their inference about which factual
account of the disputed events is the best ac-
count. Accordingly, where none is available, the
same inference of fact is nonetheless possible.

Cet article dfend

l infrence de
causalit , le fait dtablir un lien de causalit
entre les actes du dfendeur et le prjudice subi
par le demandeur en labsence de preuve
scientifique dexpert dans
le cadre dune
poursuite en responsabilit extracontractuelle.

Ce type de raisonnement, approuv en
1990 par le juge Sopinka dans larrt Snell c.
Farrell de la Cour suprme du Canada, a depuis
t vivement critiqu par la doctrine. Lauteur
le dfend par une tude de la thorie de la
preuve. Dabord, il dmontre que linfrence de
causalit sinscrit dans la mission de recherche
de la vrit du droit, puisque le but de la
recherche juridique des faits est de parvenir
aussi prs de la vrit que possible et non
datteindre la vrit absolue. Ensuite, il critique
la primaut de la preuve scientifique puisque sa
mthode et
la nature de ses conclusions
diffrent de celles de la recherche juridique des
faits. Finalement, lauteur dmontre que la
constatation des
la
constatation juridique des faits, inclut dj
linfrence. La preuve scientifique nest quun
lment parmi plusieurs que le juge des faits
analyse pour dduire le meilleur compte rendu
possible des faits en litige. Ainsi, quand aucune
preuve scientifique ne peut tre tablie, il est
nanmoins lgitime de recourir linfrence.

faits, en particulier

* Associate Professor, Faculty of Law, University of Alberta. I am indebted to Duane Sza-
fron, Professor of Science at University of Alberta, for an illuminating discussion of sci-
entific fact-finding. I also thank two anonymous peer reviewers, Allan Beever, Vaughan
Black, David Cheifetz, Ted DeCoste, Lindsey Ehrman, Lewis Klar, Moin Yahya, and
Bruce Ziff for their helpful comments. I am also grateful for the comments of partici-
pants at the Perspectives on Causation conference, University of Aberdeen, June
2009.

Citation: (2010) 55 McGill L.J. 1 ~ Rfrence: (2010) 55 R.D. McGill 1

Russell Brown 2010

(2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

2

Introduction

I.

The Jurisprudence and the Objections
A. Inference Causations Place in Canadian Jurisprudence
B. Anticipating Objections and Looking to Evidence Theory

II.

Veritism

III.

Scientific Fact-Finding
A. Differentiating Scientific Fact-Finding from Legal Fact-Finding
B. The Generality of Scientific Fact-Finding and the

Specificity of Legal Fact-Finding

IV.

The Inferential Quality of Legal Fact-Finding
A. Infusing Evidence Theory into Causal Inference
B. The Structure of Inference Causation

3

4
4
8

13

18
18

23

30
30
35

Conclusion

45

 THE POSSIBILITY OF INFERENCE CAUSATION

3

Introduction
My aim in this paper is to justify what I shall refer to as inference
causation in the tort of negligence. At stake is the proposition that,
where scientific expert evidence is unable to verify a causal link between
a defendants creation of risk and a plaintiffs suffering, a legal fact-finder
may infer such a link.
My account of inference causation will avoid the well-worn paths of
causation theory, drawing instead from theoretical considerations of evi-
dence and of its role in the fact-finding process. Canadian legal tradition
has not embraced (or given much consideration to) a theory of evidence as
it pertains to legal fact-finding.1 Moreover, tort lawyers have shown no in-
terest in exploring how evidence theory might relate to inference causa-
tion, perhaps because evidence merely provides the basis for finding or
not finding cause-in-fact. Such indifference is, however, misplaced. Such a
convention does not, for example, explain trial phenomena.2 More to the
point, it does not account for the uncertain threshold of civil proof that
equates mere probability with certainty, nor the processes of logical rea-
soning that legal fact-finders bring to a trial. In short, it fails to account
for the distinction between the nature of cause-in-fact, and what is re-
quired to prove cause-in-fact. My rationale, then, for introducing theoreti-
cal evidence scholarship to the debate within tort law about inference cau-
sation is that evidence theory takes this distinction seriously by explicitly
accounting for the standard of proof and the epistemology of legal fact-
finding. It therefore injects an air of adjudicative reality into the discourse
on inference causation.

The express genesis of inference causation3 within Canadian law con-
stitutes only one element within Canadian torts jurisprudence out of an
emerging array of non-exclusive tests4 for assessing whether a plaintiff
has discharged the requirement of proving cause-in-fact. I should there-

1 For a notable exception, see Marilyn MacCrimmon, Developments in the Law of Evi-
dence: The 1988-89 Term: The Process of Proof: Schematic Constraints (1990) 1 Sup.
Ct. L. Rev. (2d) 345 [MacCrimmon, Law of Evidence]; Marilyn MacCrimmon, What is
Common about Common Sense?: Cautionary Tales for Travelers Crossing Disciplinary
Boundaries (2001) 22 Cardozo L. Rev. 1433 [MacCrimmon, Common Sense].

2 For a discussion of some of these trial phenomena, see text accompanying note 92.
3 I say express, as I will be arguing that inference causation has always been an imma-

nent element of legal fact-finding.

4 Vaughan Black, Decision Causation: Pandoras Tool-Box in Jason W. Neyers, Erika
Chamberlain & Stephen G.A. Pitel, eds., Emerging Issues in Tort Law (Oxford: Hart,
2007) 309 (this new attitude is what we might call the array approach at 309). See
also Kuwait Airways v. Iraqi Airways (Nos. 4 and 5), [2002] UKHL 19, [2002] 2 A.C. 883
at 1106, [2002] 2 W.L.R. 1353, Lord Hoffman.

(2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

4

fore take care at the outset to distinguish what I am setting out to eluci-
date from what I am not. I will therefore begin by situating inference cau-
sation within recent Canadian jurisprudence on cause-in-fact. Having dis-
tinguished inference causation (or at least its current doctrinal expres-
sion) within that body of law, I will recount various criticisms that have
been levelled in respect of inference causation. A substantial portion of
what follows will be devoted to responding to each of those criticisms in
turn. Ultimately, however, my aim is justificatory, and to that end I will
conclude by offering an account for inference causation, grounded in the
characteristics of evidence, and in the demonstrated cognitive processing
that legal fact-finders apply to that evidence.

I. The Jurisprudence and the Objections

A. Inference Causations Place in Canadian Jurisprudence

The array of tests available in Canada to plaintiffs for proving cause-
in-fact comprise the material contribution to risk test, the material con-
tribution to harm test, and the but-for test.5 The first test, which appears
obiter dicta in the Supreme Court of Canadas recent pronouncement in
Hanke, is said to be available to a fact-finder where factors that are out-
side of the plaintiffs control such as current limits of scientific knowl-
edge make it, in part, impossible for the plaintiff to prevail using the but-
for test.6 The second test, the material contribution to harm test, was de-
scribed in Athey as being available where the but-for test is unwork-
able.7 In contrast to Hankes exclusive focus on augmentation of mere
risk, Atheys test allows a plaintiff to succeed only where the defendants
negligence materially contributed to the occurrence of the actual injury.8

5 For my conceptualization of these tests, see Russell Brown, Material Contributions
Expanding Hegemony: Factual Causation after Hanke v. Resurfice Corp. (2007) 45
Can. Bus. L.J. 432 [Brown, Expanding Hegemony]. For the material contribution to
harm test, see ibid. at 434-38. See also Athey v. Leonati, [1996] 3 S.C.R. 458, 140 D.L.R.
(4th) 235 [Athey cited to S.C.R.]; Walker Estate v. York-Finch General Hospital, 2001
SCC 23, [2001] 1 S.C.R. 647, 198 D.L.R. (4th) 193. For the material contribution to risk
test, see Resurfice Corp. v. Hanke, 2007 SCC 7, [2007] 1 S.C.R. 333, 278 D.L.R. (4th) 643
[Hanke].

6 It must be clear that the defendant breached a duty of care … exposing the plaintiff to
an unreasonable risk of injury, and the plaintiff must have suffered that form of injury:
ibid. at para. 25.

7 Athey, supra note 5 at 466.
8 Ibid. This conception of material contribution is distinct from a nearly forgotten, more
orthodox conception that was expressed as recently as Myers v. Peel County Board of
Education ([1981] 2 S.C.R. 21, 123 D.L.R. (3d) 1). See David Cheifetz, The Snell Infer-
ence and Material Contribution: Defining the Indefinable and Hunting the Causative

 THE POSSIBILITY OF INFERENCE CAUSATION

5

In Canadian tort jurisprudence, however, the third testthe orthodox,
counterfactual, sine qua non but-for test requiring proof on a balance of
probabilities9retains primacy. Even within Athey and Hanke, the but-for
test was not only affirmed as the presumptive test for proving cause-in-
fact,10 but was also applied. To the extent that any post-Hanke trend is
discernible, it is characterized by a marked judicial reluctance to conclude
that the but-for test is unworkable (per Athey)11 or impossible (per
Hanke),12 thereby dooming the claims of plaintiffs unable to meet its more
stringent requirement of showing a probable and necessary causal link
between their suffering and a defendants negligence.13

The ongoing primacy of the but-for test is largely due to the subsisting
influence of the 1990 pronouncement of Justice Sopinka for the Supreme
Court of Canada in Snell v. Farrell,14 and his explicit recognition of infer-
ence causation as the means by which the but-for test is to be applied in
cases of factual uncertainty. In Snell, the plaintiff, in her seventies by the
time of trial, underwent surgery to remove a cataract. A retrobulbar hem-
orrhage that was visible to the defendant surgeon occurred during the
surgery. Although the hemorrhage itself was not a result of negligence,
the trial judge found that the defendants failure to abort the surgery
upon discovering the hemorrhage was unreasonable. By the time the

Snark (2005) 30 Advocates Q. 1 at 83 [Cheifetz, Causative Snark]; Jane Stapleton,
Lords aLeaping Evidentiary Gaps (2002) 10 Torts L.J. 276 at 282-83 [Stapleton,
Lords aLeaping]; Brown, Expanding Hegemony, supra note 5 at 309. This older
conception recognized that factors additional to the defendants negligence that also
contributed to the plaintiffs injury would not, in and of themselves, preclude the defen-
dants liability.

9 Re B. (Children), [2008] UKHL 35, [2009] 1 A.C. 11, [2008] 4 All E.R. 1; F.H. v.

McDougall, 2008 SCC 53, [2008] 3 S.C.R. 41, 297 D.L.R. (4th) 193.

10 Athey, supra note 5 at 466 (the general test); Hanke, supra note 5 at para. 21 (the

basic test).

11 Athey, supra note 5 at 466.
12 Hanke, supra note 5 at para. 25.
13 Indeed, by giving no serious consideration to the meaning of these threshold terms, Ca-
nadian courts skirt the thorny normative questions of whether and when the but-for
test ought to be abandoned in certain cases. Even in the relatively few instances where
plaintiffs have been able to overcome evidentiary gaps due to scientific uncertainty,
they have done so in most cases not on the basis of the alternative tests for proving
cause-in-fact, but rather because they were viewed as having discharged the but-for
test. See Russell Brown, Material Contributions Expanding Hegemony (Or, Where
Are We and How Did We Get Here?) in Continuing Legal Education Society of British
Columbia, ed., Causation in Tort After Resurfice (Vancouver, June 2008) 2.1.1 at 2.1.11-
14.

14 [1990] 2 S.C.R. 311, 72 D.L.R. (4th) 289 [Snell cited to S.C.R.].

(2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

6

plaintiffs vitreous chamber cleared (nine months later), the plaintiffs op-
tic nerve had atrophied to the point of blindness.

The cause-in-fact issue in Snell lay in the uncertain etiology of the op-
tic nerve atrophy. While it can result from retrobulbar hemorrhage, other
possible causes of optic nerve atrophy include three conditions from which
the plaintiff suffered: high blood pressure, diabetes, and severe glaucoma.
Neither partys expert was able to express an opinion on the cause of the
atrophy. The trial judge applied Lord Wilberforces judgment in McGhee v.
National Coal Board15 (where the defendant employers negligent failure
to provide washing facilities for their workmen was one of several possible
causes of the plaintiffs dermatitis) to reverse the onus of proof and impose
liability upon the defendant.16 Both the result and the reasoning were ap-
proved at the New Brunswick Court of Appeal.17
While affirming the judgment of the lower court, Justice Sopinka, for
the Supreme Court of Canada, reviewed the five speeches in McGhee as
well as the following speech of Lord Bridge of the House of Lords in Wil-
sher v. Essex Area Health Authority:

The conclusion I draw … is that McGhee v. National Coal Board
laid down no new principle of law whatever. On the contrary, it af-
firmed the principle that the onus of proving causation lies on the
pursuer or plaintiff. Adopting a robust and pragmatic approach to
the undisputed primary facts of the case, the majority concluded
that it was a legitimate inference of fact that the defenders negli-
gence had materially contributed to the pursuers injury.18

Elaborating on the desired robust and pragmatic19 approach to making
common sense inferences, Justice Sopinka added that the dissatisfaction
with the traditional approach to causation stems to a large extent from its
too rigid application by the courts in many cases. Causation need not be
determined by scientific precision.20 Justice Sopinka quoted Lord Salmon
from Alphacell Ltd. v. Woodward: it is essentially a practical question of
fact which can best be answered by ordinary common sense rather than
abstract metaphysical theory.21 Justice Sopinka continued:

15 (1972), [1973] 1 W.L.R. 1, [1972] 3 All E.R. 1008 (H.L.) [McGhee].
16 Snell v. Farrell (1986), 77 N.B.R. (2d) 222, 195 A.P.R. 222 (Q.B.T.D.).
17 Snell v. Farrell (1988), 84 N.B.R. (2d) 401, 214 A.P.R. 401 (C.A.).
18 [1988] 2 W.L.R. 557 at 569, [1988] 1 All E.R. 871 (H.L.) [emphasis added, references

omitted, Wilsher].

19 Ibid.
20 Ibid.
21 Alphacell Ltd. v. Woodward, [1972] A.C. 824 at 847, [1972] 2 All E.R. 475 [emphasis

added], cited in Snell, supra note 14 at 328.

