Establishing legal causation[edit]
Notwithstanding the fact that causation may be established in the above situations the law often intervenes and says that it will nevertheless not hold the defendant liable because in the circumstances the defendant is not to be understood in a legal sense as having caused the loss. In the United States this is known as the doctrine of proximate cause. The most important doctrine is that of novus actus interveniens which means a ‘new intervening act’ which may ‘cut the chain of causation’.
intervenes：干涉

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proximate ：近似的，最近的

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Proximate cause[edit]
Main article: Proximate cause
The but-for test is factual causation and often gives us the right answer to causal problems but sometimes not. Two difficulties are immediately obvious. The first is that under the but-for test almost anything is a cause. But for a tortfeasor's grandmother's birth the relevant tortious conduct would not have occurred. But for the victim of a crime missing the bus he or she would not have been at the site of the crime and hence the crime would not have occurred. Yet in these two cases the grandmother's birth or the victim's missing the bus are not intuitively causes of the resulting harm. This often does not matter in the case where cause is only one element of liability as the remote actor will most likely not have committed the other elements of the test. The legally liable cause is the one closest to or most proximate to the injury. This is known as the Proximate Cause rule. However this situation can arise in strict liability situations..
tortfeasor：侵权的人

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Intervening cause[edit]
Imagine the following. A critically injures B. As B is wheeled to an ambulance she is struck by lightning. She would not have been struck if she had not been injured in the first place. Clearly then A caused B's whole injury on the ‘but for’ or NESS test. However at law the intervention of a supervening event renders the defendant not liable for the injury caused by the lightning.
supervening：随后发生 附加

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renders：提供 表现

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The effect of the principle may be stated simply:
if the new event whether through human agency or natural causes does not break the chain the original actor is liable for all the consequences flowing naturally from the initial circumstances. But if the new act breaks the chain the liability of the initial actor stops at that point and the new actor if human will be liable for all that flows from his or her contribution.
Note however that this does not apply if the Eggshell skull rule is used. For details see the article on the Eggshell Skull doctrine.

Independent sufficient causes[edit]
When two or more negligent parties where the consequence of their negligence joins together to cause damages in a circumstance where either one of them alone would have caused it anyway each is deemed to be an "Independent Sufficient Cause" because each could be deemed a "substantial factor" and both are held legally responsible for the damages. For example where negligent firestarter A's fire joins with negligent firestarter B's fire to burn down House C both A and B are held responsible. (e.g. Anderson v. Minneapolis St: P. & S. St. R.R. Co. 146 Minn. 430 179 N.W. 45 (1920).) This is an element of Legal Cause.

Summers v. Tice Rule[edit]
The other problem is that of overdetermination. Imagine two hunters A and B who each negligently fire a shot that takes out C's eye. Each shot on its own would have been sufficient to cause the damage. But for A's shot would C's eye have been taken out? Yes. The same answer follows in relation to B's shot. But on the but-for test this leads us to the counterintuitive position that neither shot caused the injury. However courts have held that in order for each of the defendants to avoid liability for lack of actual cause it is necessary to hold both of them responsible See Summers v. Tice 33 Cal.2d 80 199 P.2d 1 (1948). This is known simply as the Summers v. Tice Rule.

Concurrent actual causes[edit]
Suppose that two actors' negligent acts combine to produce one set of damages where but for either of their negligent acts no damage would have occurred at all. This is two negligences contributing to a single cause as distinguished from two separate negligences contributing to two successive or separate causes. These are "concurrent actual causes". In such cases courts have held both defendants liable for their negligent acts. Example: A leaves truck parked in the middle of the road at night with its lights off. B fails to notice it in time and plows into it where it could have been avoided except for want of negligence causing damage to both vehicles. Both parties were negligent. (Hill v. Edmonds 26 A.D.2d 554 270 N.Y.S.2d 1020 (1966).)

Foreseeability[edit]
Legal Causation is usually expressed as a question of 'foreseeability'. An actor is liable for the foreseeable but not the unforeseeable consequences of his or her act. For example it is foreseeable that if I shoot someone on a beach and they are immobilized they may drown in a rising tide rather than from the trauma of the gunshot wound or from loss of blood. However it is not (generally speaking) foreseeable that they will be struck by lightning and killed by that event.

This type of causal foreseeability is to be distinguished from foreseeability of extent or kind of injury which is a question of remoteness of damage not causation. For example if I conduct welding work on a dock that lights an oil slick that destroys a ship a long way down the river it would be hard to construe my negligence as anything other than causal of the ship's damage. There is no novus actus interveniens. However I may not be held liable if that damage is not of a type foreseeable as arising from my negligence: The Wagon Mound (No 1) [1961] AC 388 (Privy Council). That is a question of public policy and not one of causation.[original research?]