“BUT FOR” RULE
In the law of NEGLIGENCE, a principle that provides
that the defendant’s conduct is not the cause
of an injury to the plaintiff, unless that injury
would not have occurred except for (“but for”) the
defendant’s conduct.
In order to be liable in negligence, the defendant’s
conduct must constitute the proximate
cause, or direct cause, of the plaintiff ’s injury.
The concept of proximate cause encompasses both legal cause and factual cause, and the “but
for” rule pertains to the latter. It is also referred
to as the sine qua non rule, which means “without
which not,” or an indispensable requirement
or condition. The “but for” rule is a rule of
exclusion, in that the defendant’s conduct is not
a cause of the event, if the event would have
occurred without it.
The “but for” rule explains most cases when
limited solely to the issue of causation, but it
does not resolve one type of situation: if two
causes concur to bring about an event, and
either one of them, operating independently,
would have been sufficient to cause the identical
result, some other test is required. This situation
arises, for example, when the defendant sets a
fire that unites with a fire from some other
source, and the combined fires burn the plaintiff
’s property, although either fire alone would
have been sufficient to do so. In such cases, each
cause has actually played so significant a role in
achieving the result that responsibility must
attach to it. Neither may be relieved from that
responsibility on the basis that identical harm
would have occurred without it, or no liability at
all would ensue.
In order to rectify the frequently problematic
application of the “but for” rule, some jurisdictions
have applied a broader rule, which
provides that the defendant’s conduct is a cause
of the event if it was a material element and a
substantial factor in bringing about the event.
The jury ascertains whether such conduct constitutes
a substantial factor, unless the issue is so
unambiguous that it is appropriate for judicial
determination. The prevailing view is that “substantial
factor” is a phrase sufficiently comprehensible
to the layperson to supply an adequate
guide in instructions to the jury, and that it is
neither possible nor beneficial to simplify it.
In addition to resolving the aforementioned
case, the substantial factor test resolves two
other types of situations that have proved troublesome,
where a similar, but not identical,
result would have followed the defendant’s act
or where one defendant has made an obvious
but insignificant contribution to the result. The
application of the two rules can achieve the
same result in some instances, since, except as
indicated, no case has been encountered where
the defendant’s act could be deemed a substantial
factor when the event would have transpired
without it. In addition, cases seldom arise where
the defendant’s conduct would not be such a
substantial factor yet was so indispensable a
cause that the result would not have ensued
without it.
If the defendant’s conduct was a substantial
factor in causing the plaintiff ’s injury, he or she
will not be absolved from liability simply
because other causes have contributed to the
result, since such causes are always present.
However, a defendant is not necessarily relieved
of liability because the negligence of another
person is also a contributing cause, and that person,
too, is to be held liable for the harm
inflicted. The principle of joint tortfeasors is
based primarily upon recognition of the fact
that each of two or more causes may be charged
with a single result.
