Journal of Philosophy, Science & Law
10, April 12, 2010
The Flexibility of Description and NESS Causation
Barrister and Solicitor, Toronto, Canada
Two arsonists start separate fires to the same house. Each fire is sufficient to destroy the house
on its own. Neither is therefore sine qua non necessary to the loss. A growing number of scholars suggest that the sufficiency of the two fires deems them causes-in-fact of the loss. The most influential sufficiency analysis of recent years is the NESS test developed by
Hart and Honoré
and Richard Wright.
The NESS test of
causation popularized by Professor Richard Wright, is emerging as the new
supplement to the but-for test for the twenty first century.
That test is as follows:
[A] particular condition was a cause of
(condition contributing to) a specific consequence if and only if it was a necessary element of a set of
antecedent actual conditions that was sufficient
for the occurrence of the consequence.
Wright boldly declares that the NESS test “not only resolves but also
clarifies and illuminates the causal issues in the problematic causation cases
that have plagued tort scholars for centuries”
and that it “is the essence of the concept of causation”.
The suggestion that the NESS test resolves all outstanding causation issues is
overstated. As this paper argues, the
test provides a way for judges to allocate responsibility in the face of
prohibitively complex or indeterminate causal processes. It argues that there are essentially three
situations where the law might viably substitute a necessity for a sufficiency test:
(a) where the law must attribute responsibility for a
legally indivisible entity that, in nature, is composed of infinitely smaller
organic constituent parts (such as house destroyed by fire);
(b) where the law must attribute responsibility for
the passing of a legal or natural threshold (such as intoxication or death); or
(c) where it is impossible for a court to determine
the necessary causes in a particular process because the process is too complex
or microscopic for explication in a courtroom.
The reliability of a sine qua non or NESS analysis depends on the causation question. Causes and effects can be framed in varying
degrees of detail. This paper argues the
degree of detail given to an effect affects how we determine causes. Where generalized descriptions are given to
an outcome, more than one causal factor can appear as independently sufficient
to produce the result. If more than one
cause appears as sufficient, none of these sufficient causes appear as
necessary. In these cases, the sine qua non test does not work and a
sufficiency test may be required.
On the other hand, if the description of the
result is highly detailed, a greater number of the attending circumstances will
be deemed necessary to that particularly described result. In these cases, the sine qua non test works fine.
The contribution of this paper is, first, to clarify that the NESS test
only improves upon the sine qua non test
where we are concerned with broadly or generally described outcomes. This point is neglected by proponents of NESS
causation and leads to confusion. Except
where broad descriptions are
employed, the NESS test adds nothing. On
the other hand, broad descriptions
are often essential and necessary to legal decision-making. The second contribution of this paper is to
suggest that broad descriptions are needed where courts are concerned with
something considered indivisible, such as a threshold, or with prohibitively
complex processes. The NESS test
suitably responds to these cases.
Necessity and Sufficiency
Since Hume, the modern approach to the question of
causation has been to disregard the possibility of unobservable “forces” or
“powers” acting on the elements of a causal sequence. Rather, Hume argues causation is only
understood through generalizing from our empirical evidence. Where two things occur in “constant
conjunction” within a “regular sequence” the factors come to be experienced as
“cause” and “effect”.
The thesis was revised and reformulated by J.S. Mill
in System of Logic. Mill accepted
that causation is not discoverable from innate properties or forces operating
within or on the elements of an event. He rejected, on the other hand, Hume’s suggestion that the “conjunction”
between elements to a sequence need be universal and “constant”.
Mill argued that a plurality of factors can
bring about a particular type of occurrence. A “cause” does not need to precede an “effect” in every set of circumstances. Causation can be established if the preceding element was necessary for the event in the
particular circumstances of the
occurrence.The theory developed in System of Logic leaves us with
the sine qua non test used to determine causation in law today.
The NESS test extends the plurality thesis further. While Mill argued that a plurality of factors
or circumstances can produce a type of outcome, H&H and
Wright suggest a number of concurrent factors can be sufficient to produce a
particular occurrence in a specific
instance. The NESS test responds
to instances where there appears to be a multiplicity of sufficient
causes. There are a number of famous
examples in the cases and literature:
A fire is set by the engine of
a train. This fire unites with a second
fire sparked by the engine of a different train owned by a different railroad
company. The fire destroys a plaintiff’s
Two motorcyclists drive
noisily past a horse-drawn wagon. The
sound of the motorcycles spooks the horse and causes a plaintiff injury.
Two hunters are grouse hunting. They simultaneously shoot and strike a third
hunter who instantly dies. It is
impossible to say which bullet strikes first.
The accumulation of five units
of pollution is sufficient to cause an injury. Seven defendants, acting independently, each discharge one unit of
In these cases, our intuitions suggest that each of the defendants is
responsible. It would seem
counterintuitive to suppose that neither fire destroyed the property or neither
bullet fatally wounded the third hunter, for example. On the other hand, the sine qua non test would relieve both railroad companies and both hunters
of responsibility. We are not able to
say that, but for the negligence of
the first railroad company, the plaintiff's property would have gone
unharmed. If the first engine hadn't
sparked a fire, the second spark would have. Neither of the motorcycles is necessary to stir the horse. If the first hadn’t, the second would have
and vice versa. If the first
hunter’s bullet hadn't fatally struck the victim, the second would have and vice
versa (similarly, for each individual polluter). The sine
qua non test leaves these plaintiffs without recovery, notwithstanding that
the sources of their injuries are more or less clear.
