In criminal law, it is defined as the actus reus (an action) from which the specific injury or other effect arose and is combined with mens rea (a state of mind) to comprise the elements of guilt.
Even the youngest children quickly learn that, with varying degrees of probability, consequences flow from physical acts and omissions.
There are many ways in which the law might capture this simple rule of practical experience: that there is a natural flow to events, that a reasonable man in the same situation would have foreseen this consequence as likely to occur, that the loss flowed naturally from the breach of contractual duties or tortuous actions, etc.
In New South Wales, this requirement exists in s 5D of the Civil Liability Act 2002 (NSW),[2] reinforcing established common law principles.
The courts have generally accepted the but for test notwithstanding these weaknesses, qualifying it by saying that causation is to be understood "as the man in the street" would,[4] or by supplementing it with "common sense".
H. L. A. Hart and Tony Honoré, and later Richard Wright, have said that something is a cause if it is a "necessary element of a set of conditions jointly sufficient for the result".
This arguably gives us a more theoretically satisfying reason to conclude that something was a cause of something else than by appealing to notions of intuition or common sense.
On the other hand, hunter A's gunshot, being a deliberate human intervention in the ordinary state of affairs, is elevated to the status of "cause".
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).
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.
The most important doctrine is that of novus actus interveniens, which means a 'new intervening act' which may "cut the chain of causation".
Yet in these two cases, the grandmother's birth or the victim's missing the bus are not intuitively causes of the resulting harm.
[2][5] However, at law, the intervention of a supervening event renders the defendant not liable for the injury caused by the lightning.
However, courts have held that in order to prevent each of the defendants avoiding liability for lack of actual cause, it is necessary to hold both of them responsible.
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.
Because causation in the law is a complex amalgam of fact and policy, other doctrines are also important, such as foreseeability and risk.
Particularly in the United States, where the doctrine of 'proximate cause' effectively amalgamates the two-stage factual then legal causation inquiry favoured in the English system, one must always be alert to these considerations in assessing the postulated relationship between two events.
If B was left in a position that any reasonable person would consider safe but a storm surge caused extensive flooding throughout the area, this might be a novus actus.
Fault lies not only in what a person actually believes, but also in failing to understand what the vast majority of other people would have understood.
Hence, the test is hybrid, looking both at what the defendant actually knew and foresaw (i.e. subjective), and at what the reasonable person would have known (i.e. objective) and then combining the conclusions into a general evaluation of the degree of fault or blameworthiness.
In cases involving the partitioning of damages between multiple defendants, each will be liable to the extent that their contribution foreseeably produced the loss.
Sometimes the reverse situation to a novus actus occurs, i.e. factual causation cannot be proved but the court nevertheless does want to hold the defendant liable.
[11] The medicine, later recalled from the market, caused the defendant to develop a malignant bladder tumor due to its negligent manufacture.
[13] They departed from traditional notions of pure cause and adopted a "risk based" approach to liability.
To be acceptable, any rule of law must be capable of being applied consistently, thus a definition of the criteria for this qualitative analysis must be supplied.
If the first incident merely damaged B's leg so that he could not move, it is tempting to assert that C's driving must have been the more substantial cause and so represents a novus actus breaking the chain.
Hence, if A leaves B on the road with knowledge of that risk and a foreseeable event occurs, A remains the more proximate cause.
The fault which caused the initial injury is compounded by the omission to move B to a safer place or call for assistance.
A difficult issue that has arisen recently is the case where the defendant neither factually causes the harm, nor increases the risk of its occurrence.
In Chester v Afshar [2004] 4 All ER 587 (HL), a doctor negligently failed to warn a patient of risks inherent in an operation, specifically cauda equina syndrome.