murder is a "cause" (on the but for or NESS test). So is the accomplice's act in driving the principal to the scene of the crime. However, the causal contribution is not of the same level (and, incidentally, this provides some basis for treating principals and accomplices differently under criminal law). Leon Green and Jane Stapleton are two scholars who take the opposite view. They consider that once something is a "but for" (Green) or NESS (Stapleton) condition, that ends the factual inquiry altogether, and anything further is a question of policy. Establishing legal causation 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’. 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. Intervening cause 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,