Substantial Factors and
In response, American tort law adopts a “substantial factor”
test and Anglo-Canadian law a “material contribution”
test. These tests supplement the shortcomings of sine qua non causation. In
instances of where intuitively causal factors are not deemed independently
necessary for the loss, a court can instead ask whether the defendant’s conduct
was a “substantial factor” or “materially contributed” to the circumstances of
a loss. The test seems clear. Both fires, both gunmen and both
motorcycles can easily be said to have “materially contributed” as “substantial
factors” to the losses in issue. On the other hand, it is far from clear
what phrase “material contribution” or “substantial factor” actually
means. Wright has attacked the substantial
factor test as vacuous rhetoric providing little instruction to judges in
The tests suggest that a factor is
“substantial” or “materially contributes” to a loss if it helps bring about the
loss. A factor which helps cause an occurrence, is thus considered one of its causes. The language is clearly circular.
Imagine that there was a fourth hunter who simply fired a blank
round. Imagine that at the time the two
fires engulfed the plaintiff's property the local firefighters were sleeping on
shift, but that the fire station was an hour away and that the plaintiff’s
property was completely destroyed in less than an hour. We would intuitively say the blank round and
sleeping firefighters did not “materially contribute” to those losses. Why is this so? Normally we would say that this is so because
they were not necessary ingredients in bringing about the loss. If the third gunman hadn't fired the black
round, death still would have ensued. If
the firefighters hadn't been sleeping, they still would have arrived at the
scene too late to save the building. On
the other hand, the second gunmen’s bullets and the second steam engine were
equally unnecessary to the losses. As
were the first. The “material
contribution” and “substantial factor” tests do not provide much in guidance to
distinguish between these intuitively causal and intuitively non-causal
The NESS Test
The NESS test provides a means to handle these types of problems. We can say that the third gunmen, the
sleeping firefighters or the electric car were not “causes” as they were, both, unnecessary to produce the loss and insufficient to do so. The spark from the first engine, however, was
a necessary element in starting the first fire and was sufficient in the
circumstances to bring about the loss. The spark was, thus, a “cause”. Similarly, the first motorcycle or the first shot were necessary parts
of sufficient sets and, thus, “causes” under the NESS test.
General and Specific Descriptions
The principal virtue of the NESS test is in its ability to make up for
the shortcomings of the sine qua non test where there appears to have been multiple sufficient causes.
Wright also declares the NESS test as an
advance over sine qua non causation
for its ability to resolve instances of “preemptive causation”. Examples in the literature include:
Two assassins independently
plot to kill a desert traveller. The
first poisons the water in the traveller's canteen. The second, not knowing about the poison,
empties the canteen. When the traveller
opens her canteen to have a drink, she finds it empty and ultimately dies of
A mechanic fails to make the
required repairs to a set of car brakes. The driver of the car approaches an intersection. He does not attempt to use the brake pedal
and strikes the car ahead of him.
In a case of “preemptive causation”, a factor that could have been
sufficient to produce a loss is “preempted” by a second that actually brings
about the loss. The attempt of the first
assassin is preempted by the actions of the second. The traveller dies of thirst, not of poison.
Similarly, the negligence of the garage in
repairing the brakes is said to have been preempted by the negligence of the
Notably, in both examples, the preempted set of factors was sufficient to produce the loss. The NESS test,
however, is more specific. It requires
that the set of conditions sufficient to produce the loss actually be
operating at the time of the loss. If a man dies in a hospital the day before a nuclear attack kills
everyone in his town, we would not say that the nuclear attack, though
sufficient to kill him had he survived, was a “cause” of his death. It was not actually
operating at the time. Where this
qualification is added, the NESS test can be used to resolve the examples
Nevertheless, this analysis is not an advance over sine qua non. Both tests operate in the same way. Suppose a man is fatally shot and lies
bleeding on the street, beyond the hope of treatment. While lying on the street, another attacker
stabs him until death.
If we ask who is responsible for the victim's death, the second
attacker seems to be exculpated under the sine
qua non test. The victim would have
died as a result of the shooting regardless of the actions of the second
attacker. On the other hand, if we ask
who was responsible for the death in question or his death under the
circumstances it is clear that both assailants are responsible. The victim would have survived longer had it
not been for the second attacker. Both
factors were, thus, necessary to bring about the specific death in question.
This is essentially the same qualification Wright adds to the NESS test
when he requires that only sufficient sets actually operating at the
time of an occurrence be considered.
assailants' actions are necessary and sufficient to produce the loss suffered in the circumstances. It is only where a general description
of the events is used to describe an occurrence that a sufficiency test
is required. Mackie makes this
observation regarding the desert assassins example. He writes:
If Y is the fact that the traveller died
of thirst, then the puncturing of the can both caused and was necessary in the
circumstances for Y. If Y is the fact that the traveller
died on this journey, then the puncturing of the can neither caused nor was
necessary in the circumstances for Y.
Lawyers might consider whether a death would have occurred, “but for”
cause Y. However, this question contains implicit
limits. Everyone dies. If Y doesn’t
kill a person, an infinite amount of other things might and at least one thing
will. A death needs to be framed with some degree of specificity in order for a
factor to appear as a necessary sine qua
non or sufficient NESS cause actually operating at the time of the
death. Wright, thus, requires that
sufficient sets actually be operating at
the time of loss in order to be deemed NESS causes. As is discussed below, this qualification
leaves the NESS test without anything to add to sine qua non causation in these circumstances.
