Causation is the "causal relationship between the defendant's conduct and end result". In other words, causation provides a means of connecting conduct with a resulting effect, typically an injury. 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. Causation only applies where a result has been achieved and therefore is immaterial with regard to inchoate offenses.
Background concepts
Legal systems more or less try to uphold the notions of fairness and justice. If a state is going to penalize a person or require that person pay compensation to another for losses incurred, liability is imposed according to the idea that those who injure others should take responsibility for their actions. Although some parts of any legal system will have qualities of strict liability, in which the mens rea is immaterial to the result and subsequent liability of the actor, most look to establish liability by showing that the defendant was the cause of the particular injury or loss.
Even the youngest children quickly learn that, with varying degrees of probability, consequences flow from physical acts and omissions. The more predictable the outcome, the greater the likelihood that the actor caused the injury or loss intentionally. 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. However it is phrased, the essence of the degree of fault attributed will lie in the fact that reasonable people try to avoid injuring others, so if harm was foreseeable, there should be liability to the extent that the extent of the harm actually resulting was foreseeable.
Relationship between causation and liability
Causation of an event alone is insufficient to create legal liability.
Sometimes causation is one part of a multi-stage test for legal liability. For example, for the defendant to be held liable for the tort of negligence, the defendant must have owed the plaintiff a duty of care, breached that duty, by so doing caused damage to the plaintiff, and that damage must not have been too remote. Causation is just one component of the tort.
On other occasions, causation is the only requirement for legal liability (other than the fact that the outcome is proscribed). For example, in the law of product liability, the courts have come to apply to principle of strict liability: the fact that the defendant's product caused the plaintiff harm is the only thing that matters. The defendant need not also have been negligent.
On still other occasions, causation is irrelevant to legal liability altogether. For example, under a contract of indemnity insurance, the insurer agrees to indemnify the victim for harm not caused by the insurer, but by other parties.
Because of the difficulty in establishing causation, it is one area of the law where the case law overlaps significantly with general doctrines of analytic philosophy to do with causation. The two subjects have long been intermingled.
Establishing causation
Where establishing causation is required to establish legal liability, it usually involves a two-stage inquiry, firstly establishing 'factual' causation, then legal (or proximate) causation. Factual causation must be established before inquiring into legal or proximate causation.
Establishing factual causation
The usual method of establishing factual causation is the but-for test. The but for test inquires "But for the defendant's act, would the harm have occurred?" A shoots and wounds B. We ask "But for A's act, would B have been wounded?" The answer is "No." So we conclude that A caused the harm to B. The but for test is a test of necessity. It asks was it "necessary" for the defendant's act to have occurred for the harm to have occurred. In New South Wales, this requirement exists in s 5D of the Civil Liability Act 2002 (NSW), reinforcing established common law principles.
One weakness in the but-for test arises in situations where each of several acts alone are sufficient to cause the harm. For example, if both A and B fire what would alone be fatal shots at C at approximately the same time, and C dies, it becomes impossible to say that but-for A's shot, or but-for B's shot alone, C would have died. Taking the but-for test literally in such a case would seem to make neither A nor B responsible for C's death.