THE POSSIBILITY OF INFERENCE CAUSATION

7

The legal or ultimate burden remains with the plaintiff, but in
the absence of evidence to the contrary adduced by the defendant, an
inference of causation may be drawn although positive or scientific
proof of causation has not been adduced.22

For Justice Sopinka and the Court, our limited scientific understand-
ing of what must have happened in a given occasion does not pose an in-
surmountable hurdle to the plaintiff. The absence of affirmative scientific
evidence can be legitimately overcome by an inference of cause-in-fact
from the undisputed primary facts of a case.23 More specifically, a causal
link may be inferred where such an inference is supported by the avail-
able evidence, assessed in a robust and pragmatic fashion. We are told
that this correction of our understanding of how to treat evidence should
furnish a salutary amelioration of the too rigid application of the tradi-
tional approach to the but-for test.24 Therefore, instead of perpetuating a
historical insistence upon a (curiously ambiguous) blend of scientific pre-
cision and metaphysical theory, Snell tells us that we should view the
question of whether or not the evidence reveals a causal link between risk
and suffering as essentially a practical question of fact.25

In sum, and irrespective of the ambiguity in his account of how cause-
in-fact used to be determined, Justice Sopinkas statement made it clear
that cause-in-fact would henceforth be determined by a process of inferen-
tial reasoning. Such reasoning would, in turn, draw from evidentiary
treatment that would be both pragmatic and robust. Justice Sopinka
then concluded in Snell that the evidence supported an inference that the
defendant surgeons negligent failure to abort the surgery after discover-
ing the hemorrhage was the cause-in-fact of the optic nerves atrophy.

In this paper, I shall treat Justice Sopinkas holding in Snell as the
substance of inference causation,26 with the caveat that I will neither de-
fend nor rely upon Snells empty references to an approach that is robust
and pragmatic, questions of fact that are practical, or common sense27

22 Snell, supra note 14 at 330.
23 Ibid. at 324.
24 Ibid. at 328.
25 Ibid.
26 The reasoning in Snell has since twice been affirmed by the Supreme Court of Canada.
See Laferrire v. Lawson, [1991] 1 S.C.R. 541, 78 D.L.R. (4th) 609; St-Jean v. Mercier,
2002 SCC 15, [2002] 1 S.C.R. 491, 209 D.L.R. (4th) 513.

27 Lara Khoury has attempted to rehabilitate a role for common sense in legal fact-
finding as reflecting the desired [judicial] attitude … towards the evidence, but in do-
ing so affirms that it is no rationale for actual causal determination: Lara Khoury, Un-
certain Causation in Medical Liability (Oxford: Hart, 2006) at 202-203.

(2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

8

that is ordinary.28 That is, I have deprived myself of the rhetorical fudg-
ing that afflicts judicial discourse on cause-in-fact generally and on infer-
ence causation specifically,29 leaving me with only two reference points:
the evidence, and whether it might (or might not) permit an inference of
causal linkage to be drawn where the doctors cannot identify the process
of causation scientifically.30

B. Anticipating Objections and Looking to Evidence Theory

At present, and even within Canada where Snell still governs, legal
scholars have expressed little support for inference causation generally31
and Snell specifically.32 Although there has been substantial debate about
the merits of the alternative tests of material contribution to harm and
material contribution to risk,33 academic commentary has been almost

28 Here I am agreeing with Cheifetz: Causative Snark, supra note 8 at 49-51.
29 See Vaughan Black, Book Review of Uncertain Causation in Medical Liability by Lara
Khoury, (2008) 47 Can. Bus. L.J. 145 [Black, Book Review of Uncertain Causation]
(Robust and pragmatic are fudge words of the first degree at 151). See also Mac-
Crimmon, Common Sense, supra note 1 (Although frequently relied upon, common
sense is seldom defined, nor is reliance on common sense justified at 1435); Jane Sta-
pleton, Cause-in-Fact and the Scope of Liability for Consequences (2003) 119 Law Q.
Rev. 388 at 388, citing Fairchild v. Glenhaven Funeral Services Ltd., [2002] UKHL 22,
[2003] 1 A.C. 32 at para. 45, [2002] 3 W.L.R. 89, Lord Nicholls [Fairchild]; Jane Staple-
ton, Occams Razor Reveals an Orthodox Basis for Chester v. Afshar (2006) 122 Law
Q. Rev. 426 at 426.

30 Snell, supra note 14 at para. 22, citing Wilsher, supra note 18 at 567.
31 For exceptions, see H.L.A. Hart & Tony Honor, Causation in the Law, 2d ed. (Oxford:
Clarendon Press, 1985) at 1, 23-61; Richard W. Wright, Causation, Responsibility,
Risk, Probability, Naked Statistics, and Proof: Pruning the Bramble Bush by Clarifying
the Concepts (1988) 73 Iowa L. Rev. 1001 at 1003, 1009, 1018 [Wright, Bramble
Bush].

32 There are some prominent exceptions: Khoury, supra note 27 at 202-203; Allan Beever,
Rediscovering the Law of Negligence (Oxford: Hart, 2007) at 489-92 [Beever, Rediscover-
ing].

33 For support of these alternative tests, see Allen M. Linden & Bruce Feldthusen, Cana-
dian Tort Law, 8th ed. (Toronto: LexisNexis Butterworths, 2006) at 120-23; Mitchell
McInnes, Causation in Tort Law: Back to Basics at the Supreme Court of Canada
(1997) 35 Alta. L. Rev. 1013; Craig Jones, Riffing on Mass Torts, Risk & Uncertainty:
Resurfice Corp. v. Hanke and Proof of Causation in Populations in Continuing Legal
Education Society of British Columbia, Causation in Tort After Resurfice (Vancouver,
June 2008) 4.1.1. In opposition to these alternative tests, see Brown, Expanding He-
gemony, supra note 5; Cheifetz, Causative Snark, supra note 8; Gillian Demeyere,
The Material Contribution Test: An Immaterial Contribution to Tort Law: A Com-
ment on Briglio v. Faulkner (2000) 34 U.B.C. L. Rev. 317; Lewis Klar, Downsizing
Torts in Nicholas J. Mullany & Hon. Allen M. Linden, eds., Torts Tomorrow: A Tribute
to John Fleming (Sydney: LBC Information Services, 1998) 305 at 311; Stapleton,
Lords aLeaping, supra note 8.

 THE POSSIBILITY OF INFERENCE CAUSATION

9

universally critical of inference causation.34 And, while Snells references
to common sense and robust[ness] are seen as supplying the vocabu-
lary for baseless rationalization,35 the objections go beyond inference cau-
sations semantics. Put generally, the criticism is that either by design or
in effect, inference causation excuses legal fact-finders from offering clear
reasoning for a plaintiffs recovery in the absence of evidence proving
cause-in-fact to a probable standard. Intuitive fact-finding thus smacks of
the same problem that Peter Birks discerned in intuitive law-finding
specifically that [i]t frees the judge from the shackles of traditional legal
rationality.36 The criticism levelled by Lewis Klar is generally represen-
tative of the scope of the various arguments, although more severe than
most.37 Describing inference causation as a more liberal and relaxed ap-
proach,38 he explains:

The effect of Snell v. Farrell on proving causation in cases where the
scientific and expert evidence cannot establish a probable connection
between a defendants negligence and a plaintiffs injury has been
significant. To allow an inference of cause to be drawn even where
there is no scientific evidence of a probable connection between neg-
ligence and injury is in effect to accept the essential principle of
McGhee via a different route.39

The most recent edition of Klars critique omits the following passage,
which appeared in an earlier edition:

34 Vaughan Black, The Transformation of Causation in the Supreme Court: Dilution and
Policyization in Todd Archibald & Michael Cochrane, eds., Annual Review of Civil
Litigation 2002 (Toronto: Carswell, 2003) 187 at 195 [Black, Policyization]; Cheifetz,
Causative Snark, supra note 8 at 49-54; John G. Fleming, Probabilistic Causation in
Tort Law (1989) 68 Can. Bar Rev. 661 at 670; Lewis N. Klar, Tort Law, 4th ed. (To-
ronto: Thomson Carswell, 2008) at 447 [Klar, Tort Law]; Stephen N. Pincus, Progress
on the Causal Chain Gang: Some Approaches to Causation in Tort Law and Steps To-
ward a Linguistic Analysis (1986) 24 Osgoode Hall L.J. 961 at 983; Jane Stapleton,
The Gist of Negligence, Part II: The Relationship Between Damage and Causation
(1988) 104 Law Q. Rev. 389 at 404 [Stapleton, The Gist of Negligence]. A notable ex-
ception is Khoury (supra note 27).

35 Black, Book Review of Uncertain Causation, supra note 29 at 151.
36 Peter Birks, Three Kinds of Objection to Discretionary Remedialism (2000) 29
U.W.A.L. Rev. 1 at 17 [emphasis added]. I will, however, suggest in concluding this pa-
per that inference causation is an instance of judgment, as distinguished from intuition.
See text accompanying notes 213-15.

37 Khoury describes Klars criticism (see text accompanying note 39) as more assertive

(Khoury, supra note 27 at 166).

38 Klar, Tort Law, supra note 34 at 447, n. 91.
39 Ibid. at 446-47 ^×.. By the essential principle of McGhee (ibid. at 447),
Klar is referring to Lord Wilberforces imposition in McGhee of a reverse onus, requiring
the defendant to prove that his or her negligence did not cause the plaintiffs injury
(ibid. at 441-42). See also Khoury, supra note 27 at 166.

10 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

While [inference causation] may produce a pragmatic solution to a
plaintiffs dilemma in difficult causation cases, it does depart from
the traditional but for test, and the balance of probability stan-
dard.40

This criticism of inference causation essentially advances three gen-
eral objections. First, it implicitly claims that inference causation is indif-
ferent to the laws concern for veritism in fact-finding.41 Cause-in-fact is
being established even in the absence of a probable connection between
negligence and suffering.42 The point here is that, inasmuch as the law ac-
cepts as true only what is shown to be probable, any reasoning process
that is indifferent to probability is necessarily indifferent to the laws veri-
tistic aims. Second, it is argued that scientific evidence is privileged as
being inherently reliable, or at least more reliable than an inference made
without the benefit of scientific evidence.43 And third, critics argue that no
legal fact-finder can legitimately infer cause-in-fact in the absence of such
scientific evidence. Any defence of inference causation must take these ob-

There has been substantial debate about the meaning to be ascribed to McGhee,
and particularly to Lord Reids speech; two mutually opposing views exist. See Staple-
ton, Lords aLeaping, supra note 8 at 286-87; Beever, Rediscovering, supra note 32 at
466-67. It is unnecessary for me to resolve this here, as I take Sopinka J.s reasons in
the later decision of Snell as my reference point. Nor do I propose to engage Stapletons
additional criticism in The Gist of Negligence (supra note 34) in reference to the facts
of McGhee, that it is not clear why … the common sense inference is in favour of a cu-
mulative cause mechanism rather than an alternative cause mechanism (ibid. at 404).
My justification of inference causation is indifferent to whether liability is cumulative or
apportioned with other material sources of harm.

40 Lewis Klar, Tort Law, 3d ed. (Toronto: Thomson Carswell, 2003) at 403. As a matter of
positive law, this is an incorrect statement. Inference causation is no derogation from
the but-for test but is rather an epistemological instantiation of it. See text accompany-
ing note 22 (passage from Snell); David Cheifetz, Materially Increasing the Risk of In-
jury as Factual Cause of Injury: Fairchild v. Glenhaven Funeral Services Ltd. in Can-
ada (2004) 29 Advocates Q. 253 (Snell is a but-for case at 263). However, given the
text that precedes it, I take this as stating that inference causation has, in effect, re-
laxed the plaintiffs evidentiary burden. Indeed, without explanation, Linden and Feld-
thusen describe this supposedly relaxed quality as lending it a more balanced and
humane quality (supra note 33 at 119).

41 Veritism is the laws concern for evaluating a factual proposition by reference to its con-
formance to absolute truth. This term is devised from the work of Alvin Goldman, who
specified that our drive to know presupposes a desire for truth (or at least for the closest
approximation of truth)a desire he labelled veritistic. Veritistic epistemology is
such a special field, where the selected good is knowledge and the selected bad are error
and ignorance: Alvin I. Goldman, Knowledge in a Social World (New York: Oxford
University Press, 1999) at 6.

42 See MacCrimmon, Common Sense, supra note 1 (Justice … totally dependent on un-
examined common sense is, to my mind, a fertile breeding ground for miscarriages of
justice at 1434-35).

43 This is also the argument of Lord Rodger in Fairchild (supra note 29 at para. 150).

 THE POSSIBILITY OF INFERENCE CAUSATION

11

jections seriously, and in this paper, I will argue that such defences reflect
a misunderstanding of the epistemic processes that are integral to scien-
tific fact-finding and legal fact-finding.
More specifically, in response to the first criticism that inference cau-
sation is indifferent to veritism in fact-finding, I will argue that this criti-
cism is simply wrong, inasmuch as it is grounded in a misapprehension of
what it means for legal fact-finding to be veritistic. In addressing the sec-
ond criticismthat scientific evidence is a necessary precondition to re-
liable legal fact-findingI will argue that scientific fact-finding, though
potentially helpful to legal fact-finders, cannot be determinative of legal
outcomes because it fails to account for the cognitive processes that are
necessarily brought to bear upon evidence. Inference causation, I will
show, is an unavoidable process in any causal inquiry because we never
know with certainty whether the mechanism by which a risk could mate-
rialize into suffering was instantiated in a given case: legal fact-finders in-
fer (or do not infer) that it was. Inference causations immanence within
legal fact-finding also helps answer the final criticismthat in the ab-
sence of scientific evidence, no legitimate inference of cause-in-fact can be
made. I will, however, also attempt to bolster my response with reference
to various criteria that have been emphasized by, inter alia, evidence
theorists, psychologists, and epidemiologists for making reliable infer-
ences of causal association.

To some extent, this paper will merely affirm the legitimacy of current
judicial practice: despite criticism, resort to inference causation has per-
sisted in Canadian jurisprudence. And, as Richard Wright reminds us,
judges and juries … consistently have demonstrated an ability to make
intuitively plausible factual causal determinations, resulting in causal
judgments that have inter se enjoyed remarkable agreement.44 I will be
guided, however, by Wrights caution that intuitions not conjoined with
theory when searching for underlying principles are often inadequate for
the hard cases and sometimes may mislead even in the easy cases;45 my
task in addressing the final criticism will be not merely to reiterate judi-
cial practice, but also to theorize (albeit generally) the application of this
shared but undefined causal conception.

In answering these criticisms of inference causation, I will take a
slightly different path from that of Wright, whose theory explains how in-
ference causation is applied but does not seek to justify the reliance upon

44 Wright, Bramble Bush, supra note 31 at 1018.
45 Ibid. at 1018-19.

12 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

it per se (except to show how it might be justly and rationally applied).46
For Wright, to accept inference causation is to accept an epistemological
standpoint that reasonable conclusions (or at least not obviously false, ar-
bitrary, or irrational conclusions) can be drawn from reliable evidence and
rational processes of reasoning. This theory makes sense when one con-
siders that such processes occur within an adjudicative context that im-
poses a less-than-certain and merely probable threshold for proof. This is
not to suggest that context should displace a serious analysis, but rather
that its bar-setting function influences the epistemological project of ex-
plaining how inference qualifies as knowledge.

