In tort cases, generally it is of course true that the claimant is required to discharge the burden of showing that the breach of which he complains caused the damage for which he claims and to do so by showing that but for the breach he would not have suffered the damage. Once the claimant discharges the breach, it will switch to the defendant to discharge the liability of causing the damage to the claimant. In order to establish negligence, it must be proved that the defendant’s of duty actually caused the damage suffered by the claimant and that the damage caused was not too ‘remote’ from the breach. 
Causation is established by proving that the defendant’s breach of duty was, as a matter of fact, a cause of the damage. To decide this issue the first question to be asked is whether the damage would not have occurred but for the breach of duty; this is known as the ‘but for’ test, or in Latin causa (or condition) sine qua non.
The operation of the test can be seen in Barnett v Chelsea and Kensington Hospital Management Committee (1968)  .
“A night-watchman arrived early in the morning at the defendants’ hospital, suffering from nausea after having a cup of tea at work. The nurse on duty telephoned the casualty doctor, who refused to examine the man, and simply advised that he should go home, and consult his own doctor if he still felt unwell in the morning. The man died five hours later, of arsenic poisoning: he had been murdered.”  The hospital was sued for negligence, but the action failed. According to NEILD J,
“[having found that ‘the defendants [the Hospital Management Committee] were negligent and in breach of their duty in that they or their servants or agents did not see and did not examine and did not admit and did not treat the deceased’, continued:]… It remains to consider whether it is shown that the deceased must have died in any event.  However, the breach did not cause his death. There was evidence that even if he had examined, it was too late for any treatment to save him and therefore it could not be said that but for the hospital’s negligence, he would not have died.” 
Besides that, there is another case that using but-for test device is in Chester v Afshar (2002)  . In this case, the claimant had been operated on by the defendant surgeon to treat a back problem when recommending the surgeon; the surgeon had made no mention of any risk of things going wrong. After the operation, the claimant suffered severe nerve damage, which caused paralysis in one leg. She later discovered that this was a known, if unusual, risk of the surgery. She sued the doctor. The Court of Appeal found that the doctor had not been negligent in the way he carried out the operation; the paralysis was something that could happen even when the surgery was carried out properly, is it had been negligent in not warning the claimant of the risk. Providing such information has therefore part of the doctor’s duty of care.
The ‘but for’ test- the defendant’s breach of duty is a cause of the damage if that damage would not have occurred but for it- is widely applied, but does not solve all problems particularly where there are multiple causes. In some cases, there may be more than one cause of damage, with the defendant’s conduct one among them. This has proved a difficult area for the courts and the cases are contradictory. One approach can be seen in McGhee v National Coal Board (1973)  where there pursuer developed dermatitis and alleged that it had been caused by the defenders’ failure to provide washing facilities at the workplace. The defenders admitted negligence in failing to provide these facilities but medical knowledge about the causes of dermatitis was such that it was not possible to say that had washing facilities been provided the pursuer would have escaped the disease. All that could be said was that the defenders’ failure had materially increased the risk of dermatitis but no figure could put on that contribution to the risk. 
Based on the fact, where a defendant’s negligence made a substantial contribution to the injury they could liable. The defendant could be liable even if it was not the sole cause. The decision was motivated as much by policy as by logic for, as Lord Wilberforce candidly said,
“If one asks which of the parties, the workman or the employers, should suffer from this inherent evidential difficulty, the answer as a matter of policy or justice should be that it is the creator of the risk who, ex hypothesi, must be taken to have foreseen the possibility of damage, who should bear its consequence”. 
In Page v Smith (NO 2) (1996)  , the Court of Appeal referred back to McGhee test. The case arose from the action originally discussed in the section on psychiatric shock, where the claimant argued that his physical condition, ME, was reactivated and made worse by the mental shock of the car crash caused by the defendant. In this case, the defendant appealed on the issue of causation, arguing that the trial judge had been wrong in applying a test of causation derived from McGhee:
…did the accident, on the balance of probabilities, cause or materially contribute to or materially increase the risk of development or prolongation of the symptoms which the defendant currently suffers? 
The Court of Appeal held that this was the correct test and dismissed the appeal.
“Defendant failed to provide adequate after work washing facilities so that employees could not remove from themselves, at the end of the working day, the brick dust to which they were innocently exposed during the working day. The brick dust to which claimant’s skin was exposed caused him to contrast dermatitis. But it was unclear whether claimant would have contrasted dermatitis if he had not been exposed to the dust for the prolonged period attributable to defendant’s failure to provide proper washing facilities. Claimant, in other words, could not invoke the usual but-for test. The House of Lords nonetheless found for claimant.” 
In this case, the House of Lords were at least able to say that the tortuous dust had made his illness worse. The claimant could recover full damages from the defendant. In McGhee, by contrast, there was no evidence to the effect that the claimant’s disease had been caused by occupational exposure to brick dust. Therefore, there was greater evidential uncertainty in McGhee than in Bonnington case. But the House of Lords, while acknowledging the difference, was prepared to treat the two types of evidential uncertainty as if they were the same. Lord Salmon said:
“In the circumstances of the present case, the possibility of a distinction existing between having materially increased the risk of contracting the disease, and (b) having materially contributed to causing the disease may no doubt be a fruitful source of interesting academic discussion between is, however, far too unreal to be recognized by the common law.” 
A different approach was taken in Wilsher v Essex Area Health Authority (1988)  . The claimant Martin Wilsher, was born three months prematurely, and suffered from a variety of problems associated with being born too early. He was put on an oxygen supply and, as a result of a doctor’s admitted negligence, was twice given too much. He eventually suffered permanent blindness, and the hospital was sued. However, medical evidence suggested that although the overdoses of oxygen could have caused Martin’s blindness, it could also have been caused by anyone of five separate medical conditions which he suffered from. The House of Lords held that the defendant’s breach of duty was a material cause of the injury; it was not enough to prove the defendant had increased the risk that the damage might occur. This was not sufficient to prove causation.
In Gregg v Scott  case has taken the same approach on a balance of probabilities as Wilsher v Essex Area Health Authority (1988)  . In the case of Gregg v Scott, the claimant went to see Dr Scott about a lump under his arm which the doctor thought was a collection of fatty tissue. Some time later the claimant went to another doctor and he was then diagnosed as suffering from non-Hodgkin’s lymphoma (a cancer). The claimant argued that the misdiagnosis by Dr Scott had delayed the treatment for cancer by nine months. Expert evidence showed that the effect of the delay was to reduce his chances of surviving for more than 10years from 42% to 25%, and the defendant accordingly argued that even if there had been no delay the probability was that the claimant would not have survived anyway. Held: the defendant was not liable as the claimant was unable to show that on a balance of probabilities he would have been cured if the doctor had not been negligent. 
Matters become more complicated where the claimant is affected by two successive events. In other cases it may not be easy to determine whether the later event obliterates the causative effect of the defendant’s act. In Baker v Willoughby (1969)  , the defendant negligently injured the claimant’s leg. He was later shot in that leg during a robbery and the leg had to be amputated. The defendant therefore argued that his liability was limited to the loss suffered before the date of robbery. On that reasoning the defendant’s liability was not deceased by the supervening act, whose own liability would be limited to the consequences of injury to an already disabled leg. The person responsible for the supervening act was a bank robbery, who could not be traced; even if found he might prove to be “a man of straw” and unable to meet any judgment made against him.  A different approach was taken in Jobling v Associated Dairies (1982)  , in which the supervening event was ‘natural’ one, that is, a disease which caused further injury to the claimant. The defendant was found liable by the House of Lords only up to the point at which the disease took over, when it effectively
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