In tort law "cause" is a term of art. A legal cause of injury, damage, loss or harm is something that is a substantial factor in bringing about an injury, damage, loss or harm. "An actor may be liable if his negligence is a substantial factor in causing an injury." (Vesely v. Sager (1971) 5 Cal.3d 153.)


The question of legal responsibility is commonly considered in terms of "proximate cause," which is ordinarily concerned, not with the fact of causation, but with the various considerations of policy that limit an actor's responsibility for the consequences of his conduct. (See, Prosser, Torts, pp. 311-313.) A proximate cause of an injury is a cause which, in natural and continuous sequence, produces the injury, and without which the injury would not have occurred. "The concept of proximate causation has given courts and commentators consummate difficulty and has in truth defied precise definition." (State Compensation Ins. Fund v. Ind. Acc. Comm. 176 Cal.App.2d 10, 20.)

In the Polemis Case (1921, 3 K. B. 560), defendant's worker carelessly allowed a plank to fall into the hold of the ship. The falling plank struck something and thereby caused a spark which in its turn ignited gasoline vapor in the hold. The vapor caused a fire which destroyed the whole ship. Held: The fire was not a reasonably foreseeable consequence of allowing the plank to fall. However, it was reasonably foreseeable that the falling plank would cause some form of damage to the vessel. Because of this, the court established defendant's negligence. Whether the particular damage caused by the fire was recoverable depended solely on it being a direct consequence of the negligent act. Although the damage by fire could not have reasonably been foreseen as a consequence of dropping the plank, Defendant was therefore liable for the loss of the ship by fire.

In Wagon Mound 1 (Overseas Tankship (U.K.) Ltd. v. Morts Dock and Engineering Co. Ltd. [1961] 1 All E.R. 404; [1961] A.C. 388; [1961] 2 W.L.R. 126; (P.C.) and Wagon Mound 2 (Overseas Tankship (U.K.) Ltd. v. The Miller Steamship Co. Pty. (1966), [1966] 2 All E.R. 709; [1967] 1 A.C. 617; [1966] 3 W.L.R. 498 (P.C.) the wind and tide carried the oil beneath a wharf where welding operations were being carried on by employees. After being advised that they could safely weld, the employees continued their work. Some 55 to 60 hours after the original discharge, molten metal set some waste floating in the oil on fire. The flames quickly developed into a large fire which severely damaged the wharf. Liability turned on the question of whether the risk of fire was foreseeable, since furnace oil has such a high boiling point it is unlikely to catch fire under normal circumstances. In Wagon Mound 1 and 2, the two sequential claimants argued the risks of fire in opposite ways. Each of these diametrically different presentations of the risk of fire was accepted by the very same court as equally true and valid facts. The upshot of the two Wagon Mound cases is that a defendant will only be liable for damage that is reasonably foreseeable as a consequence of the negligent act. Foreseeable damage being "real or very likely " - not far-fetched or fanciful."


There may be more than one cause of an injury. When negligent or wrongful conduct of two or more persons or negligent or wrongful conduct and a defective product contributes concurrently as causes of an injury, the conduct of each is a cause of the injury regardless of the extent to which each contributes to the injury. A cause is concurrent if it was operative at the moment of injury and acted with another cause to produce the injury. It is no defense that the negligent wrongful conduct of a person not joined as a party was also a cause of the injury.


If a plaintiff establishes all of the facts necessary to prove (1) that each of the defendants was negligent, and (2) that the negligent act of one of the defendants was a cause of plaintiff's injury, and (3) that the injury was such that it could only result from the negligent act of one of the defendants, and (4) that from the circumstances of the accident the plaintiff cannot reasonably establish which defendant's negligence was a cause of the injury, then each defendant is liable for plaintiff's injury. Where a group of persons are on a hunting party, or otherwise engaged in the use of firearms, and two of them are negligent in firing in the direction of a third person who is injured thereby, both of those so firing are liable for the injury suffered by the third person, although the negligence of only one of them could have caused the injury. "When we consider the relative position of the parties and the results that would flow if plaintiff was required to pin the injury on one of the defendants only, a requirement that the burden of proof on that subject be shifted to defendants becomes manifest. They are both wrongdoers--both negligent toward plaintiff. They brought about a situation where the negligence of one of them injured the plaintiff, hence it should rest with them each to absolve himself if he can. The injured party has been placed by defendants in the unfair position of pointing to which defendant caused the harm. If one can escape the other may also and plaintiff is remediless. Ordinarily defendants are in a far better position to offer evidence to determine which one caused the injury." (Summers v. Tice (1948) 33 Cal.2d 80)

However, under such circumstances, a defendant is not liable if he or she establishes all of the facts necessary to prove that his or her negligence was not a cause of plaintiff's injury.

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