This dramatic modification of the rules of causation here would have created a situation where the 'but for' test is effectively redundant, as the claimant only has to prove that the defendant's breach of duty 'increased the risk' of injury rather than caused it. Wilsher v Essex AHA *7 rejected this and clarified that the McGhee decision did not create a new principle of law or reverse the burden of proof.
Lord Bridge stated that McGhee judges simply took account of the specific facts of the case and applied 'a common sense inference of fact that the additional exposure to brick dust had probably materially contributed to the plaintiffs dermatitis'. Bridge's analysis proves that the judgement in McGhee did meet the demands of justice, not by changing the existing rules of causation, but by applying a common sense judgement to the undisputed facts. The problem of complex facts in causation is crucially evident in the case of Fairchild v Glenhaven *8.
The claimants suffered from mesothelioma almost certainly caused by the negligent exposure to asbestos fibres at work. The claimants worked for multiple employers who all exposed them to this asbestos risk. Mesothelioma is not a cumulative disease, but caused by a single malignant cell, which could have developed while working for any one of the employers but not all of them. The original verdict in the Court of Appeal held that the claimants appeal failed as they could not prove on a balance of probabilities that the guilty fibres where the result of any particular defendant's breach of duty.
The question of what would be a just result is difficult. According to the 'but for' principle, it would be unjust to punish any one of the employers, if the claimant was unable to prove the causal link between their negligence and the damage caused. Similarly, it would be unjust to not award damages to the claimants, simply because they could not prove which breach of duty had caused the damage (if they were a single tortfeasor as in Wilsher and McGhee, damages would be recoverable).
The House of Lords reversed the decision. Despite relaxing the conventional 'but for' rule, the demands of justice prevailed as stated by Lord Bingham 'such injustice as may be involved in imposing liability on a duty-breaking employer in these circumstances is heavily outweighed by the injustice of denying redress to the victim' In conclusion, the cases discussed reveal the extent to which judges are willing to modify the rules of causation in different situations with varying facts.
In cases like Fairchild, established rules may require modification to reach a just result. The courts use their 'instinctive, common sense' to decide an issue and use rules of causation as guides. The attribution of legal responsibility to one, other or both of the factual causes is essentially a matter for judicial discretion. Ultimately the rules concerning causation are frequently modified, as seen in the progression from Bonnington to Fairchild (and recently Chesire v Afshar) to meet the demands of justice as perceived by the courts.