I therefore do not dispute Wrights defence of inference causation. My
argument instead delves a little more deeply into, inter alia, its presuppo-
sition of an uncertain threshold of civil proof within adjudication in gen-
eral, and legal fact-finding in particular. My account of how evidence is
cognitively processed and juristically assessed will lead me to consider
that a justification for the application of inference causation in legal fact-
finding might be found not in a theory of causation, but rather in a theory
of evidence and its role in the fact-finding process. The inference of cause-
in-fact from evidence of possible correlation has been, after all, a substan-
tial epistemic concern among twentieth-century legal scholars of evi-
dence.47 It began with what William Twining called the rationalist tradi-
tion, which emphasized the role of generalizations in inferential fact-
finding,48 through to the new or mathematicist evidence scholarship,
which emerged in the 1960s and 1970s and maintained that inferential
reasoning in the face of uncertainty is a species of probabilistic reason-
ing.49 Therefore, the advantage of drawing from theoretical evidence
scholarship to inform tort lawyers debates about inference causation is
that such a theory might account for the cognitive processing and juristic
assessment of evidence in an adjudicative context. Here, I am taking as a

46 Wright comes closest to justifying intuitional causal judgment, as I describe it, in his re-
sponse to Mark Kelmans pragmatic objections to his NESS test (ibid. at 1037). While
he concedes that our imperfect knowledge means that legal fact-finders are sometimes
unable to determine whether a condition is more than a mere condition such that it con-
tributed to the result, Wright argues that this point is irrelevant to the appropriate
theory of actual causation (ibid.). Instead, [a]s lawyers, judges, jurors, or lay persons,
we do the best that we can (ibid.).

47 Peter Tillers, Mapping Inferential Domains (1986) 66 B.U.L. Rev. 883 at 883.
48 William Twining, The Rationalist Tradition of Evidence Scholarship in Enid Camp-
bell & Louis Waller, eds., Well and Truly Tried: Essays on Evidence in Honour of Sir
Richard Eggleston (Sydney: Law Book Company, 1982) 211.

49 See especially John Kaplan, Decision Theory and the Factfinding Process (1968) 20
Stan. L. Rev. 1065; Michael O. Finkelstein & William B. Fairley, A Bayesian Approach
to Identification Evidence (1970) 83 Harv. L. Rev. 489; Richard O. Lempert, Modeling
Relevance (1977) 75 Mich. L. Rev. 1021 [Lempert, Modeling Relevance].

 THE POSSIBILITY OF INFERENCE CAUSATION

13

given that such context, while plainly relevant to empirical theorizing
about legal proof, is also relevant to normative theorizing about how the
evidence can be said to permit the inference of a fact, including cause-in-fact.

This is not to suggest that evidence theory per se supplies the answer;
these weaknesses are also shared by the theory of evidence presupposed
by Klars criticisms, which offer a probabilistic account privileging statis-
tical information for assessing the reliability of evidence.50 As I will show
in my primary inquiry in this paper, probabilistic theory denies subtle
characteristics of both evidence and legal fact-finding, and its statistical
expression is of dubious reliability on its own terms. This will lead me to
conclude by considering an emerging body of work within evidence theory
(drawing particularly but not exclusively from the work of Ronald J. Al-
len) that emphasizes the legal fact-finding process as entailing the as-
sessment and continuing reassessment of competing versions of events.
That process, put succinctly, is seen as resting upon the fact-finders in-
ference to the best explanation. My claim will be that, allowing for cer-
tain important corrections, such an understanding offers tort lawyers a
generally promising normative justification for inference causation by ac-
counting for both the epistemology of legal fact-finding and the adjudica-
tive context in which it takes place.

II. Veritism

I turn, then, to the first criticism of inference causationthat it does
not conform to the laws veritistic objectives. Inference causation, the ar-
gument goes, is a rhetorical proxy for lowering the standard of proof in
cases where there are gaps in the evidence marshalled by the plaintiff.51
As such, it is an instance of a more general policyization52 of cause-in-
fact, affording by way of the vacuous incantations of robustness and
common sense a crutch for fact-finders who cannot point to affirmative
evidence explaining why a plaintiff ought to recover.53 In short, it has lit-
tle to do with truth and more to do with the fact-finders own preferences.
This concern draws strength from the language that Justice Sopinka used
in Snell, which demonstrated tepid commitment to veritism at best, inso-
far as he speculated that the causal inference could be drawn with very

50 Klar, Tort Law, supra note 34 at 447, n. 91. See infra note 152 and accompanying text.
51 See text accompanying note 38.
52 Black, Policyization, supra note 34 at 195. See also Salvatore Mirandola, Lost
Chances, Cause-in-Fact, and Rationality in Medical Negligence (1992) 50 U.T. Fac. L.
Rev. 258 at 260; Ernest J. Weinrib, A Step Forward in Factual Causation (1975) 38
Mod. L. Rev. 518 at 532 [Weinrib, A Step Forward].

53 Snell, supra note 14 at 328.

14 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

little affirmative evidence on the part of the plaintiff.54 As I have already
noted, his references to robust[ness] and common sense55 amplify the
sense that the truth of the matter, relative to the particular outcome that
a fact-finder might desire, is unimportant.

Such minimization of veritism is troubling, as truth-seeking is an ele-
mental aspiration of our legal order. Accurate fact-finding is essential,
particularly if we understand the rule of law as a substantive limitation
on the states ability to manipulate facts to its own ends, even where that
manipulation is for the ostensibly benevolent purpose of compensating in-
jured plaintiffs.56 Within the framework of a tort action, then, the law in-
sists on linking a finding of actual wrongdoing by the defendant to the
plaintiffs suffering. This linkage supplies the particular feature about
the defendant that singles him out from the generality of those available
for the shifting of the plaintiffs loss.57 A serious account of inference cau-
sation must therefore distinguish its underlying epistemic processes from
emotivism or any similarly non-cognitive, meta-ethical point of view such
as the popular concept of truthiness,58 or the benevolent principle that
Lord Nourse discerned in Fitzgerald v. Lanea concept that smiles on …
factual uncertainties and melts them all away.59 In such a mindset,
where all evaluative judgments … are nothing but expressions of prefer-

54 Ibid. at 328.
55 Ibid.
56 Birks, supra note 36 at 15:

The suggestion that judges should be free to apply whatever remedy they
think best for the trouble in hand emanates from our taking the rule of law
for granted. Benevolent power is in one sense more dangerous than malevo-
lent power. It undermines vigilance. We easily drop our guard.

57 Ernest J. Weinrib, Causation and Wrongdoing (1987) 63 Chicago-Kent L. Rev. 407 at

412.

58 Truthiness is a term coined in 2005 by political satirist Stephen Colbert that refers to
the quality of stating concepts or facts one wishes or believes to be true, rather than
concepts or facts known to be true: Stephen Colbert, The Word-Truthiness, The Col-
bert Report (17 October 2005), online: Colbert Nation . See also Stephen Colbert, Colbert Roasts Bush (Talk presented
at the White House Correspondents Dinner, Washington, D.C., 29 April 2006), online:
YouTube . As Colbert put it:

Were not some brainiacs on the nerd patrol. Were not members of the fact-
inista. We go straight from the gut. … Thats where the truth lies, right down
here in the gut. Do you know you have more nerve endings in your gut than
you have in your head? You can look it up. Now I know some of you are going
to say, I did look it up, and thats not true. Thats because you looked it up
in a book. Next time look it up in your gut. … I call it the no-fact-zone
[transcribed by author].

59 [1987] 3 W.L.R. 249 at 262, [1987] 2 All E.R. 455 (C.A.).

 THE POSSIBILITY OF INFERENCE CAUSATION

15

ence, expressions of attitude or feeling,60 there is no difference between
stating that the defendant acted wrongly by causing the plaintiffs injury
and claiming that I like the plaintiff more than I like the defendant.

The question of how we link together two persons and two moments in
timethe defendant and the moment of his or her negligence, and the
plaintiff and the moment of his or her sufferingmust therefore be an-
swered in a manner that reconciles the inferential leap with the laws
veritistic aims. Inference causation cannot be a matter of merely cobbling
together narratives taken out of thin air with a view to expressing what a
fact-finder feels; rather, it must presuppose an adjudicative procedure
that is veritistic. I will later seek to do so with reference to what Alvin
Goldman conceived of as the epistemological principle of total evidence,
being that a fact-finder must fix his beliefs or subjective probabilities in
accordance with the total evidence in his possession at the time.61 I shall
assume that Goldmans direction is uncontroversial because its reference
point of evidence affirms a veritistic imperative. There is a difference be-
tween being involuntarily disposed to feel that the defendants negligence
has caused the plaintiffs suffering, and voluntarily accepting based upon
the evidence that it has.62 By being tied to an evidentiary reference point,
inference causation can be distinguished from the fudging that is said to
permit legal fact-finders to find in favour of sympathy-inducing plaintiffs
because it feels right.63
While Goldmans total evidence presupposes veritism, it also pre-
supposes that a fact-finders belief will be decided by reference only to
such evidence in his possession at the time.64 The fact-finders truth-
seeking obligation, then, is unavoidably conditioned upon and discharged
in a state of incomplete evidence. What evidence is before the fact-finder
may and often will lead not to some absolute truth, but rather to an ap-
proximation of events. Lord Wilberforce remarked in Air Canada v. Secre-
tary of State for Trade65 that adjudication often entails imperfect or even
withheld evidence, and legal fact-finding therefore occurs without knowl-
edge of the whole truth. [Y]et, he continued, if the decision has been in

60 Alasdair MacIntyre, After Virtue: A Study in Moral Theory, 3d ed. (Notre Dame, Ind.:

University of Notre Dame Press, 2007) at 11-12 [emphasis in original].

61 Alvin I. Goldman, Epistemic Paternalism: Communication Control in Law and Soci-

ety (1991) 88 Journal of Philosophy 113 at 113 [Goldman, Epistemic Paternalism].

62 L. Jonathan Cohen, Should a Jury Say What It Believes or What It Accepts? (1991) 13

Cardozo L. Rev. 465 at 468 [Cohen, Should a Jury].

63 Cf. Cheifetz, Causative Snark, supra note 8 (The common sense method does not

mean that the fact finder may consider [propositions] that are not in evidence at 35).

64 Goldman, Epistemic Paternalism, supra note 61 at 113.
65 [1983] 2 A.C. 394, [1983] 2 W.L.R. 494 (H.L.) [Air Canada cited to A.C.].

16 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

accordance with the available evidence and with the law, justice will have
been fairly done.66

The underlying presupposition here is that veritism is not the exclu-
sive goal of adjudication, nor would it be even if it were theoretically pos-
sible to achieve absolute truth after painstaking inquiry. What we strive
for is justice achieved through a process reasonably designed to ascertain
the truth, [and] in ways consistent with … other ends of the legal system.67
Indeed, Air Canada goes so far as to suggest that [t]he principal, if not
the only, purpose of civil litigation is not truth-seeking, but rather the
resolution of disputes.68 One can think of even more non-veritistic objec-
tives such as public confidence,69 acceptance of the rule of law, pacification
of parties or simply removing a sense of injustice.70 Concerns for proce-
dural efficiency and thrift, or for fostering relationships such as those be-
tween solicitors and clients, between spouses, or between parties gener-
ally,71 also operate as competitors to veritism. So does the exclusion of
evidence going to character, to subsequent remedial measures, or to at-
tempts to compromise a claim. The new emphasis in civil justice reform
on mandatory alternative dispute resolution mechanisms and the en-
shrinement of formal settlement procedures in most rules of court (com-
plete with cost incentives) is similarly indifferent to truth-seeking. More-
over, this systemic anti-veritism is perpetuated in turn by the functus
rule, which precludes further reflection upon the evidence.72

66 Ibid. at 438 [emphasis added].
67 John Rawls, A Theory of Justice, rev. ed. (Cambridge, Mass.: Harvard University Press,

1999) at 210 [emphasis added].

68 J.A. Jolowicz, Civil Litigation: Whats It For? (2008) 67 Cambridge L.J. 508 at 514.
This was echoed by Blackmun J.s observation in Daubert v. Merrell Dow Pharm. that
the legal system aspires to quick, final, and binding … judgment (509 U.S. 579 at 597
(1993) [Daubert]).

69 Laurence H. Tribe, Trial by Mathematics: Precision and Ritual in the Legal Process

(1971) 84 Harv. L. Rev. 1329 at 1376.

70 Patrick Devlin, The Judge (Oxford: Oxford University Press, 1979) at 3. More pluralist
understandings of the aims of civil process have also embraced economizing of re-
sources, inspiring confidence, supporting independent social policies, permitting ease in
prediction and application, adding to the efficiency of the entire legal system, and tran-
quilizing disputants: Jack B. Weinstein, Some Difficulties in Devising Rules for De-
termining Truth in Judicial Trials (1966) 66 Colum. L. Rev. 223 at 241.

71 Kronman views the imperative of understanding relations between parties and those
who identify with or support them as being so fundamental to adjudication that it
speaks to the ethics of judging: Anthony T. Kronman, Living in the Law (1987) 54 U.
Chicago L. Rev. 835 at 864.

72 The general [functus rule is] that a final decision of a court cannot be reopened:
Chandler v. Alberta Association of Architects, [1989] 2 S.C.R. 848 at 860, 101 A.R. 321,

 THE POSSIBILITY OF INFERENCE CAUSATION

17

I do not mean to suggest that some of these rules are without prob-
lems; indeed, objectives other than truth are replete with interesting phi-
losophical implications. My point here, however, is that all of these rules,
which remove from the fact-finders consideration the information rele-
vant to drawing pertinent inferences, augment the likelihood of inaccu-
rate fact-finding. They demonstrate that rectitude of factual determina-
tion in civil process inevitably clashes with other objectives.73 Veritism
simpliciter, taken as the evaluation of a factual proposition by exclusive
reference to its conformance to absolute truth, is simply never in the cards
in a system where truth-seeking may be constrained or outright sacrificed
by legal desiderata, which have little, if anything, to do with truth.
At the same time, indifference to truth seems less troubling when it is
employed to achieve settlement, for example, than when it governs legal
fact-finding. The former does not implicate a norm that privileges judg-
ments that are understood by the fact-finder as being at least as correct as
possible (allowing for the impossibility of absolute correctness given im-
perfect information). It is surely wrong, however, to associate legal fact-
finding with extreme relativism. We want not mere finality, but just final-
ity, and substantial justice requires a premium on truth. On the rare oc-
casion when veritism is expressly abandoned in adjudicating cause-in-
fact,74 one might ask: Why bother having a trial? Why hear witnesses?
Why insist upon testimony in the language of perception and not of inter-
pretation? To conclude that none of these things matter would be to view
the institution of the trial as an elaborate charade. While veritism sim-
pliciter is not the goal and not empirically achievable, the law nonetheless
aspires to connect causally a defendants liability with a defendants neg-
ligent act or omission. Fact-finding thus requires truth in at least a suffi-
cient amount.75
While veritistic aims subsist in legal fact-finding, any theory of legal
fact-finding generally, and of finding cause-in-fact in particular, must also
account for the uncertainty contemplated by the structure of proof within
which evidence is processed; that is, how one determines that data repre-

Sopinka J., majority. It is based … on the policy ground which favours finality of pro-
ceedings (ibid. at 862).