The Flexibility of Description
There is a great deal of flexibility with respect to the generality or
specificity that can used to define an occurrence. At the highest point of specificity, a
sequence of events will have both been necessary to produce the exact set of
circumstances which took place and, moreover, the only sufficient
sequence which could have produced those circumstances.
Nothing besides the life of Elvis Presley,
for example, could have produced the exact circumstances of his
death. As the description of the event
becomes more generalized, other factors appear sufficient to produce the
generally-described type of loss. Where
factors operate concurrently to
produce a generally-described loss, the NESS test can be used to assign a
causal status to the sufficient, though unnecessary factors.
Suppose, for example, two children, without planning or coordination,
simultaneously swipe at a large bowl of salad and knock it off a kitchen
table. Suppose we describe the result as
“the bowl falling on the floor”. Neither
of the swipes at the bowl is necessary for this result. If the first child hadn't struck the salad
bowl, the other would have. The sine qua non test seems to suggest that
neither child caused the loss. We need
to resort to NESS causation. Both of the
swipes at the bowl, on the other hand, are independently sufficient to cause
the bowl to fall on the floor. The
children were, thus, NESS but not sine qua non contributors.
If, on the other hand, the description of the result includes the exact arrangement
of the salad on the floor, both swipes at the bowl become sine qua non factors. Both are necessary for sending the
bowl flying off the table with a particular velocity and trajectory and thus
are necessary to produce the specifics of how the salad lands on the floor.
Wright offers two arguments against the idea that the generality
employed in a description of an effect affects the necessity of contributing
factors and, thus, the relevance of the NESS test. He argues that it is circular to, first,
describe an event in detail (“as it came about”) and then to suggest that the
details were necessary to the event, so described.
Wright's argument, however, demonstrates the
same circularity. Wright suggests that
irrelevant circumstances which happen to contextualize a cause-effect
relationship can be discounted as “causes” if we limit ourselves to considering
the sufficient sets actually operating in the circumstances. But how do we determine whether a factor is actually operating? Wright provides no
guidance. He writes, referring to the
“desert traveller” example:
How this result came about is the very issue to
be resolved. To include how the result
came about in the description of the result is to assume an answer to the
causal question before it is posed. Thus, in the desert traveller example, the question of whether the
traveller died by thirst, by poisoning, by both, or by neither, is precisely
the issue that the causal inquirer is supposed to resolve. To describe the death as 'death by thirst' is
to assume an answer to that question before it is posed...
He suggests that the assumption that the victim “died by thirst” rather
than simply “died on this journey” decides the answer to the causal question
before it is asked. This is only partly
true. We might infer that the victim
“died by thirst” when we discover a dehydrated body. This link in the causal chain is assumed. However, that is not what is in issue. It is simply part of the narrative. The causal puzzle is whether the first or
second assassin played a causal role in that particular death. The question is “Why was the victim's body
dehydrated?” or “Was the conduct of one or both of the actors necessary in
bringing this about?” Describing the
death in detail does not assist in telling us which of the actors is
Second, Wright argues that using specific descriptions of an
event makes every circumstance or factor situating an event a “cause” and does
not allow us to distinguish between causes and correlations. Wright uses the
example of two shooters whose bullets simultaneously strike a victim's heart.
Wright asks us to imagine that a third party was climbing Mount Everest at the
time. He argues that employing a specific description of the heart wound leaves
it impossible to distinguish between causal and correlative factors situating
the attack such as the third party's ascent of Everest:
None of the three conditions by itself was a
but-for-cause of the victim's death, but the cluster composed of all three of
them was a but-for-cause. The aggregate
but-for test does not distinguish the actual causes-- the two bullets-- from
completely irrelevant conditions such as B's climbing Mt. Everest.
It is true that employing a description which
specifies every circumstance situating an event leaves it impossible to
separate causal from merely correlative factors. As suggested above, every factor will be “but
for” necessary to produce that precise description. On the other hand, Wright overstates Mackie's
point. There is a clear middle ground
between employing a perfect description that includes every circumstance of an
event and one that is hopelessly vague. There is clearly, for example, a middle
ground between asking “Why all the circumstances on the earth appear as they
did at the moment of this victim's death, including B's climbing of
Everest?” and asking “Why do people die?” We might ask why
the victim died at that time. Posing the
question in this manner leads to overdetermined causation, given that
the two sufficient bullets pierced the victim's chest at the same
time. On the other hand, we might ask
why the victim died of two bullet wounds. Here, both bullets are sine qua non necessary
to the latter question. Both questions
are legitimate. The first is slightly
more generalized than the second. However, both questions exist somewhere between perfect specificity and
perfect generality. Every causal
question put to the courts will be framed in this middle ground.
The NESS test, however, has a fatal problem with
broadly-described consecutive factors. If the description of an event's duration is extended far enough, we may
be required to label things as “causes” that are intuitively non-causal. Imagine the victim dies immediately upon
being shot. The second assailant
however, does not know this and, like before, stabs the by-now-deceased
victim. If our test is sufficiency and we ask what factors were sufficient to cause the loss that day, the
stabbing, as a sufficient factor, is a cause of the death. Clearly this is wrong. The man was already dead. Wright, accordingly, requires the
qualification that a NESS factor be operating at the time of a loss—or,
that the time frame under consideration be stated with specificity and
precision. If we state the time of death
with precision, however, the sine qua non test is perfectly acceptable
and the NESS test adds nothing. Sine qua non like the NESS test, can
explain that the stabbing party's act was necessary to precipitate and,
thus, cause the death of the victim in the first scenario. It is unnecessary to the causal
sequence in the second scenario where the stabbing party attacks an already
dead victim. The NESS test is relevant
to circumstances where an event is described so generally as to allow for multiple concurrently-operating factors sufficient to a produce a loss. It does not assist with “preemptive
causation” questions where sufficient factors occur one after the other.
