"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 plain-tiff'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.