73 Lisa Dufraimont, Evidence Law and the Jury: A Reassessment (2008) 53 McGill L.J.

199 at 203-204.

74 Hymowitz v. Eli Lilly & Co., 539 N.E.2d 1069 (N.Y. 1989) (there should be no exculpa-
tion of a defendant who, although a member of the market producing DES for preg-
nancy use, appears not to have caused a particular plaintiffs injury at 1078 [emphasis
added]).

75 James R. Olchowy, Battling for the Judges or Jurys Imagination: Evidence, Storytel-

ling, and Effective Trial Advocacy (2003) 16 Windsor Rev. Legal Soc. Issues 1 at 2.

18 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

sents proof of a factual proposition.76 To return to my earlier point, the ob-
jectives of evidence law are epistemic and concern the rational reasoning
methods by which conclusions are reached. Both reference points of total
evidence77 and of the civil standard of proof influence the epistemological
project of explaining how the cognitive inferences drawn by fact-finders
qualify as knowledge: though legal fact-finders may seek truth from the
evidence, the most they will find is a likelihood of truth. For juristic pur-
poses, something short of absolute knowledgeincluding inferential
knowledgemust therefore still carry veritistic value. The first criticism
is thus unconvincing: by adopting an evidentiary reference point, infer-
ence causation proves to be no less veritistic than adjudication itself.

III. Scientific Fact-Finding

I now progress to the second inquiry by examining the criticism that
inference causation disregards a supposed need for scientific evidence
linking the defendants negligence with the plaintiffs suffering. Specifi-
cally, I advance two reasons for which scientific uncertainty cannot justly
halt the fact-finders inquiry in its tracks. First, scientific fact-finding is,
simply put, not the same thing as legal fact-finding. They are not mutual
substitutes. Second, scientific fact-finding, which operates at a general
statistical level, tells us only about the causal link that might or might not
exist in general, and not whether such a link is instantiated in the par-
ticular instance of the defendants negligence and the plaintiffs suffer-
ing.78

A. Differentiating Scientific Fact-Finding from Legal Fact-Finding

The first reason why the absence of scientific evidence should not de-
feat a legal finding of causation implicates the contrasting standards of
proof and underlying epistemic processes entailed by science and law re-
spectively. While both courts79 and academics80 rightly aver the diver-

76 Ronald J. Allen, Factual Ambiguity and a Theory of Evidence (1993) 88 Nw. U.L. Rev.
604 at 606 [Allen, Factual Ambiguity]; James Q. Whitman, The Origins of Reasonable
Doubt: Theological Roots of the Criminal Trial (New Haven: Yale University Press,
2008) at 20-21.

77 Goldman, Epistemic Paternalism, supra note 61 at 113.
78 See Hart & Honor, supra note 31 (In the sciences causes are often sought to explain

not particular occurrences but types of occurrence at 33 [emphasis in original]).

79 Snell, supra note 14 (Causation need not be determined by scientific precision at 328;
Medical experts ordinarily determine causation in terms of certainties whereas a
lesser standard is demanded by the law at 330). See also Daubert, supra note 68
(there are important differences between the quest for truth in the courtroom and the
quest for truth in the laboratory at 596-97).

 THE POSSIBILITY OF INFERENCE CAUSATION

19

gence in standards of proof, its significance lies not in the existence of two
thresholds but in the intervening gap.

The point becomes more plain when expressed mathematically.81 The
legal burden of juridical proof is conventionally understood to be a prob-
ability measure of {>0.50}, meaning that the fact, while less than certain
to represent absolute truth, is more likely true than not.82 The scientific
threshold for proof, however, is substantially higher. Statistically signifi-
cant conclusions normally fall within an interval of confidence, which is
the range within which a study parameter lies 95% of the time.83 Thus, a
scientific expert would deny causal association between potential causes
and effects unless the connection would not occur due to chance (or some
other potential cause) more than 5% of the time.84 This leaves a range of
{<0.449} between the two thresholdsthe probability measure of proposi- tions that satisfy legal fact-finders but not scientific fact-finderswithin which the plaintiffs case ought to succeed. To side with the defendant whenever a scientific fact-finder will not opine on cause-in-fact therefore creates a one-sided inquiry in the defendants favour.85 It is simply unjust for a legal fact-finder to maintain that a scientific fact-finders refusal or inability to discern the necessary causal linkage between risk creation 80 Beever, Rediscovering, supra note 32 at 491-92; Glanville Williams, Causation in the Law [1961] Cambridge L.J. 62 at 66-69 [Williams, Causation in the Law]. 81 The reducibility of burdens of proof to numerical terms appears to be widely accepted, although it remains controversial in some circles. See e.g. Robert J. Rhee, Probability, Policy and the Problem of Reference Class (2007) 11 Intl J. of Evidence & Proof 286 at 287-88; Craig R. Callen, Kicking Rocks with Dr. Johnson: A Comment on Professor Al- lens Theory (1991) 13 Cardozo L. Rev. 423 at 428 [Callen, Kicking Rocks]; Ronald J. Allen & Craig R. Callen, The Juridical Management of Factual Uncertainty (2003) 7 Intl J. of Evidence & Proof 1 at 7 [Allen & Callen, Juridical Management]. My pur- pose here, however, is simply to use statistical reference points to demonstrate the gap between legal and scientific fact-finding thresholds. 82 Allen, Factual Ambiguity, supra note 76 at 604; Ronald J. Allen & Michael S. Pardo, The Problematic Value of Mathematical Models of Evidence (2007) 36 J. Legal Stud. 107 at 111 [Allen & Pardo, Mathematical Models]. See also Fennell v. Southern Mary- land Hosp. Ctr., 580 A.2d 206 at 206 (Md. 1990); Russell v. Subbiah, 500 N.E.2d 138 at 141 (Ill. App. 3 Dist. 1986). 83 Richard Goldberg, Causation and Risk in the Law of Torts: Scientific Evidence and Me- dicinal Product Liability (Oxford: Hart, 1999) at 105; Margaret A. Berger & Lawrence M. Solan, The Uneasy Relationship between Science and Law: An Essay and Introduc- tion (Symposium, A Cross-Disciplinary Look at Scientific Truth: Whats the Law to Do?, Brooklyn Law School, 2 March 2007) (2008) 73 Brook. L. Rev. 847 at 852. 84 Troyen A. Brennan, Helping Courts with Toxic Torts: Some Proposals Regarding Al- ternative Methods for Presenting and Assessing Scientific Evidence in Common Law Courts (1989) 51 U. Pitt. L. Rev. 1 at 23-24 [Brennan, Helping Courts]; Berger & So- lan, supra note 83 at 852. 85 Beever, Rediscovering, supra note 32 at 471.  20 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL and suffering is determinative of the outcome of a tort action.86 All that this failure tells us is that the factual proposition is not provable to a probability measure of {0.95}. The juristic inquiry must continue. The question is how. The divergence in thresholds is less revealing than the divergence in underlying methodologies. Since the intellectual tasks of the scientist and jurist are distinct inter se, they call for distinct intellectual processes. In science, where we know what has happened but are left with indetermi- nate data that fail to explain why it happened, experiments are repeated until the event can be satisfactorily replicated. The underlying process of fact-finding, then, involves further refinement of the deductive structure of extant knowledge by eliminating anomalies whether empirical or theo- retical, thereby modifying or even replacing the overall conceptual struc- ture of that kind of event. In this way, the scientific process seeks to sim- plify phenomena, usually by controlling as many variables as possible. Until that occurs, determinations of a causal link between risk augmenta- tion and suffering must always remain provisional:87 counterexamples, or the possibility of confounding factors, can never be ruled out.88 Legal fact-finders, by contrast, do not replicate events to determine why something happened. Instead, parties offer up various, and typically inconsistent versions of events, all of which are assessed in drawing a conclusion about the event.89 In part, this difference in process exists be- cause of the practical imperatives in the law that preclude waiting for cer- tainty;90 whereas science is ongoing, law needs finality, and so the fact- finding inquiry must stop at some point.91 More fundamentally, there are 86 [T]he law prefers a 50 per cent[] chance of doing justice to the certainty of doing injus- tice: Weinrib, A Step Forward, supra note 52 at 524, citing Glanville L. Williams, Case Comment on Cook v. Lewis, (1953) 31 Can. Bar Rev. 315 at 317. This is also the law on criminal causation. See R. v. Smithers, [1978] 1 S.C.R. 506, 75 D.L.R. (3d) 321 (The weight to be given to the evidence of the experts was entirely for the jury. In the search for truth, the jury was entitled to consider all of the evidence, expert and lay, and accept or reject any part of it S.C.R. at 518). 87 Chris Miller, Causation in Personal Injury: Legal or Epidemiological Common Sense? (2006) 26 L.S. 544 at 547. 88 See Berger & Solan, supra note 83 at 851-52. 89 See generally Allen, Factual Ambiguity, supra note 76. 90 Miller, supra note 87 at 547. 91 Alex Stein, The Refoundation of Evidence Law (1996) 9 Can. J.L. & Jur. 279 at 286 [Stein, Refoundation]. Indeed, further investigation may be futile; the notion that there may be [c]ompleteness of evidence is only a scientific ideal (Dufraimont, supra note 73 at 206). Because factual uncertainty derives from gaps in evidence instead of known-but-unavailable evidence, nobody can predict the fact that would be proven by the missing evidence, were it available. Causal determinations, then, entail underly-  THE POSSIBILITY OF INFERENCE CAUSATION 21 simply too many variables in legal fact-finding to be accounted for, includ- ing factors for which scientific deduction cannot account. Trial phenom- ena, such as lawyers and witnesses conduct, demeanour, emotive quali- ties, and personal idiosyncrasies fit uneasily into scientific fact-finding be- cause homing in on their meaning involves complex mental activities that have their own peculiar logic and pattern.92 These phenomena instantiate soft variables, which are resistant to ready quantification. Nearly forty years ago, Laurence Tribe saw them as undermining scientific claims to normatively superior fact-finding properties.93 Confronted with the neces- sity of reaching just verdicts with such uncertain evidence, legal fact- finders reason inductively. For scientific fact-finding to be substitutional for legal fact-finding, it would have to be able to make deductive sense of soft variables, with the necessary and sufficient conditions for drawing a conclusion set out in ad- vance. For example, if the expert witness smiles at her interrogator more at one end of the mouth than the other, twitches her eyebrows, and averts her gaze, she lacks confidence in her answer. This deduction is of course impossible, because the cognitive rules by which soft variables are proc- essed defy definitive articulation.94 What I am suggesting here is that le- gal fact-findings inductive structure is a necessary instantiation of the more general observation of Martha Nussbaum and Hilary Putman: [M]ental states are not only compositionally plastic but also computa- tionally plastic, that is, [there are] reasons to believe that physically possible creatures ... have an indefinite number of different pro- grams, and that the hypothesis that there are necessary and suffi- cient conditions for the presence of such a belief in computational, or computational-cum-physical, terms is unrealistic in just the way [that] the theory that there is a necessary and sufficient condition for the presence of a table stateable in phenomenalist terms is unrealis- tic: such a condition would be infinitely long, and not constructed ac- cording to any effective rule.95 ing reasoning that would have to account for unrealized and unknowable forensic pos- sibilities. See Alex Stein, An Essay on Uncertainty and Fact-Finding in Civil Litiga- tion, with Special Reference to Contract Cases (1998) 48 U.T.L.J. 299 at 300 [Stein, Uncertainty]. 92 Tillers, supra note 47 at 921; MacCrimmon, Common Sense, supra note 1 at 1436. See also Faryna v. Chorny (1951), [1952] 2 D.L.R. 354, 4 W.W.R. (N.S.) 171 (B.C.C.A.). 93 Tribe, supra note 69 at 1361, 1393. 94 Allen, Factual Ambiguity, supra note 76 at 626-27. 95 Martha C. Nussbaum & Hilary Putnam, Changing Aristotles Mind in Martha C. Nussbaum & Amelie Oksenberg Rorty, eds., Essays on Aristotles De Anima (Oxford: Oxford University Press, 1995) 27 at 48 [emphasis added].  22 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL Therefore, the most that we can say is that such information, when put before a fact-finder, may or may not influence the outcome drawn. It de- pends on the sum total of [the fact-finders] experiences at the moment of decision,96 which would, by that time, include counsels submissions on the meaning of such information and all other observations generated during the trial. In an illuminating psychological study of the cognitive processes of le- gal fact-finding where jurors are employed, experimental psychologists Nancy Pennington and Reid Hastie empirically demonstrated these cogni- tive processes at work.97 The juror, they conclude, is a sense-making in- formation processor who, confronted with evidentiary uncertainty includ- ing soft variables, strives to create a meaningful summary of the evi- dence available that explains what happened.98 This meaningful sum- mary is achieved by imposing a narrative story form upon the evidence, informed by the evidence itself,99 knowledge about events similar in con- tent to the case under consideration, and generic expectations about what makes a complete story.100 In short, explanations of events take the form of stories constructed from deductive and inductive reasoning pro- cedures applied to the evidence and world knowledge, including infer- ences about events, and causal relations between them.101 Where there is factual uncertainty, individuals attempt to make sense of the available evidence by composing the most compelling plausible explanation possi- ble. The understanding of legal fact-finding as entailing the composition of a narrative is hardly new.102 In fact, it is consistent with the advice that 96 Allen, Factual Ambiguity, supra note 76 at 627. 97 Nancy Pennington & Reid Hastie, A Cognitive Theory of Juror Decision Making: The Story Model (1991) 13 Cardozo L. Rev. 519 [Pennington & Hastie, Cognitive Theory]. See also Reid Hastie, Steven D. Penrod & Nancy Pennington, Inside the Jury (Cam- bridge, Mass.: Harvard University Press, 1983); Reid Hastie & Nancy Pennington, The O.J. Simpson Stories: Behavioral Scientists Reflections on The People of the State of California v. Orenthal James Simpson (1996) 67 U. Colo. L. Rev. 957. Causal cognition is in fact a substantial topic in psychological research, although Pennington and Hasties work represents a rare instance of empirical insight. Even neuroscience has not shed insights. See David Danks, The Psychology of Causal Perception and Reason- ing in Helen Beebee, Christopher Hitchcock & Peter Menzies, eds., Oxford Handbook of Causation (Oxford: Oxford University Press, 2007). 98 Pennington & Hastie, Cognitive Theory, supra note 97 at 519. 99 Ibid. at 522 (case-specific information acquired during the trial). 100 Ibid. 101 Ibid. at 524. 102 See Hart & Honor, supra note 31 at 28-30; Robert P. Burns, A Theory of the Trial (Princeton: Princeton University Press, 1999) [Burns, Theory]; Richard Lempert, Tell-  THE POSSIBILITY OF INFERENCE CAUSATION 23 senior trial counsel impart to junior counsel as to the most effective way of presenting a case,103 and has led to theoretical reconceptualizations of the trial.104 Nonetheless, these findingswhich, after twenty-five years, have yet to be challengedare fundamentally disconcerting to some. If stories are dependent, even partly, on a fact-finders life experience, then differ- ent fact-finders may presumably derive different (and inconsistent) ac- counts from identical facts. Further, while it might be tempting to consign this problem to the specific case of civil juries, which are still commonly employed in Canada,105 the implications of such cognitive processes in judge-alone cases are actually more troubling. Jurors must persuade and be persuaded by one another, while a judge need only persuade him- or herself. Disconcerting as this may be, however, the point remains from Pennington and Hasties work that this process is an empirically demon- strated phenomenon. B. The Generality of Scientific Fact-Finding and the Specificity of Legal Fact-Finding The danger of completely subsuming legal fact-finding into scientific fact-finding is also obvious when one considers that scientific method em- phasizes the indispensability of statistical explanations.106 That is, it at- tempts to derive statistically significant correlations or associations be- tween outcomes and exposures to variables, such as risk augmentation. This ostensibly formal methodology may be intuitively appealing to legal fact-finders, who must otherwise rely on indeterminate and therefore fal- lible human intuition and intelligence to reconstruct a past that may or may not have occurred as they have determined.107 It has, as Tribe ob- served, the lure of objectivity and precision.108 This allure, however, is ing Tales in Court: Trial Procedure and the Story Model (1991) 13 Cardozo L. Rev. 559; Linda R. Hirshman, It Will Be Pleasanter to Tell You a Story (1991) 13 Cardozo L. Rev. 445. 103 MacCrimmon, Law of Evidence, supra note 1 at 345. 104 Burns, Theory, supra note 102; Olchowy, supra note 75. 105 Civil juries are particularly common in British Columbia, perhaps due to the frequent resort to juries by British Columbias public motor vehicle insurer. See The Honourable John C. Bouck, Civil Jury TrialsAssessing Non-Pecuniary DamagesCivil Jury Re- form (2002) 81 Can. Bar Rev. 493 at 513. 106 Troyen A. Brennan & Robert F. Carter, Legal and Scientific Probability of Causation of Cancer and Other Environmental Disease in Individuals (1985) 10 J. Health Pol. 33 at 38-39. 107 However, legal fact-finders typically do not accept statistical analysis as determinative of anything. See Wright, Bramble Bush, supra note 31 at 1050-51. 108 Tribe, supra note 69 at 1331.  24 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL false because mathematical probability is rarely absolutely certain.109 Consider Jonathan Cohens celebrated gatecrasher paradox: where 501 out of 1000 spectators gatecrashed their way into a rodeo, a purely statis- tical analysis would lead to the absurd injustice of the impresario being able to recover admission fees from all 1000 spectators, based solely on the probability {>0.50} that every one of them was a gatecrasher.110 Even
if we were to increase the number of gatecrashers to within the scientific
fact-finding thresholdsay, 951 out of 1000the impresarios claim
would still seem absurd.
False mathematical certainty aside, the imposition of a purely statisti-