Indivisible Entities and
Uncertain Causal Processes
As I have argued, the degree of generality or specificity given to the
description of an occurrence affects whether the sine qua non or a NESS
test for causation better supports our “causal intuitions”. If a description is broad enough to include
more than one sufficient causal factor or set of factors, certain factors will
appear redundant, unnecessary and thus, not sine qua non “causes”. In such circumstances, the NESS test can be
used to assign a causal role to the redundant factors.
This raises a further question. If a sine qua non test provides an adequate explanation of
causation where descriptions of events are specified with precision, why would
the law ever employ generalized descriptions? I suggest three reasons. First,
the law must often assign responsibility for legally or putatively indivisible
losses or injuries. Second, lawyers and
judges often need to deal with minute or intricately complex causal processes
unsuited for explication in the court room. Where it is intuitively clear that a factor contributed to a loss, but
impossible, due to the complexity of a causal process, to state the nature of
that contribution, the law can employ a sufficiency test to ensure that a
causal contributor is held liable. The
third reason borrows from the other two. Where a reaction results after some threshold is breached and
more than one party is responsible for the breach, a sufficiency test, again,
can be used to hold parties accountable.
Let's slightly modify an example from above. Two independently-acting arsonists, both
unaware of the presence of the other party, start a house on fire. One arsonist starts a fire at the north side
of the house; the other, from the south side. Both fires are sufficiently powerful to destroy the house in its
entirety. Each fire, however, consumes
one part of the house alone. The fires
join in centre to destroy the remains. Both arsonists are held jointly and severally liable for the entire
This result, in one sense, seems
fictive. While both fires are
responsible for the destruction of the entire house, the first fire destroys
one part of the house and the other fire, another. The fires only destroy part of the house
together. A more specific (and accurate)
description of the damage would detail the portions of the house destroyed by
Nevertheless, the house may be treated, for the purposes of law, as an
indivisible entity and not as the aggregate of its constituent parts. If the law were to simply hold the parties to
account for damage to the constituent parts of the thing, the sine qua non test works fine. We ask, “but for fire A, would this part of the house have been destroyed?”. On the other hand, where the law holds
parties to account for a thing as a whole the sine qua non” test leads to causal overdetermination. Both fires are sufficient to destroy the
house and, thus, neither are necessary. Here, the NESS test is useful. This accords with our general observation that broader descriptions of
events can allow for multiple sufficient causes and, thus, create situations of causal overdetermination resolvable using the NESS test.
It is, thus, the conceptual or legal existence of a “house” as either a
complete entity or a collection of its parts that dictates the procedure used
to identify causes. Wright, although far
from admitting that the NESS test is something to be used as a matter of policy
or exigency, seems, on the other hand, to partly concede the point. He quotes
Betch and Miller's comments on the redundant fires example:
Wright responds, as follows:
Again this assertion begs the question. Under this approach the defendant's fire was
a but-for cause only of the precise “positions of the smoke, ashes, and some
parts of the ruins,” not of the destruction of the house, for which these
precise details are irrelevant. The
detailed description will be useful only if the plaintiff wishes to recover for
damages that would not have occurred but for the precise position of the smoke
and debris—a most unlikely lawsuit.
It is, thus, the pleadings, rather than the facts,
themselves, that determine the applicability of the NESS test.
It would likely be very difficult for a court to determine the precise
measure of damage caused by each fire in this example. It would certainly be so if the two fires
joined prior to reaching the house and consumed the entirety of the house
together. When the fires meet, the
energy of one fire becomes an inseparable part of the energy of the other. Pollution is another important example. In situations where pollutants mix together,
one source becomes impossibly mixed with another. Determining the individual impact of either
In such circumstances, it is intuitively
clear that both sources contribute to the injury. Each source forms an inseparable part of the
same injuring force. Where the law is
unable to distinguish between the contributions of one party to an injuring
force and those of another, issues of causal overdetermination may
If two polluters each dump a sufficient
amount of pollutants into a body of water to cause a plaintiff injury, neither
act is sine qua non necessary for the
injury. Yet both clearly
contribute. NESS causation is useful
A related problem of indivisibility arises in cases evidencing
what Michael Moore has described as asymmetrical overdetermination.
A causal process is asymmetrically
overdetermined if one of the contributing factors plays a lesser causal
role and is, in itself, insufficient to produce a loss, while another plays a
larger, independently sufficient, role. Moore uses the fires example:
[S]uppose the fire set by the
defendant is much smaller than the second fire; the two join as before and the
resultant fire destroys the structure. The second fire would have been sufficient by itself to have destroyed
the structure, but the defendant's small fire would not have been, since it
would have been extinguished by the available equipment before it could have
destroyed the structure.
The causal ramifications are, again, different
depending on whether we employ a rich or a broad description of
the loss. If we employ a rich
description detailing the exact measure and magnitude of the injuring force
and the exact nature and scale of the loss suffered, all of the constituent
elements, both big and small, will be collectively necessary and
sufficient. The amount of pollutants
released by one of the lesser contributing parties forms a necessary part of
the nature and scope of the injury and the level of toxicity created by the
conduct of all the parties. The sine qua non test aptly handles this
situation. If a broad description of the loss is used, in contrast, the lesser injuring force is not a cause on
the application of either the
NESS or sine qua non test. If the type of damage suffered from exposure
to pollution is described in a broad way, analogous to the destruction of a
“house” as a singular totality, the release of a small, independently
insufficient amount of pollution is neither necessary nor sufficient to
produce the loss.
