The last of the three elements of the action in negligence is, then, that of causation. Even when a duty of care has been established and the defendant has been negligent, the claimant still must establish (on the balance of probabilities, that is: more likely than not) the link between his injury and the defendant’s actions.
The question of causation can be divided into two issues: firstly, causation in fact; and secondly, causation in law, or remoteness.
This first has been described as ‘primarily a matter of historical mechanics’. As its name states, it involves establishing the facts of how something came about at a given time. Causation, in fact, may not establish all, or even the main, causes, but it will permit exclusion of certain factors or persons from having contributed to a particular outcome.
A hypothetical test is traditionally used to begin the process of establishing causation in fact. Known as the ‘but for’ test, it involves asking the question, ‘But for the defendant’s breach of duty, would the claimant’s damage still have occurred?’ If the answer is ‘yes’, then the defendant’s breach generally can be eliminated as a factual cause of the damage. If the answer is ‘no’, then we know that the defendant’s breach is at least one of the contributing causes of the damage. A case which illustrates this is: Barnett v Kensington & Chelsea Hospital Management Committee, a case concerning arsenic poisoning.
You have learned that the but-for test has some serious deficiencies, the most important of which arise when there are unknowns, that is: it is not possible to answer the but-for question. This may be because of lack of scientific knowledge, as in Fairchild v Glenhaven Funeral Services, or because of an uncertainty around the hypothetical behaviour of the claimant, as in Chester v Afshar. The legal concepts of ‘material contribution to injury’ and ‘material contribution to risk’ have been discussed. If we think of factual causation as represented by a chain of events, then it is important to understand the impact of the novus actus interveniens or intervening act: either by the claimant, a third party or due to an act of nature.
Causation in law is often referred to as remoteness. This is concerned with the scope of a defendant’s duty. Even when there is a factual link between the defendant’s act and the claimant’s loss, the law must draw a line somewhere, not on the grounds of pure logic, but simply for practical reasons. Often this issue arises when the defendant’s negligence results in an unanticipated outcome, or one which occurs in an unusual way. The test which is applied today to determine whether an event is too remote to incur liability was established in the Wagon Mound (No 2) and is, simply, ‘reasonable foreseeability’. That is, given the defendant’s negligence, was this type of damage which occurred reasonably foreseeable? It is not necessary that the exact way it came about be foreseeable, and that was established in Hughes v Lord Advocate.
Factual causation is one of the more challenging aspects of the law of negligence.