cal methodology upon legal fact-finding also carries both normative and
practical difficulties.111 The normative problem lies in the detraction from
individualized justice represented by a statistical approach. As Glanville
Williams observed, the scientist is concerned with causal generalisations.
But in historical and legal statements this notion of generalisation and
reproducibility hardly figures at all.112 This is because statistics tell us
only about the frequency of increased risk translating into suffering, while
the attribution of individual suffering to the risk creator is always inde-
terminate.113 The laws veritistic aims, however, presuppose a normative
ideal that adjudication involves attempting to find what happened during
specific, unique events.114 Given this ideal, legal fact-finders seek to pro-
nounce individualized judgments about whether the risk creator actually
caused the suffering.115 And so, they investigate particular phenomena
[a]rmed with … knowledge of general physical laws and other data such
as eyewitness testimony of behaviour,116 and then apply [their] data (in-
cluding [their] knowledge of general physical laws) to an investigation of a

109 Randolph N. Jonakait, Stories, Forensic Science, and Improved Verdicts (1991) 13

Cardozo L. Rev. 343 at 350.

110 L. Jonathan Cohen, The Probable and the Provable (Oxford: Clarendon Press, 1977) at

74-75.

111 Wright, Bramble Bush, supra note 31 (Probabilistic causation is philosophically and

pragmatically insupportable at 1003).

112 Williams, Causation in the Law, supra note 80 at 66.
113 Brennan, Helping Courts, supra note 84 at 24; Khoury, supra note 27 at 49-50.
114 See Richard W. Wright, Causation in Tort Law (1985) 73 Cal. L. Rev. 1735 at 1822-23

[Wright, Causation].

115 Rabbi Paul Bergman & Reverand Al Moore, Mistrial by Likelihood Ratio: Bayesian

Analysis Meets the F-Word (1991) 13 Cardozo L. Rev. 589 at 591.

116 Jane Stapleton, Choosing What We Mean by Causation in the Law (2008) 73 Mo. L.

Rev. 433 at 435.

 THE POSSIBILITY OF INFERENCE CAUSATION

25

particular completely specified phenomenon occurring on a specific occa-
sion.117

The problem is even more fundamental for juristic purposes. Statisti-
cal evidence suffers from an inherent flaw that demands caution even
when it is employed solely as a basis for estimating general frequencies.118
Consider that the event in question can be modelled in an infinite number
of ways, and that each privileges certain characteristics that will generate
a particular probability. The dependency of the resulting probability on
the class from which it is drawn generates an epistemological problem:
different classes will yield different probabilities. Furthermore, there is no
a priori method for determining whether any particular class is relatively
closer or further from the objective truth.119 Considering Cohens gate-
crasher paradox, we can create subsets of rodeo attendees by, for example,
distinguishing church-goers from atheists (on the assumption that the
former are more likely to be honest about paying admission); or wealthy
from poor (on the assumption that the former have no incentive to gate-
crash); or old from young (on the assumption that the former have more
respect for the law). In short, it is not enough to know how many atten-
dees there are in order to formulate generalized causal probabilities. We
need to know the circumstances that are relevant to the statistical prob-
ability of someone bilking the impresario.

This problem, which is typically conceptualized as one of reference
classes,120 is implicated in factual causation. In a case like Snell, science
could generate some conditional probabilities, such as the chance of going
blind without the defendants negligence, contrasted with the chance of
going blind with the defendants negligence. From that, science could ul-
timately generate an equation showing the probability that the blindness
was caused by the defendants negligence. I have already observed that,
being a mere probability, this evidence is unhelpful in determining indi-

117 Ibid. at 438.
118 Vern R. Walker, Theories of Uncertainty: Explaining the Possible Sources of Error in
Inferences in Marilyn MacCrimmon & Peter Tillers, eds., The Dynamics of Judicial
Proof: Computation, Logic, and Common Sense (Heidelberg: Physica-Verlag, 2002) 197
(Statistical associations among sampling data may warrant a conclusion or finding
about generic causation, but that conclusion has an inherent risk of causal error at
223).

119 See Michael S. Pardo, Reference Classes and Legal Evidence (2007) 11 Intl J. of Evi-

dence & Proof 255 at 256; Wright, Bramble Bush, supra note 31 at 1047.

120 John Maynard Keynes, A Treatise on Probability (London, U.K.: Macmillan, 1921) at
94; David Papineau, For Science in the Social Sciences (London, U.K.: Macmillan, 1978)
c. 3 at 50ff.; Wright, Bramble Bush, supra note 31 at 1046-47; Paul Roberts, From
Theory into Practice: Introducing the Reference Class Problem (2007) 11 Intl J. of Evi-
dence & Proof 243.

26 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

vidualized causation.121 My point here, however, is that the probability, as
a probability, is of questionable value unless we know much more about
the reference class from which it is derived. The reference class can be
gerrymandered in an infinite number of ways. It could be as narrow as
someone of the gender, age, and identical medical history as the plaintiff
in Snell, as broad as all patients whose case histories are canvassed in the
scientific literature, or somewhere in betweenperhaps the experts own
patients, or subsets comprising female patients or female patients with
particular pre-existing conditions.
The relevance, or probative value, of statistical evidence thus depends

upon the selected group of persons, or reference class, as the overall popu-
lation from which probabilities are extracted, and in respect of which fac-
tual generalizations are made. The problem is not just that some refer-
ence classes will lead to the probabilistic conclusion that negligence
caused suffering while other reference classes will not permit that conclu-
sion in statistical terms. It is also that the value of probability assess-
ments varies with the chosen reference class: the less specific the refer-
ence class is to the question at hand, the less reliable the derived probabil-
ity assessments are.122 The ostensibly objective and methodically rigorous
quality of probability assessments is therefore illusory.
Moreover, the reference class problem is not merely a question of ob-
structing attempts to find a true cause of an event. Because the refer-
ence class is the basis on which correlation is attributed to causality, an
overly broad reference class will lead to false causality.123 This is particu-
larly germane to epidemiology, which is based not upon experimentation,
but upon statistical causation, denoting correlations between risk expo-
sures and outcomes.124 To consider a simple example, suppose we wish to
determine whether the cause-in-fact of a plaintiffs exposure to a particu-
lar disease lies with her choice of residence between Edmonton and Cal-
gary. And suppose that, thanks to researchers who recently considered
this question by examining one hundred residents of Edmonton and one

121 See text accompanying note 106-10.
122 For this reason, Wright (Bramble Bush, supra note 31 at 1047) cites the work of Pap-

ineau (supra note 120).

123 I am grateful to Duane Szafron for our discussion of this point. For a recent and rare
example of a trial court discerning a reference class problem in the opinion evidence of a
scientific expert, see Duncalf v. Capital Health Authority, 2009 ABQB 80, 4 Alta. L.R.
(5th) 201 at para. 96, 64 C.C.L.T. (3d) 237.

124 Brennan, Helping Courts, supra note 84 at 23; Brennan & Carter, supra note 106 at
44. On statistical causation generally, see Margaret A. Berger, Eliminating General
Causation: Notes Towards a New Theory of Justice and Toxic Torts (1997) 97 Colum.
L. Rev. 2117 at 2125-26; Steve Gold, Causation in Toxic Torts: Burdens of Proof, Stan-
dards of Persuasion, and Statistical Evidence (1986) 96 Yale L.J. 376 at 384.

 THE POSSIBILITY OF INFERENCE CAUSATION

27

hundred residents of Calgary, we now have access to generalized data.
Assume the data show that fifty of the Calgarians and twenty of the Ed-
montonians share a propensity to the disease. Without refining their ref-
erence class of city of residence, we might conclude that living in Calgary
increases the propensity to that disease by a factor of two or greater. Be-
cause, however, the researchers found their Edmontonians living in the
suburbs and their Calgarians living downtown, their chosen reference
class has led us into false causality. That is, were we simply to take the
figures as given, we would be wrongly connecting propensity to disease
with city of residence, instead of urban living (or other potentially influen-
tial reference points such as ethnicity, health, or economic status). The
reference class problem thus poses the risk that we might misidentify the
cause by failing to distinguish a genuine causal influence from a spurious
covariation.125

In addition to the normative objection for detracting from individual-
ized justice, there are lingering pragmatic concerns over relying upon sta-
tistical probabilities for the purposes of legal fact-finding. One practical
reason for eschewing statistical analysis of cause-in-fact lies in the com-
plex nature of trial evidence, of which discrete bits accumulate and inter-
sect with one another, forcing fact-finders to perform continual adjust-
ments of their perceptions both of the parties dispute, and of particular
evidence in light of new discoveries. Most probabilistic attempts to answer
the objection about accounting for the complexities of accumulating evi-
dence have relied upon Bayesian decision theory.126 In essence, Bayess

125 I should concede that the reference class problem is not unique to expressly probabilis-
tic or other formal mathematical expressions of evidence. All evidence is inherently
probabilistic (United States v. Shonubi, 895 F. Supp. 460 at 514 (E.D.N.Y. 1995)), be-
cause any factual generalization necessarily implies a reference class. The reference
class problem is therefore an unavoidable concomitant of inference causation to at least
some degree. The problem runs deeper, however, where naked statistical evidence is
used in isolation from ones own reasoning. Unlike other forms of evidence (whether in
documentary, testimony, or physical form), any interpretation drawn from statistical
evidence is entirely dependent upon the reference class. This is only making explicit a
pervasive and inherent feature of scientific fact-finding in cases of forensic uncertainty:
quantitative theories, such as statistical approaches, all require knowledge of the in-
terdependencies of data (Allen, Factual Ambiguity, supra note 76 at 620). That is,
elements of statistical evidence are mutually dependent in a way that must be dis-
cerned before any reliable interpretation can be derived.

126 Kaplan, supra note 49; Finkelstein & Fairley, supra note 49; Lempert, Modeling Rele-
vance, supra note 49; Richard D. Friedman, Route Analysis of Credibility and Hear-
say (1987) 96 Yale L.J. 667; Peter Tillers & David Schum, Hearsay Logic (1992) 76
Minn. L. Rev. 813; Dale A. Nance & Scott B. Morris, Juror Understanding of DNA
Evidence: An Empirical Assessment of Presentation Formats for Trace Evidence with a
Relatively Small Random-Match Probability (2005) 34 J. Legal Stud. 395.