Wright's answer to the problem of asymmetrical
overdetermination is curious. On the one hand, Wright argues that is
“useless tautology” to employ a description of an event in a degree of detail
such that every preceding circumstance becomes a necessary part of the sequence.
the other hand, Wright seems to concede that a certain degree of detail in the
description is required in order for an insufficient factor (i.e. the smaller
fire) to form a necessary part of the sequence as an NESS cause. Referring to
the concurrent fires example, he writes:
The same causal situation
exists even if there were only two fires, one of which was independently
sufficient and the other of which was not. The first fire was clearly a cause, since it was independently
sufficient. But the second fire also was
a cause. It was necessary for the
sufficiency of a set of actual antecedent conditions which included another
fire (the first) that was “at least large enough to be sufficient for
the injury if it merged with a fire the size of the second fire.” The sufficiency of this set is not affected
by the fact that the first fire was so large that it would have been sufficient
by itself... The word of the quoted
condition, “at least large enough,” is not a verbal gimmick. The condition is an actual one that existed
on the particular occasion.
Wright's answer is confusing. He argues that the condition “at least large
enough” qualifies the hypothetical sufficient set under consideration to those actually
existing at the time. However, if
the larger fire was at least large enough to destroy the house, the
second fire becomes an unnecessary part of the circumstances. It is neither sine qua non necessary, nor a necessary element in a broadly
described sufficient set. It is true
that the second fire is part of the conditions that actually existed at the
time of the occurrence. However, if the
second fire is an unnecessary part of those circumstances, it appears as a
correlative rather than a causal factor.
Suppose one of the arsonists wears a wristwatch or
that the fire is lit under a full moon. These also form part of the conditions which contextualize the
destruction of the house. The
distinction between the smaller fire and the moon or watch is only explicable
by reference to the consequences wrought by the energy of the fire. A fire, no matter the size, burns. Watches and moons do not. The smaller fire, moreover, played some role
in the burning of the house. It was not
necessary to destroy the “house” as an indivisible unit. It was, on the other hand, a necessary factor
in effecting the nature, magnitude and shape of the destruction. The moon and wrist watch had no impact on the
nature or scale of the destruction. It
is only when we employ a rich description of the fire that the smaller
fire becomes a necessary element to the sequence.
Perhaps this is what Wright is alluding to when he
The word of the quoted
condition, 'at least large enough,' is not a verbal gimmick. The condition is an actual one that existed
on the particular occasion.
The “actual one”, “at least large enough”, in
Wright's comment, refers to an injuring force effecting a very specific
injury. It is the force which destroys
an entity in a particular manner and magnitude. The description of the “cause”, again, becomes inextricably linked to
the description of the “effect”. A
smaller fire is unnecessary to effect the damage, broadly-described, but
was a necessary element in the richly-described destruction of the
house. Employing this rich
description, however, leads us to Betch and Miller's conclusion
in minute detail, it would
probably appear that the defendant's [smaller] fire was a cause, for the
positions of the smoke, ashes, and some parts of the ruins might well have been
caused by the [smaller] fire.
Again, Wright's resistance to Mackie and Betch and
Miller's observation on this point seems misplaced given the necessity of using rich descriptions in order to make a smaller, independently-insufficient
contributor a necessary element in a NESS (or equally, a sine qua non) set.
Notably, in cases of asymmetrical
overdetermination—or, where there is a contributing sufficient factor and a
contributing insufficient factor—an NESS assessment never adds anything to sine
qua non causation. If a broad
description is used, the larger fire will be both necessary and sufficient
for the loss and the smaller fire, unnecessary and insufficient. If a rich description is used, both
fires are sine qua non necessary. The NESS test adds nothing. It
only becomes useful in situations where a broad description is employed
and where there is more than one sufficient factor (or set of factors). These cases are likely easier for our
“causal intuitions”. If two fires large
enough to destroy a house combine, it is easy to intuit that both fires
“materially contributed” to the damage. The NESS test provides a sophisticated explanation of why this is so. The relevance of the test, on the other hand,
is dependent upon the how the consequences are described. Many things can burn
a house down. Only a particular set can burn a house down in a particular way.
The relevance of the NESS approach is dependent upon whether a legal inquiry is
looking to resolve a general or particular causal question.
This conversation has an important application to
causal questions that involve thresholds. Thresholds can have natural and physiological or social and legal
basis. For example, after reaching a
certain blood-alcohol threshold, a person is no longer legally able operate a
motor vehicle. While the effect of
alcohol on a human body is a physiological process, the threshold, itself, is
social. It has been set in light of the
danger that attends combining alcohol with motor vehicle transportation. On the other hand, there are also strictly
physiological thresholds. After a person
reaches a blood alcohol level of 400 milligrams of alcohol for 100 millilitres
of blood, he or she runs a serious risk of death. Death is a physiological threshold. Upon death, the heart will stop pumping blood
and electrical activity in the brain will permanently cease. Thresholds can be breached by incremental
contributions from a number of different sources. The (legal or physiological) injury incurred,
on the other hand, is indivisible.