28 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

theory furnishes a mechanism for incrementally revising probability es-
timates in light of new information, thereby allowing a fact-finder to up-
date continually an opinion about the relative likelihood of a fact. This re-
quires two pieces of data, the first being an a priori estimate of the prob-
ability that a fact is proven or not. Bayesians posit that a fact-finder be-
gins with an original estimate of the likelihood of a fact, such as a causal
link. That prior probability is then continually revised to reflect the statis-
tical impact of new, relevant information as it is received and incorpo-
rated into the probabilistic calculation of the likelihood of the fact. In
Bayesian terms, the statistical impact of new information constitutes the
second piece of data, being a likelihood ratio, which is multiplied by the
prior probability to create the posterior possibility. The process is re-
peated until finally, the fact-finder supposedly arrives at the final modifi-
cation of the probability of the causal linkage.127 The claim for Bayesian
methodology and for its superiority over intuitive fact-finding was neatly
summarized by Michael Saks and Jonathan Koehler:

The Bayesian approach can have a clarifying effect on ones thinking
about evidence. By using Bayes Theorem, we can see what informa-
tion about the evidence is needed, where the absence of data is re-
placed by assumptions of the witnesses or fact findings, and, ulti-
mately, what impact the evidence should have on our preexisting be-
liefs. In contrast, the intuitive decision-maker has few helpful guide-
posts for updating beliefs, and risks falling victim to some of the
many biases associated with the heuristic strategies intuitive deci-
sion-makers employ.128

Bayesian methodology suffers from several defects, however, making
it incompatible with legal fact-finding.129 The principal objection chal-
lenges prior probability: it is in essence a reference class, since it forms
the a priori basis from which the probability of a proposition such as a

I do not propose here to describe the mathematical underpinnings of Bayesianism,
preferring to leave them to the more technically competent; I wish instead to focus on
whether Bayess theorem furnishes an adequate model for legal fact-finding. For the
mathematical underpinnings, see generally Tribe, supra note 69 at 1351-52; Michael J.
Saks & Jonathan J. Koehler, What DNA Fingerprinting Can Teach the Law About
the Rest of Forensic Science (1991) 13 Cardozo L. Rev. 361 at 364, n. 17. See especially
Judea Pearl, Probabilistic Reasoning in Intelligent Systems: Networks of Plausible In-
ference, 2d ed. (San Francisco: Morgan Kaufmann, 1988) c. 2 at 29ff.

127 Tribe, supra note 69 at 1350. See also Saks & Koehler, supra note 126 at 364.
128 Ibid. at 364-65. See also Jonathan J. Koehler & Daniel N. Shaviro, Veridical Verdicts:
Increasing Verdict Accuracy Through the Use of Overtly Probabilistic Evidence and
Methods (1990) 75 Cornell L. Rev. 247 at 274-75.

129 Bayesian analysis was first attacked by Tribe (supra note 69). Tribe did not, however,
challenge the Bayesian epistemological claim to describe the structure of rational think-
ing about inference, but rather focused on what he saw as Bayesianisms pragmatic,
moral, and social implications.

 THE POSSIBILITY OF INFERENCE CAUSATION

29

causal link is made. Everything that followsthe continual updates of the
prior estimate in light of discovered bits of evidence relevant to cause-in-
fact, and the final modification of the assessment of the propositions like-
lihooddepends upon that starting point. Unless, however, that pre-
existing probability is truly a priorithat is, an analytic proposition that
derives from logic and reason,130 as opposed to a synthetic proposition
based on experience and observations131there is no reason to accept that
pre-existing probabilitys status within Bayesian methodology is inher-
ently reliable. And, without that reliable starting point, the posterior or
ultimate probability that is the subject of later calculation and recalcula-
tion is unreliable. In other words, the new statistical probability is just as
suspect as the prior.132 Its relevance is determined only with reference to
an initial hypothesis, formulated without the sophisticated probabilistic
thinking that is ostensibly brought to bear upon later aggregations. Given
the scientific fact-finders claim to formal objectivity, the Bayesian process
is ironically totally dependent upon the intuition that probabilists eschew
as fuzzy and unreliable, and which statistical evidence ostensibly allows
fact-finders to avoid.
Another objection to statistical evidence and to its processing in litiga-
tion in a Bayesian manner derives from the empirical limitations of hu-
man computational capacity. The notion of a fact-finder who continually
and mathematically refines accumulating data is simply implausible. The
nature of evidence itself, moreover, defies easy Bayesian statistical reduc-
tion. Take again the problem of soft variables in evidence. Bayesian
analysis cannot account for the significance to be ascribed to the meaning
of a witnesss smile in describing a possible causal sequence because both
the factual question of whether it was more a smirk than a smile, and the
contrasting hypotheses (derision or humour) pose difficulties and perhaps
impossibilities for statistical reduction. In other words, probabilistic
analysis of evidence, including evidence of cause-in-fact, is futile because
evidence does not speak for itself by stating its own probative qualities or

130 Examples of such logical analytic propositions are: one plus one equals two; only women
can give birth; and the sun rises in the east and sets in the west. It is admittedly possi-
ble, in statistical terms, to have such a proposition where the reference class is objec-
tively determinable and exhaustive, but it only seems imaginable in an experimental
setting, free of competing versions of historical events. Bergman and Moore give the ex-
ample of a sack of 100 marbles, 50 of which are red. As a matter of logic and reason,
there is a prior probability of {0.50} that the first marble removed from the sack will be
red (Bergman & Moore, supra note 115 at 597).

131 Examples of such synthetic propositions are: Norway is colder than Greece; London is

larger than Vancouver; and all owls hunt at night.

132 See Tribe, supra note 69 at 1350.

30 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

weight.133 As Keynes explained, weight … measures the sum of the fa-
vourable and unfavourable evidence, [while] probability measures the dif-
ference.134

IV. The Inferential Quality of Legal Fact-Finding

A. Infusing Evidence Theory into Causal Inference

Evidence, then, acquires legal meaning only through the complex cog-
nitive processes that fact-finders bring to bear upon it. Benjamin Cardozo
(writing extrajudicially) observed, We may try to see things as objectively
as we please. None the less, we can never see them with any eyes except
our own.135 This is particularly so for statistical evidence, which requires
first transforming it from evidence about the generality of cases to evi-
dence about the particular case before us.136 Statistics alone prove noth-
ing, because [n]o datum or object has an inferential value standing
alone.137 They are, in short, just something else to be interpreted;138 their
probative value will depend on the strength that they lend to a particular
inference at trial.139 This is the upshot of the problem Wright averred

133 Ronald J. Allen, On the Significance of Batting Averages and Strikeout Totals: A Clari-
fication of the Naked Statistical Evidence Debate, The Meaning of Evidence, and the
Requirement of Proof Beyond a Reasonable Doubt (1991) 65 Tul. L. Rev. 1093 at 1103
[Allen, Batting Averages].

134 Keynes, supra note 120 at 77 [emphasis in original]. This explains why, even where the
probability might increase as a plaintiff accumulates supportive evidence, a legal fact-
finder can still reject evidence: the truth of the underlying proposition might still be un-
reliable despite its probabilistic superiority over the competing proposition.

135 Benjamin N. Cardozo, The Nature of the Judicial Process (Mineola, N.Y.: Dover, 2005)

at 9.

136 Tribe, supra note 69 at 1346 [emphasis in original].
137 Craig R. Callen, Cognitive Science and the Sufficiency of Sufficiency of the Evidence

Tests (1991) 65 Tul. L. Rev. 1113 at 1129.

138 Allen & Pardo, Mathematical Models, supra note 82 at 135-36.
139 Indeed, it is not obvious that statistics ought to be considered by legal fact-finders at all,
let alone in isolation from other evidence. The disparity to which I have already referred
(between probability theory and human reasoning when confronted with uncertainty)
suggests that they ought to be excluded altogether, and there is support for this argu-
ment. See Callen, Kicking Rocks, supra note 81; Tribe, supra note 69 at 1350, 1365.
There is also support for the inclusion of statistics within a pluralist understanding of
what qualifies as evidence. See Allen & Pardo, Mathematical Models, supra note 82 at
136-37; Roberts, supra note 120 at 251-52; Stein, Uncertainty, supra note 91 at 301;
Tillers, supra note 47 at 888-89. Pardo also seems to have taken Robert Rhee as having
argued for exclusion (Pardo, supra note 119 at 257); however, Rhee has expressly stated
that statistics are not a question of exclusion, but rather one of appropriate weight
(Rhee, supra note 81 at 291). I do not attempt to resolve this dispute in this paper. My

 THE POSSIBILITY OF INFERENCE CAUSATION

31

when he wrote that probabilities provide, at most, evidence supporting the
applicability of different causal generalizations, but do not in themselves
indicate which of the possibly applicable causal generalizations actually
applies to the particular concrete occurrence.140
My argument here is that evidence submitted at trialparticularly
circumstantial evidence of correlation, such as the temporal proximity be-
tween surgery and blindnessis, on its own, underdeterminative of the
true significance that legal fact-finders may (or may not) ascribe to as-
pects of the historical event that is the subject matter of the lawsuit.141
Such ascription is accomplished by an inferential process in which docu-
ments, testimony, and physical exhibits become evidence through argu-
ments that interpret them with relation to that event.142 This goes to a
more fundamental point, also anticipated by Wright: particularizing
cause-in-factthat is, determining what happened on a particular occa-
sionrequires us to infer that a causal generalization and its underlying
causal law have been fully instantiated on the particular occasion.143 In
other words, inference is a ubiquitous and immanent part of causal de-
termination. Fact-finders always find facts by choosing whether or not to
infer. We infer (or do not infer) from the fact that the morning newspaper
reports that the Toronto Maple Leafs lost last nights hockey game that
they did; we infer (or do not infer) from the fact that history books repre-
sent that Horatio Nelson died in victory at the Battle of Trafalgar that he
did. Indeed, present knowledge of historical past is a construction com-
posed of what the evidence obliges us to believe.144 Historical fact, then,
is not a question of what was or what really happened, but rather
what the historian infers to have happened.145 Tort claims requiring legal
fact-finders to fill gaps in order to construct a historical account of cause-
in-fact entail the same inferential process.146

point is to defend inference causation, which is supported by both sides of the issue of
whether the evidence that scientists can offer in the face of factual uncertainty ought to
be considered.

140 Wright, Bramble Bush, supra note 31 at 1047 ^×..
141 Ibid.
142 Ariel Porat & Alex Stein, Liability for Uncertainty: Making Evidential Damage Ac-

tionable (1997) 18 Cardozo L. Rev. 1891 at 1891, 1900.

143 Wright, Bramble Bush, supra note 31 at 1049.
144 Michael Oakeshott, Experience and its Modes (Cambridge: Cambridge University Press,

1933) at 102-12.

145 Ibid.
146 This is conceded even by Klar, who goes further than I do in describing the legal fact-
finders options: Klar, Tort Law, supra note 34 (The court must guess at what would
have occurred, using its best judgment, intuition, common sense, experiences, expert

32 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

Consider a plaintiff who, at the material time, suffered from type I os-
teogenesis imperfecta, or brittle bone disease, meaning that his bones
break easily in the course of ordinary daily activities. He does not experi-
ence symptoms of having a broken arm until after he is hit by a negli-
gently operated automobile while crossing a marked crosswalk. A subse-
quent X-ray reveals that he has suffered a broken arm. No one would se-
riously suggest that a scientific expert is required in this case to demon-
strate cause-in-fact, nor would anyone seriously challenge a legal fact-
finders conclusion that the drivers negligence was the cause-in-fact of the
plaintiffs broken arm. The factual causal link is nonetheless being in-
ferred, not demonstrated. Evidential sourcesin this case, the plaintiffs
evidence as to his medical conditiondo not speak for themselves by
demonstrating the mechanical coincidence of negligence and suffering;
their significance is inherently ambiguous.147 That ambiguity is resolved,
either to the plaintiffs or the defendants benefit, by the drawing of an in-
ference.
Recall Klars criticism of inference causation, and in particular its as-
sumption that scientific evidence is inherently reliable, or at least more
reliable than an inference drawn in the absence of scientific evidence.148
For that criticism to be evaluated, it is first necessary to know what is
meant by scientific evidence. On that point we are given a hint in his ac-
companying critique of Scott (Crick) v. Mohan,149 a decision of the Alberta
Court of Queens Bench. We are told, despite the fact that no statistical
studies could provide a clear link to the causation issue, an inference of
cause-in-fact was drawn.150 It appears, then, that this critique of inference
causation presupposes that naked statistics, on their own, constitute evi-
dence. As I have shown, however, the idea of naked statistical evidence is
nonsensical in a juristic context because it presupposes an epistemological
perspective that does not conform to the epistemology that is characteris-
tic of legal fact-finding. What probabilists skip is the linkage between
propositions derived from observations of phenomena, and the implica-
tions of these propositions allowing us to draw conclusions about legal li-
ability.

evidence, and whatever else may be of assistance at 432 [emphasis added]). The text
does not attempt to reconcile this statement with his rejection of inference causation in
the absence of scientific evidence (see text accompanying notes 38-40).

147 Ronald J. Allen, The Nature of Juridical Proof (1991) 13 Cardozo L. Rev. 373 at 395

[Allen, Nature of Juridical Proof].

148 Klar, Tort Law, supra note 34 at 447, n. 91.
149 (1993), 142 A.R. 281, 41 A.C.W.S. (3d) 1148 (Q.B.).
150 Klar, Tort Law, supra note 34 at 447, n. 91 [emphasis added].

 THE POSSIBILITY OF INFERENCE CAUSATION

33

Criticisms of inference causation151 are therefore based upon an im-
poverished understanding of the nature of evidence. No account of infer-
ence causation can satisfactorily theorize precisely how the linkages be-
tween evidence of risk augmentation and factual conclusion are to be
made and justified, because evidence itself is theory laden.152 As such, it
can be interpreted in different ways, instantiating what Peter Tillers calls
the nomological structures of evidence whereby a partys theory of the
evidence becomes part of the totality of the evidence itself.153 Evidence of
causationthat is, Goldmans total evidence154might therefore be use-
fully distinguished from an evidential source.155 The former is an amplifi-
cation of the latter (as opposed to a reiteration), which, on its own, carries
no weight. Evidence of causation simply exists; it is left to the fact-finder
to ascribe significance to the evidential source, which in turn becomes
part of the evidence. Just how that is donehow weight is ascribed to
evidential sourcesis complex, and further complicated by trial condi-
tions.156 The cogency of that ascription is the nub of the problem that crit-
ics discern in inference causation: because it is non-demonstrable and in-
ductively uncertain, its acceptance depends on a theory of evidence that
affords a fact-finder the discretion of saying that she has sufficient or in-
sufficient reasons for making an inference from evidential sources. I will
conclude this paper by referring to such a theory, but my point here is
that such criticisms are futile, considering inference causations imma-
nence within legal fact-finding and its complexity that defies mathemati-
cal reduction.
An argument that might be raised in objection to all this is that my
example of the brittle-boned plaintiff is an obvious case where scientific
evidence is unnecessary. Such obviousness is presumably derived from
the observable mechanical linkages: a hitherto asymptomatic plaintiff is
struck by an automobile, he experiences symptoms of a broken arm, and
is shortly thereafter diagnosed with a broken arm. Being able to relate his
broken arm to a mechanism such as being struck by an automobile is un-
doubtedly more comforting to a legal fact-finder determining cause-in-
fact, since knowledge of a mechanism usually implies knowledge of when

151 See text accompanying note 38-40.
152 Robert P. Burns, Some Realism (and Idealism) About the Trial (1997) 31 Ga. L. Rev.

715 at 757 [Burns, Some Realism].

153 Tillers, supra note 47 (theory is built into all evidencethat nomological structures are

part of the fabric of evidence at 891).
154 See text accompanying notes 61-64.
155 See Stein, Refoundation, supra note 91 at 287, 308.
156 Ibid. at 308-309.

34 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

it is or is not likely to be active.157 Conversely, knowledge of a correlation
does not typically imply knowledge of when risk is or is not likely to mate-
rialize into suffering. The case of the brittle-boned plaintiff is, however,
not a case solved by mechanical knowledge because we do not know
whether the broken arm was caused by the mechanism of {automobile +
plaintiff} or by the pre-existing risk. Given the plaintiffs contrasting pre
and postaccident symptoms, however, we might infer correlation be-
tween the defendants negligent driving and the plaintiffs suffering. In-
deed, that is all we can ever do since we can never know with certainty
whether the mechanisms by which a risk could materialize into suffering
were instantiated in a given case.
Given the ubiquity and immanence of inference causation in legal fact-
finding, there is no obvious demarcation between so-called obvious cases
where one can safely infer a causal link between risk augmentation and
suffering, and cases requiring scientific evidence.158 The point is that fact-
finding, including fact-finding for determining cause-in-fact, always in-
volves evidentiary gaps that are filled by ascriptions of meaning to evi-
dential sources. Ex hypothesi, causal determinations can be conditioned
upon any evidential sources containing any amount of information. Since
the fact-finder does not know what evidence is not before him or her,159
[a]n argument that relevant allegations are more probable than not may
be constructed upon virtually any amount of evidence160 by bringing to
bear upon it those qualities that Cardozo identified as a stream of ten-
dency … [giving] coherence and direction to thought … [and comprising]
inherited instincts, traditional beliefs, [and] acquired convictions.161
While this carries an obvious risk of reaching a factually incorrect con-
clusion, that risk diminishes where, as Wright has described it, some
credible causal generalization links conditions of that type to results of
that type.162 As such, a legal fact-finder is persuaded that an incompletely
known causal law was probably fully instantiated on a particular occa-
sion. The question on which I conclude this paper addresses whether and
how we can give structure to that linkage generated by the causal gener-
alizationthat is, to the fact-finders reasoning processsuch that the
evidential sources and concomitant information can be assessed in rela-

157 Danks, supra note 97 at 19. See also Hart & Honor, supra note 31 at 28-30 (discussion

of Cause and Effect).

158 I am grateful to Lewis Klar for discussions of this issue.
159 I make this point at supra note 91.
160 Stein, Uncertainty, supra note 91 at 300.
161 Cardozo, supra note 135 at 8.
162 Wright, Bramble Bush, supra note 31 at 1046.

 THE POSSIBILITY OF INFERENCE CAUSATION

35

tion to all other evidential sources before the fact-finder. As I have argued
elsewhere, the devil is in the details.163

B. The Structure of Inference Causation

I have argued in this paper that the structure of juristic proof, while
veritistic, is distinct from the structure by which propositions are scien-
tifically verified. As a result, neither the existence of an evidentiary gap
nor scientific demurral from bridging that gap excuses the fact-finder
from proceeding further in the causal inquiry. Reliance on scientific de-
murral as being determinative is, quite literally, unjust.164 Moreover, be-
cause complete evidentiary knowledge is impossible (i.e., always laden
with gaps), legal fact-finding would grind to a halt were it to operate in
isolation from the human judgment inherent in the cognitive processes
brought to bear upon the evidence.165 This is particularly so where the le-
gal fact-finder is called upon to adjust her understanding of the case in
order to account for the complex nature of accumulating and intersecting
trial evidence and soft variables such as trial phenomena.
On the one hand, the reality of legal fact-finding166 requires that we
acknowledge the role of explanatory considerations as guides to the infer-
ences that legal fact-finders draw. And, where several propositions might
explain a given event, legal fact-finders are naturally inclined to infer as a
fact the proposition that best explains it. On the other hand, legal fact-
finders, in choosing to draw such an inference, must conform to the laws
concern for veritism.

I now proceed to consider whether reconciliation of these conflicting
imperativesaccounting for the rough justice that underlies how legal
fact-finders assess the relevance of particular bits of evidence, and the re-
quirement that facts that are found be not merely plausible but prob-
ablecan be achieved by applying the insights of evidence theory. In do-
ing so, I will attempt to bring some structural discipline to the cognitive
processes, which Pennington and Hastie demonstrated.167 Ultimately, I
seek to defend those processes as comprising a normative phenomenon.

163 Brown, Expanding Hegemony, supra note 5 at 442. It may be a matter of ecumenical
preference; Burns writes that God is in the details: Burns, Some Realism, supra
note 152 at 757 [emphasis added].
164 See text accompanying note 85-86.
165 I am understanding evidence here as total evidence (see text accompanying notes

61-64).

166 See especially Pennington & Hastie, Cognitive Theory, supra note 97.
167 Ibid.

36 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

Structural discipline requires a measure of veritistic rigour that is not
obvious, for example, in some of the most important, recent theorizing of
the trial as an instance of storytelling.168 Robert Burns, for example,
while viewing evidence as being necessarily organized [by legal fact-
finders] in narrative form, argues that narratives are generally persua-
sive because of normative considerations that emerge from the narra-
tives themselves.169 While Burnss positive account of legal fact-finding is
unobjectionableindeed, it simply reiterates the epistemic reality re-
vealed and explicated by Pennington and Hastie170his normative claim
arguably drifts into emotivism by privileging the legal fact-finders own
preferences. At the very least, his focus on normative considerations that
emerge from the narratives themselves171 fails to privilege the evidence
and ignores the particular question of fact to be determined. While any
purely narrative model is logically and epistemologically superior to the
probabilistic models because it dovetails with how legal fact-finders treat
evidence,172 the normative significance of any narrative must be drawn, at
least inter alia, from the fact that it accounts for evidence, and not from
the narrative itself. In short, the plaintiff should win where the legal fact-
finder thought the plaintiffs account of the facts seemed the most prob-
able and plausible of all competing accounts, and not because the plain-
tiffs story itself evoked considerations that the legal fact-finder thought to
be normatively significant. Otherwise, the plaintiff might as well succeed
on account of the cut of his jib.

For this papers purposes, however, any tenable theory must account
for the empirical fact that the process of legal fact-finding entails generat-
ing potential explanations of the evidence. Having heard the explanations,
the fact-finder then turns to selecting the best probable explanation (the
one that best explains the evidence) as more likely than all the others to
be true, and thus to stand as truth. Various evidence theoristsRonald
J. Allen in particularhave conceptualized this structure as also compris-
ing the framework of a normative process dubbed as inference to the best
explanation173 (IBE), which is shorthand for the idea that explanatory

168 This lack of rigour also opens inference causation to criticism from probabilists. See
James Brook, The Blue Bus Stop: On Professors Stories and the Stories Plaintiffs Tell
(1991) 13 Cardozo L. Rev. 621 at 623 (Comment on Allen, Nature of Juridical Proof,
supra note 147).

169 Burns, Some Realism, supra note 152 at 756-57.
170 Indeed, Burns cites Pennington and Hasties work (see ibid. at 751-53).
171 Ibid. at 757.
172 Pennington & Hastie, Cognitive Theory, supra note 97.
173 Amalia Amaya, Inference to the Best Legal Explanation in Hendrik Kaptein, Henry
Prakken & Bart Verheij, eds., Legal Evidence and Proof: Statistics, Stories, Logic

 THE POSSIBILITY OF INFERENCE CAUSATION

37

considerations guide inferences during the proof process and that these
considerations therefore themselves explain the … phenomena under dis-
cussion.174 IBE mirrors the two-stage process of legal fact-finding: poten-
tial explanations are offered by the parties or constructed by the fact-
finder, and are then assessed with reference to explanatory criteria.

[I]n civil cases fact-finders ought to infer the best explanation (and
find for the party whom it favours) from the competing explanations
offered by the parties or additional explanations fact-finders con-
struct for themselves.175

In the first stage, the parties themselves do the heavy lifting, offering
competing versions of events in order to explain the evidence. The plain-
tiff (or the party with the burden of proof) offers versions that address the
formal substantive elements comprising his claim, while the defendant of-
fers versions that omit one or more of those elements and addresses the
formal substantive elements of any affirmative defences. In the second
stage, those explanations of eventsand any other explanations that
have been constructed by the legal fact-finderare considered for plausi-
bility relative to the evidence. The defendants strongest counterargument
to the plaintiffs case, then, will be a competing narrativethat is, an-
other way to order the information into coherency.176

IBE, at least as Allen conceives it, thus conforms to the empirically
demonstrated cognitive reasoning processes of legal fact-finders. Factual
conclusions are formulated by developing narratives of events to account
for the evidence.177 These narratives will consist of various subsets of the
events that are said by one party or another to suggest a causal relation-
ship. Ultimately, based on the preferred narrative, cause-in-fact may or
may not be inferred by the legal fact-finder. The preferred narrative,
moreover, might be one constructed by the fact-finder herself; although
Allen has equivocated on this point,178 there is no reason to restrict the ac-

(Farnham, U.K.: Ashgate, 2009) 135 at 135; Allen, Factual Ambiguity, supra note 76;
Allen & Callen, Juridical Management, supra note 81; Allen & Pardo, Mathematical
Models, supra note 82; Ronald J. Allen & Michael S. Pardo, Probability, Explanation
and Inference: A Reply (2007) 11 Intl J. of Evidence & Proof 307 [Allen & Pardo,
Probability]; Pardo, supra note 119.

174 Allen & Pardo, Probability, supra note 173 at 315.
175 Ibid. at 316.
176 Jonakait, supra note 109 at 347.
177 See also Callen, Kicking Rocks, supra note 81 at 443; John Leubsdorf, Stories and

Numbers, Comment, (1991) 13 Cardozo L. Rev. 455 at 458.

178 Ronald J. Allen, A Reconceptualization of Civil Trials (1986) 66 B.U.L. Rev. 401 at

432-34. Compare Allen & Pardo, Probability, supra note 173 at 316.

38 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

ceptable alternative scenarios to those posed by the parties.179 Otherwise,
fact-finding might not be as accurate as it could be. Parties will, after all,
propose narratives that are untrue, sometimes because the truth is un-
helpful, and sometimes because they simply do not know what happened.
At the very least, parties have incentives to propose the narrative that fa-
vours their interests. Legal fact-finders should be free to accept any nar-
rative that makes sense of the evidence, even if it means accepting their
own account rather than accepting one partys version of events or strik-
ing a compromise between the parties competing versions of events. Be-
cause any conclusion by a fact-finder that contradicts her own judgment
regarding the forensic possibilities lacks epistemic justification, justice re-
quires a fact-finder to reject any account that simply does not sound
right.180
However the explanations are constructed, the essential idea remains
that the parties tender evidential sources, whose probative value depends
on the strength of these explanations that they support.181 Determining
the strength of the respective explanations is a matter of asking not only
which inferences are reasonably supported by the evidence, but also
whether the facts are compatible with one or more hypotheses. As Allen
and Pardo have explicated:

An item of evidence is relevant if it is explained by the particular ex-
planation offered by the party tendering the evidence, assuming the
explanation matters to a fact of consequence to the substantive law.
The probative value of this evidence will depend on the strength of
the pertinent explanation: the more it is explained, the more proba-
tive; the less it is explained, the less so. The strength of the desired
inference will depend on all the other relevant evidence and any
competing (contrasting) explanations.182

179 I am presupposing here that where a judicially constructed account favours the plain-
tiff, such an account can reasonably be said to fall within the factual scope of the plain-
tiffs pleadings. Otherwise, as a matter of natural justice, the defendant cannot be li-
able, since she has not had the opportunity to know the case to be met.

180 See also Allan Beever, Cause-in-Fact: Two Steps out of the Mire (2001) 51 U.T.L.J.
327 (the findings of a court should not be inconsistent with the laws of nature or of
logic at 344).

181 Tillers, supra note 47 at 917, n. 59:

The judge, when inquiring into the relative strength of [a particular hypothe-
sis against a competing hypothesis], is not a scientist trying to determine
which general laws are most strongly supported by the available [statistical]
data. The judge typically asks which factual hypotheses are most strongly
supported by the evidence in the case.

182 Allen & Pardo, Probability, supra note 173 at 317.

 THE POSSIBILITY OF INFERENCE CAUSATION

39

As Wright expresses it, we cannot infer that the causal law underlying
the causal generalization also has been instantiated unless we can rule
out competing causal generalizations.183 Here again, the reference point
must be the evidence put before the fact-finder.184
Of course, there is a circularity to IBE in that the explanation and the
underlying evidence are mutually reinforcing.185 The evidence under con-
sideration justifies the belief that the explanation is correct, while the ex-
planation is used to clarify the evidence. As Allen and Pardo have demon-
strated, however, scientific hypotheses are also circular in that they are
mutually reinforced by the same phenomena they seek to explain.186
Moreover, this understanding of the circular structure of juristic proof has
the advantage of being able to aggregate the evidence in a way that ac-
counts for actual cognitive capacities and processes, unlike Bayess theo-
rem. Potential explanations are considered in light of new evidence in a
manner that accords with a legal fact-finders provisional assessment of
the original evidence. This provisional assessment, like the Bayesians
prior probability, will not be expressed in statistically reliable terms. My
claim is not that inference causation is without problems, but that it is re-
flective of the epistemology and the (relatively) relaxed threshold of legal
fact-finding. Hence, for juristic purposes, it is normatively superior to
slavish adherence to scientific fact-finding.

Inference causations superiority can be maintained, however, only if
IBEs process of constructing and comparing narratives is undertaken
with reference to the evidence, free of idiosyncratic value judgments.
While critics of IBE (or of inference causation generally) may be suspi-
cious of the inherent risk that a legal fact-finder might fudge cause-in-fact
on emotivist grounds, that risk is an unavoidable concomitant of the epis-
temology of legal fact-finding. To reduce that risk, one can scrutinize
judgments to ensure that the underlying factual conclusions are oriented

183 Wright, Bramble Bush, supra note 31 at 1051.
184 Wright adds:

[P]articularistic evidence can negate the instantiation of one of the abstract
elements in a competing causal generalization or lower the ex post probabil-
ity that it was instantiated, thereby eliminating the competing causal gener-
alization or lowering the ex post causal probability associated with it. On the
other hand, particularistic evidence can support the competing causal gener-
alization by establishing that one or more of the abstract elements in the
competing causal generalization was actually or probably instantiated (ibid.).