Where contributing factors are independently
insufficient, but cumulatively sufficient to cause to breach a threshold, they
are sine qua non necessary in their
aggregate. Suppose, for example, a patron
at a bar is served alcohol by two different servers taking back-to-back
shifts. The first server brings the
patron a very large number of drinks sufficient to cause his death. He drinks these very fast and orders another
round immediately from a second server who is unaware of the tab with the
first. This round is sufficient, again,
in itself, to put him past the point of death. If we think of the two rounds as having been consumed consecutively, the first is a necessary and sufficient cause
of the death, and the second is not. A sine qua non test will suffice. As the first round was sufficient to cause his death, the second round
was unnecessary. If we think of them as
having been consumed concurrently, on the other hand, neither round is necessary. If he did not take the first round, the
second would kill him and vice versa. An NESS analysis is needed.
In situations of asymmetrical overdetermination, again, sine qua non causation will suffice. Suppose the first round of drinks was more modest and insufficient to
cause the patron much harm. Before these
drinks are absorbed into the patron blood stream, the second server brings a
round sufficient, in itself, to kill him. Was the first round a necessary and sufficient element in the
death? If we simply consider whether the
round contributed the patron's state of drunkenness or toxicity, the answer is
clearly 'yes'. Intoxication is
gradated. A person passes into states of
drunkenness in gradual or imperceptible degrees. The smaller, first round would serve as a
necessary element in moving the patron towards that state. Death, on the other hand, is not a matter of
degree. Its all-or-nothing nature means
that a smaller, insufficient factor will be an unnecessary attending
circumstance where death is succeeded by a larger, independently sufficient
injuring force. The first round is,
thus, an unnecessary part of the set of circumstances which caused the death
and is neither a sine qua non nor
NESS cause of the sequence.
Omissions raise a particular challenge for sine qua non causation. Even when the causal status of an omission
seems intuitively clear, in any particular instance an innumerable number of
other factors might have been capable of preventing the loss or injury suffered. Take the example referenced above. A garage does not repair the brakes on a car
as required. The driver of the car
approaches an intersection but omits to use the brake pedal. She strikes the car ahead of her. From
a non-legal or mechanical point of view, describing either party's omission as
a cause is problematic. The car was propelled to the scene of the accident by
fuel and by its engine. It collided with
the vehicle ahead as a result of this force. It is certainly possible to state that the collision would not have
occurred if the brakes had been operable and had been applied. However, the collision also would not have
occurred if the driver diverted the vehicle up a runway truck ramp or if she
simply didn't bother to drive that day, or if the vehicle was struck by
lighting or was slowed to a halt by maple syrup. The number of things which didn't but might
have occurred to stop the cars from colliding is limitless. They are, on the other hand, entirely
speculative. It is, rather, the duties which bear on the occasion that make the omissions visible as causes. These duties
exist socially. They are not a physical
part of the sequence of events. If the
driver had no duty to stop, her failure to do so would not be seen a “cause” of
the accident—or would only be a “cause” in the most arbitrary and speculative
On the other hand, once a legal duty is established, an omission can be
analyzed much in the same way as factual causes. An omission tends to have a location in time,
much like a physical factor, and its causal significance can be similarly
analyzed in relation to the rest of a factual sequence. Suppose parents owe a duty to ensure that
their children are safely fastened to their car seats. If a parent omits to strap his child into a
car seat, is subsequently involved in an accident and the child is injured, we
can ask “but for the parent's omission would the child have been injured?”
The NESS test does not assist in sorting out causal problems involving
omissions. Where an incident occurs and
there are multiple concurrently operating omissions, the NESS test is
unnecessary. If any of the omitted
preventative acts were performed in the circumstances, the accident would not
have occurred. The omissions were sine
qua non necessary to the occurrence. Suppose both of the child's parents had an independent legal duty to
ensure the child was strapped into the car seat. Had either parent strapped the child in, the
child would have been protected. Both
(temporally concurrent and independently sufficient) omissions are thus sine
qua non causes.
This paper has argued that:
The NESS test provides a useful tool for sorting out causation issues
an event is described broadly
such that there are multiple sufficient causes;
these causes operate
the causes form an indivisible
injuring force or inflict an indivisible injury such as the breach of a
The NESS test does not add anything to a sine qua non analysis
only one of the causes is
sufficient (situations of asymmetrical overdetermination);
the injuring forces operate
consecutively (situations of preemptive causation);
a rich description of
the injuring force or the injury is employed such that all of the
constituent factors become necessary to complete the description; or
where there are omissions
which are independently sufficient to cause the loss.
Both the sine
qua non and the NESS test provide relatively clear and, in most
circumstances, easily and predictably applied analytical tools for assessing causation. They are not, however, suited to every
purpose. Importantly, the sine qua non test leads to the counterintuitive
conclusion that a factor which, in itself, can cause a certain loss, does not play
a causative role if operating concurrently with another factor, equally able to
do so. In such circumstances, the NESS
test supports the intuition that both factors contribute to the loss. However, such circumstances only arise where losses
are described in a suitable degree of generality. The detail in the description given to an
injury affects whether problems of overdetermination arise and, thus, whether an NESS analysis is required.
As Wright suggests, plaintiffs generally do not need
to prove the minute details of a loss, but rather, must show a chain of
causation from the defendant’s conduct to a broadly defined legal injury. At some point, the degree of detail in the
sequence of events is irrelevant to the cause of action. The pleadings formulate a particular kind of
causal question. Depending on the degree
of detail used to describe the loss, an NESS analysis may be needed. If the description of a loss is general, more
than one factor may appear as sufficient to cause the loss and we may need to
rely upon a sufficiency analysis. If the
description of a loss is rich, a greater number of factors will be deemed
necessary to produce the loss and we will be better served by a traditional sine qua non analysis. The relevance of
NESS causation, as this paper has argued, depends entirely upon how we
formulate our question. While Wright's contribution to the literature is
significant, his claim to have found a panacea to resolve “the causal issues in
the problematic causation cases that have plagued tort scholars for centuries”
HLA Hart and Tony Honoré,
“Causation in the Law” 2nd ed. (Oxford, 1985).
Richard Wright, “Causation in
Tort Law,” 73 California Law Review 1735 (1985); “Actual Causation vs.
Probabilistic Linkage: The Bane of Economic Analysis,” Journal of Legal Studies,
14 (1985): 435-548; “Causation, Responsibility, Risk, Probability, Naked
Statistics, and Proof: Pruning the Bramble Bush by Clarifying the Concepts,” Iowa Law Review, 73
Fischer continues: “Momentum in favour of the NESS test is
building. Indeed, several prestigious
scholars now advocate some version of the NESS test, and the American Law
Institute’s new Restatement (Third) of Torts: Liability for Physical Harm replaces
the substantial factor test with a version of the NESS test” in D. Fischer,
“Insufficient Causes” Kentucky Law
Journal 94:277 (2005). See, also,
Jane Stapleton, “Choosing what we mean by ‘Causation’ in the law”, Missouri Law Review, 73:433 (2008); Jane
Stapleton, “Legal Cause: Cause-in-Fact and the Scope of Liability for
Consequences” Vanderbilt Law Review 54:941 (2001); and Tony Honoré, “Necessary and Sufficient Conditions in Tort Law”, Philosophical Foundations of Tort Law,
D. Owens, ed., (Claredon, 1997).
Wright, “Causation in Tort Law”, supra note
2 at 1788-1813; Wright, Pruning, supra note 2 at 1019.
Wright, “Causation in Tort Law”, supra note 2 at 1802.
Other authors have noted this
point. See Becht and F Miller, The
Test of Factual Causation in Negligence and Strict Liability Cases, at 18,
quoted by R. Wright, “Causation in Tort Law”, supra note 2 at 1779; and J.L.
Mackie, The Cement of the Universe: A
Study of Causation (Oxford: Oxford
University Press, 1974) at 44-6.
David Hume, A Treatise of Human
Nature (Clarendon Press, 1888) at 167. Hume argued that in order to
understand causal relationships we ought not to look to uncover something
inherent to the properties of substances interacting in any particular
occurrence or, in his words, the “power and necessity... in objects”.
Hume argued that, because the human
mind has no way to interpret this “necessity” in objects, causal proofs are
felt rather than found. The “experience”
of causation, he suggests, is the most we can expect to discover about “causes”
and represents the epistemic limits of causal inquiries. Ibid at 247.
While Mill spoke of an “invariable
order of succession” and a “general uniformity of the course of nature”, his
inductive method recognizes that a plurality of causes can produce a particular
outcome, notwithstanding that a specific outcome may have specific antecedents
in a particular instance. John Stuart
Mill, A System of Logic, Ratiocinative and Inductive, (Oxford, Longmans,
Greens & Co., 1884), bk IIi, ch. X, §.
For example, Prosser and Keeton on the Law of Torts reads: “The defendant's conduct is a cause in fact
of the event if the event would not have occurred but for conduct; conversely,
the defendant's conduct is not a cause of the event, if the event would have
occurred without it.” Prosser and Keeton on the Law of Torts § 41 (5th ed.) (West Group, 1984) at 266.
Wex Malone, "Ruminations on
Cause-in-Fact," Stanford Law Review 9 (1956): 60-99 at 90; Wright,
“Causation in Law”, supra note 2 at 1792. The hypothetical is based upon the facts of
the case of Anderson v. Minneapolis, St.
P. & S. Ste. M. Ry. 146 Minn. 430, 179 N.W. 45 (1920); Kingston v.
Chicago & NW Ry. (1927) 191 Wis. 610, 211 NW 913.
Corey v. Havener, 182 Mass. 250, 65, N.E. 69
(1902). Also discussed by Malone, ibid.
Suppose both bullets pierce a vital organ. This example is modified from the facts of Cook v. Lewis,  S.C.R. 830 (S.C.C.) and Summers v. Tice (1948) 33 Cal. 2D
80. In Cook and Summers, however, the courts were unable to
ascertain which of the bullets struck.
Wright, “Causation in Law”, supra
note 2 at 1793. See, for example, Michie v. Great Lakes Steel Division
National Steel Corporation 495 F2d 213 (6th cir. 1974)
Institute, Restatement (Second) of Torts, §§ 431-433 (1965); Sindell v. Abbott Laboratories, 26 Cal.
3d 588, 607 P.2d 924, 163 Cal. Reptr. 132, cert.
denied, 101 S. Ct. 286 (1980).
Athey v. Leonati  3 S.C.R. 458.
Wright, “Causation in Law”, supra note 2 at 1782-4.
In Wright's words, the “duplicative causation”
cases. Supra, note 8 at 1791
J.A. McLaughlin, “Proximate Cause,” Harvard Law Review 39 (1925) at 155. Discussed by H&H,
“Causation in Law”, supra note 4 at 239-41; R. Wright, “Causation
in Tort Law”, supra note 2 at 1802; J.L. Mackie, supra note 7 at 44-6, Tony Honore, Responsibility and Fault (Oxford:
Hart Publishing, 1999) at 111-112; Jane
Stapleton, “Unpacking 'Causation'” in Relating to Responsibility: Essays for
Tony Honore on his Eightieth Birthday, P. Cane and John Gardner, eds.,
(Oxford and Portland: Hart Publishing, 2001) 178-9.