185 Michael S. Pardo & Ronald J. Allen, Juridical Proof and the Best Explanation (2008)

27 L. & Phil. 223 at 233 [Pardo & Allen, Juridical Proof].

186 Ibid. at 233 (The law of gravity explains why everyday objects fall and the speed at
which they do so; these observable events justify belief in the accuracy of the theory un-
der ordinary conditions).

40 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

toward the evidence. Assuming that sources of evidence and inference are
dual bases for conclusions about past events,187 evidentiary sources must
be taken seriously because they immunize the process from pure caprice.
What happens, however, when both parties offer improbable explana-
tions? In this event, the solution advanced by Allen and Pardo drifts from
the legal imperative of veritism: If both parties offer poor explanations,
jurors should find for the better of the two parties explanations (unless
they can construct a better one for themselves, based on the evidence they
have heard).188 More recently, they have explained, [T]he key point is
the comparative aspect of the process. A verdict will (and should) be ren-
dered for the better (or best available) explanation, whether one of the
parties or another constructed by the fact-finder.189 The sole reference
point, according to Allen (here writing alone) is the commitment to ra-
tionality.190 A verdict is therefore entered for whichever party offers the
more plausible account, however implausible it may be, and irrespective
of whether it satisfactorily accounts for the totality of the evidence at
trial.191

This solution is deeply flawed. As I have already pointed out, legal
fact-finding occurs within a range below a total probability of {1.0}. A
court will never have before it all the explanatory hypotheses; thus all the
known possibilities, taken together, will never add up to a total probabil-
ity of {1.0}. The legal fact-finding challenge is not, then, to ensure that the
account chosen in aggregate adds up to {1.0}. It remains necessary, how-
ever, for the account to meet a threshold of probability. The problem with
Allen and Pardos solutionthat we infer to the best explanation, even if
that explanation is not even likely to be trueis that it does not conform,
or at least does not necessarily conform, to the legal imperative of meeting
a probability threshold. I have attempted to show that inference causation
is not only immanent in a particular process of rational reasoning, but is
also normatively desirable because it is veritistic. It takes seriously the
reference point of evidence, albeit subject to the inherently uncertain civil
standard of proof that legal fact-finders apply. Inference causation is
therefore insufficient to form the substance of an inference of fact with an

187 Jerome Frank, Courts on Trial: Myth and Reality in American Justice (Princeton:
Princeton University Press, 1950) at 80 (the trial process involves an intelligent in-
quiry based on the evidence).

188 Allen & Pardo, Probability, supra note 173 at 316.
189 Pardo & Allen, Juridical Proof, supra note 185 at 238 [emphasis in original].
190 Allen, Factual Ambiguity, supra note 76 at 629.
191 Allen, Batting Averages, supra note 133 at 1093. Jolowicz makes a similar proposition
to the effect that the legal fact-finder need only be satisfied that there is enough evi-
dence to rule out that a particular proposition is not incorrect (supra note 68 at 518-19).

 THE POSSIBILITY OF INFERENCE CAUSATION

41

explanation that is merely the most easily justified in reference to the in-
ferential criteria. The fact-finder must view the chosen explanation not
only as a rational explanation or the best among several possible rational
explanations, but also as a probable account of the linkage (or absence
thereof) between the defendants negligence and the plaintiffs suffering.
In short, IBE ought to be IBPE: inference to the best probable explana-
tion.
What if the explanations are equally good (or bad)? Again, Allen and
Pardo offer a method of deciding that, with a qualification, is acceptable:
If the explanations are so bad (or good) that the jurors cannot decide be-
tween them, then the decision should go against the party with the bur-
den of persuasion on the issue.192 It follows from this method that the
judgment should go against the party with the burden of persuasion not
only when all explanations are equally bad, but also when they are un-
equally bad, yet still bad enough that none of them meets the threshold of
probability. If an explanation is not even likely to be true, it must be re-
jected.

To be found liable, the defendant must have somehow increased the
risk to the plaintiff under circumstances that allow us to infer that he
probably caused the plaintiffs sufferingthat is, under circumstances
that allow us to compensate for our lack of knowledge of whether causal
mechanisms were actualized in a particular case by applying our knowl-
edge of a correlation between risk and suffering.193 But what are those cir-
cumstances? More specifically, what are the criteria that tend to make
one particular narrative more likely to be true than its competitor,
thereby making an inference of cause-in-fact more reliable than its nega-
tion? A complete answer is impossible. If Wrights observation that cause-
in-fact is a complex and subtle concept that long has resisted efforts to
articulate a precise definition194 is correct, it follows that a Rosetta stone
decrypting the cognitive processes by which the mind can reassure it-
self195 in determining cause-in-fact must be even more elusive. Evidence
theorists and others have postulated various (and occasionally inconsis-
tent) factors, including simplicity,196 consistency,197 coherence or consil-

192 Allen & Pardo, Probability, supra note 173 at 316.
193 See text accompanying note 157.
194 Wright, Causation, supra note 114 at 1826.
195 Weinrib, A Step Forward, supra note 52 at 523.
196 Allen & Callen, Juridical Management, supra note 81 at 28 (reference to Occams Ra-

zor); Allen & Pardo, Probability, supra note 173 at 315.

197 Ibid.

42 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

ience,198 coverage,199 uniqueness,200 and the temporal association be-
tween risk and suffering.201 Some of these qualities have been emphasized
by Pennington and Hastie, whose work has demonstrated that, as an em-
pirical matter, jurors appear to focus on coherence, completeness, and
uniqueness in selecting the preferred narrative.202 David Danks, a compu-
tational cognitive scientist whose work involves developing models to pre-
dict linkages between causal observations and inference, has emphasized
inter alia the temporal association between risk and suffering.203 Epide-
miologists have also devised criteria for when an inductive leap from sta-
tistical association to causation might be justified, including some of the
foregoing, including temporality.204 Temporal precedence of the risk
relative to the suffering is also emphasized by computer scientist Judea
Pearl and engineer T.S. Verma as being significant and possibly necessary
for linking statistical association and causation, although they caution
that, on its own, temporal precedence may be insufficient to distinguish
genuine cause-in-fact from spurious associations caused by unknown fac-
tors.205

Temporal precedence suggests an explanation for the divergent treat-
ment of the respective plaintiffs in Snell and in the House of Lords deci-
sion in Wilsher,206 the case that Justice Sopinka relied on (in addition to
McGhee) as an authority for inference causation. In Wilsher, the prema-
ture infant, while only hours old, was negligently oxidized through his
umbilical vein instead of an artery, resulting in his being supersaturated

198 Coherence or consilience refers to the absence of internal contradictions, whether with
regard to elements of the narrative or the evidence found to be true (ibid.). See also
Cohen, Should a Jury, supra note 62 at 476; Burns, Some Realism, supra note 152
at 753; Stein, Refoundation, supra note 91 at 312 (refers to cogency, although this is
tautological, since a factor is by definition cogent as it renders the truth of a conclu-
sion more probable).

199 Coverage refers to the extent to which all the evidence is explained by the narrative.

See Burns, Some Realism, supra note 152 at 753.

200 Uniqueness refers to the absence of a plausible competing narrative (ibid.).
201 Judea Pearl & T.S. Verma, A Theory of Inferred Causation (Paper presented to the
Second International Conference on the Principles of Knowledge Representation and
Reasoning, Cambridge, Mass., April 1991) [unpublished]; Danks, supra note 97 at 18;
Wright, Bramble Bush, supra note 31 at 1014.

202 Pennington & Hastie, Cognitive Theory, supra note 97.
203 Danks, supra note 97 at 18.
204 Miller, supra note 87 at 547, citing Sir Austin Bradford Hill, The Environment and
Disease: Association or Causation? (1965) 58 Proc. R. Soc. Med. 295 (listing nine crite-
ria). See also Mervyn Susser, The Logic of Sir Karl Popper and the Practice of Epide-
miology (1986) 124 Am. J. Epidemiology 711.

205 Pearl & Verma, supra note 201.
206 Supra note 18.

 THE POSSIBILITY OF INFERENCE CAUSATION

43

with oxygen. From one week to nearly six weeks after birth, the plaintiff
was monitored negligently; as a result, he was again exposed to danger-
ously high levels of oxygen. The plaintiff subsequently developed retrolen-
tal fibroplasia (RLF), resulting in blindness. While RLF could have been
caused by the overoxydization, it was likely (but not certain) that the
plaintiff suffered from one or more of five possible conditions, any of which
might have caused RLF on its own. In the House of Lords, Lord Bridge
made the observation that the majority in McGhee had concluded that it
was a legitimate inference of fact that the defenders negligence had ma-
terially contributed to the pursuers injury.207 On the facts of Wilsher,
however, it was not possible to make that inference.208
Why were there different outcomes as to whether it was possible to
judge where the balance of probabilities lay in Snell and Wilsher? Al-
though the reasoning in both cases is peremptory, the facts and outcomes
suggest that the temporal association between the defendants risk and
the plaintiffs suffering may have been seen as significant. In Snell, we
are not told how long the plaintiff had suffered with the pre-existing
sources of risk, but given her advanced age, it is likely that she had suf-
fered from them for a substantially longer period of time than the few
hours of lifespan that preceded the negligent treatment of the plaintiff in
Wilsher.

In considering the factor of temporal association, then, a legal fact-
finder might be less reluctant to infer a causal link from facts resembling
those in Snell than she would be from those in Wilsher. In Snell, perhaps
sufficient time had passed since the onset of the pre-existing sources of
risk for the temporal association between the defendants negligence and
the plaintiffs blindness to support a finding of cause-in-fact. Even if the
temporal association did not support a definitive causal link, it might
have still been possible to conclude that the facts of Snell were more sug-
gestive of a causal link than those of Wilsher, where there was no case
history and therefore no information about whether the pre-existing
sources of risk had ever become symptomatic.

I do not, however, intend to suggest that the criteria for inference cau-
sation can be reduced to temporal association, or even to any of the other
criteria that I have mentioned. Nor do I propose to consider in this paper
the individual or collective strengths and weaknesses of these criteria. If
the cognitive dynamics surrounding proof are complex, then debates over

207 Ibid. at 566. Lord Bridges observation was later rejected by a majority of the House of

Lords in Fairchild (supra note 29).

208 Moreover, Lord Bridge held that the trial judge wrongly applied the test for causation

and ordered a retrial.

44 (2010) 55 MCGILL LAW JOURNAL ~ REVUE DE DROIT DE MCGILL

the criteria according to which those dynamics support inferences of proof
are even more so. The work of evidence scholars like Allen has led to
doubts about the feasibility of describing or reproducing those processes of
cognitive dynamics.209 Moreover, efforts to borrow from the insights of
other disciplines inevitably suffer from severe problems of communica-
tion, not only because of mutual inaccessibility but because of divergent
understandings of what is relevant: What is viewed as interesting or vi-
tal in one discipline may be viewed as tedious and boring in another.210

I can only note that these criteria might usefully serve as qualities
against which the narratives constructed from the evidence can be meas-
ured, thereby reconciling the veritistic reference point of the evidence
with the epistemology of legal fact-finding. Briefly put, these criteria may
contribute to what H.L.A. Hart and Tony Honor describe as special co-
gency in the evidence.211 This statement might invite the objection that
such special cogency is not enough because it still allows fact-finders to
fudge cause-in-fact. This objection, however, falls flat, and not merely be-
cause of the need for finality in adjudication or the unavoidably inferen-
tial nature of legal fact-finding. Complaints about fudging also fail be-
cause they are grounded upon the dubiously pessimistic assumption that
legal fact-finders do not act in good faith by observing the ethical impera-
tive of applying good judgment to the evidence. Judgment, as Anthony
Kronman reminds us, is not the same as intuition:

If judgment is conceived of as a process of reflection followed by a
moment of intuitive insight, then our assessment of the soundness of
a particular judgment can never depend on the reasons given to
support it, since … [its quality] will be a function of its intuitive bril-
liance and originality and these are qualities that, by assumption, no
reasoned argument can express.212

This conclusion is wrong because for good judgment to be seen as such, it
must provide not only insight but also a compelling account. In other
words, judgments argumentative dimension distinguishes it from intui-
tion,213 bringing it within the category of what Hart described as charac-

209 MacCrimmon, Common Sense, supra note 1 at 1436-37. See also Mirjan Damaka,

Truth in Adjudication (1998) 49 Hastings L.J. 289 at 292.

210 MacCrimmon, Common Sense, supra note 1 at 1438.
211 Hart & Honor, supra note 31 at 422.
212 Kronman, supra note 71 at 849.
213 Ibid. at 850.

 THE POSSIBILITY OF INFERENCE CAUSATION

45

teristic judicial virtues that judges should use in cases requiring discre-
tion.214

Justly applied, then, IBE is no more an instance of intuition than it is
of deductive, scientific reasoning. Rather, it entails drawing and ulti-
mately justifying a factual determination, including one of cause-in-fact.
IBE requires simply that a legal fact-finder give each alternative its due
and entertain all the possibilities by feeling for himself what is most at-
tractive in each.215 In the end, consigning cause-in-fact to a judgment call,
literally speaking, should not leave an observer in the dark about why
cause-in-fact is or is not found in a given instance. Legal fact-finders must
be able to offer some rational explanation that earns public acceptance of
the factual determination being made. For example, the temporal associa-
tion between the defendants negligence and the plaintiffs blindness in
Snell, combined with the prior asymptomology of the pre-existing sources
of risk, might offer a rational, coherent, and deliberative (and therefore
acceptable, if not universally agreed upon) grounding for a causal link.

Conclusion
Critics of inference causation may dislike its lack of determinacy, but
indeterminacy is a plainly unavoidable concomitant of legal fact-finding,
not a defect in inference causation. The mere fact that inference causation
leaves open an opportunity for substituting emotivist fudging for good-
faith, deliberative fact-finding is, on its own, a feeble indictment; tort law
is replete with opportunities to fudgeon the reasonableness of the de-
fendants conduct, on the closeness and directness of her relationship in
law to the plaintiff, and on the foreseeability of the suffering. The alter-
native espoused by critics of inference causationrelying upon scientific
fact-finding, which operates at a more severe thresholdhas been shown
to be untenable. Moreover, critics insistence upon probabilistic evidence
connecting risk with suffering has obscured the mutually distinct episte-
mologies of legal and scientific fact-finding. In the end, even though en-
couraging legal fact-finders to confine inference causation to the best
probable explanation leaves room for indeterminacy, it may be the closest
we can come to facing epistemic reality without falling into emotivist error.

214 Hart specifically listed among such virtues a concern to deploy some acceptable general
principle as a reasoned basis for decision: H.L.A. Hart, The Concept of Law (Oxford:
Oxford University Press, 1961) at 200.

215 Kronman, supra note 71 at 853.