Saunders Sys. Birmingham Co. v.
Adams, 217 Ala 621 So. 72 (1928). Discussed, for example, by W.
Prosser, Handbook of the Law of Torts, § 41 at 239-40 (4th ed. 1971); Leon Green, “The Causal Relation Issue in Negligence Law,” Michigan
Law Review, 543 at 569; R. Wright, “Causation in Tort Law”, supra note 2 at 1801.
Though, see dissenting opinion of Stapleton, supra note
21 at 180.
Michael Moore, “Causation and Responsibility” Social
Philosophy and Policy 16:2 (1999) at
11. For a related example see Frye v.
City of Detroit, 256 Mich. 466, 239 N.W. 886 (1932) where a pedestrian was
hit by an automobile, thrown into the path of a street car, and then struck
again. His widow was denied recovery as
she could not establish which impact killed him.
A modified sine qua non test requires we consider an element's “but for” role only with respect to the
specific occurrence—or, with respect to its role in the set of conditions actually
operating at the time of the occurrence.
Mackie, supra note 7 at 46.
“Strict sufficiency”, Wright, Pruning, supra note 2 at 1020.
Becht and Miller similarly write respecting the fires
example: “in minute detail, it would
probably appear that the defendant's fire was a course, for the positions of
the smoke, ashes, and some parts of the ruins might well have been caused by
the defendant's fire.” In A. Becht and F Miller, supra note 7 at 18,. See also, Cole, who argues that the degree of
specificity in describing the “counterfactual” employed in a “but for” test
affects the viability of the test. Where
a highly specified counterfactual is employed, there is limited empirical basis
to test the validity of the counterfactual assertion and the “but for” test
becomes more speculative. “Windfall and
Probability: A Study of 'Cause' in Negligence
Law: Part I. Uses of Causal Language” California Law
Review 52:3 (Aug 1964) 459.
Wright, Pruning, supra note
2 at 1025.
Mackie's argument was that if the injury is
specifically described, the volley of both bullets together represents the “but
for” cause of the injury. Supra note 7 at 47; Wright, Pruning, supra note 2 at 1027. Wright makes the
same argument in the article, “Causation in Tort Law” in response to a similar
observation to Mackie's by Ronald Perkins, supra, note 2 at 1778.
Wright, Pruning, supra note
2 at 1027.
v. Minneapolis, St.
P. & S. Ste. M. Ry. 146 Minn. 430, 179 N.W. 45 (1920); Kingston v. Chicago & NW Ry. (1927) 191 Wis. 610, 211 NW 913.
A. Becht and F Miller, supra note 7 at 26.
Wright, “Causation in Tort Law”, supra note 2 at 1779.
Michie v. Great Lakes Steel Division National
Steel Corporation, 495
F.2d 213. 6 ERC 1444, 4 Envtl. L. Rep. 20324.
There is a need, here, however, to make an
easily-glossed-over distinction between two types of uncertainty in cases which
deal with multiple negligent parties. First, often there is a singular injury which clearly was the
consequence of a single source, but the law is unable, due to the negligence of
more than one party, to determine the responsible party. In such cases, there is an uncertainty as to
the source of the injury. There are a number of famous examples: Two hunters go grouse hunting
together knowing that are other hunters in the area. As some grouse fly into
the air, both hunters simultaneously take a shot. A nearby hunter is shot in the face. It is clear that the victimized hunter was
shot by one of the two. It is, however,
impossible to determine who was responsible. This example is similar to the grouse example noted above except that in
this example it is unclear which bullet struck the victim. In the above example (of causal overdetermination) both bullets strike the victim at the
same time. The present example reflects
the facts of Cook v. Lewis,  S.C.R. 830 (S.C.C.); Summers v.
Tice (1948) 33 Cal. 2D 80. As
another example, a woman is negligently hit by a bus, which then flees
the scene of the accident. There are two
bus companies in town and it is, thus, clear, that one of the bus companies was
vicariously liable for her injuries. It
is, on the other hand, impossible to say which of the two was responsible. Smith v. Rapid Transit Inc. 317
Mass. 469 (1945). A number of drug
manufacturers sell the same drug. The
drug is later found to cause medical complications. In the circumstances, there is no real way
for any party injured by the drug to identify which manufacturer produced the
dose that he or she was injured by. Palsgraf v. Long Island Railroad Co., 162 N.E. 99 (N.Y. 1928). Notably, however, neither the “but
for” nor the NESS test is equipped to deal with situations where an injury results from a single, albeit
uncertain, source. If it is clear that
one bullet struck the victim, but unclear which hunter was responsible, we
cannot say with certainty that either bullet necessary or sufficient in
the circumstances to have produced the loss. The bullet which struck the victim was both necessary and sufficient to
produce the injury. The bullet which
missed was neither necessary nor sufficient. These are not cases of causal
Moore, “Causation and
Responsibility”, supra note 24 at 10.
Wright, Pruning, supra note
2 at 1025.
Wright, “Causation in Tort Law”, supra note 2 at
The conflation of factual causation and policy has
led some commentators to suggest that omissions are not properly referred to as
causes at all. See, for
example, Michael Moore, “Causation and Responsibility”, supra note 24 at