But-for and NESS in simple cases

The but-for test is illustrated clearly by the case of Barnett v Chelsea and Kensington Hospital

Management Committee50 _{where the claimant’s husband became ill after drinking a cup of tea. He}

went to hospital where the staff were negligent in their failure to examine and treat him, instead sending him home with advice to see his GP the next day. The victim died from arsenic poisoning in his tea but despite the negligence of the hospital staff his claim failed at the causation stage because but-for their negligence he would still have died. Given the time that had elapsed between drinking the tea and arriving at hospital, the time it would have taken to examine and diagnose the illness and begin treatment, the treatment would have failed to save him. Given the circumstances, including the stage of advancement of the poisoning, the doctor’s negligence was not necessary for the outcome, it made no difference. Herein lies the appeal of the but-for test, it answers the question ‘did the defendant’s act make a difference?’.51 _{Yet it is}

widely recognised that the but-for test is of limited use because as well as excluding irrelevant

48 _{Wright, ‘Pruning’ (n4) 1021}

49 _{Mackie’s INUS test (‘insufficient but necessary part of an unnecessary but sufficient set’ test) incorporates the}

plurality of causes since it recognises that a particular sufficient set of conditions need not be necessary for the outcome. See JL Mackie, The Cement of the Universe: A Study of Causation (Clarendon Press 1974). For any particular instance of causation, however, it only recognises the occurrence of one sufficient set which, as Wright explains, converts the INUS account into the sine qua non (‘but-for’) account. See Wright, ‘The NESS Account’ (n3) 288. Since it is applied in the same way as the but-for test it will not be analysed separately.

50 _{[1968] 2 WLR 422 (QBD).} 51 _{Moore (n23) 84.}

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conditions ‘it can also exclude others that are relevant’52 _{in cases where an outcome is ‘over-}

determined’ as in the classic double-hit hunters scenario discussed in the next section.

The NESS test was developed by Richard Wright in response to the weaknesses of the but-for test and built upon ideas initially developed by Hart and Honoré in Causation in the Law. In this account of causation something is a cause if it was a Necessary Element of a Sufficient Set: ‘a

particular condition was a cause of (condition contributing to) a specific consequence if and only if it was a necessary element of a set of antecedent actual conditions that was sufficient for the occurrence of the consequence.

(Note that the phrase “a set” permits a plurality of sufficient sets.)’.53 _{The first part of this is}

uncontroversial, since causes are ‘complex’ any outcome is caused by the coming together of a set of conditions which are together sufficient to cause it. The novelty of the NESS test is that it fully incorporates plurality, so while in most instances one particular outcome will only have been preceded by one of these possible sets, the NESS test recognises that multiple sets may actually occur.

Where only one set actually occurs the NESS test would not complicate the way the law currently works. For example in Barnett there are many ways that the claimant’s husband’s death could have been caused but only one, poisoning with arsenic, actually occurred. The doctor’s negligence was not necessary for the sufficiency of the set of conditions that actually occurred because the treatment would still have been too late even if the doctor had examined the patient. In these straightforward cases the NESS test would effectively collapse into the but-for test; the but-for test is shorthand for the NESS test where there is only one set of conditions. This makes it clear that widespread adoption of the NESS test would not have the effect of complicating cases generally which should allay possible fears that adopting the NESS test would lead to increased litigation.

52 _{Honoré (n39) 363-4.}

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Since strong necessity (but-for) is more stringent, a factor that is strongly necessary will also satisfy a test of weak-necessity. But the reverse is not true – a factor that fails the test of strong- necessity may still be a cause because causation only requires weak-necessity. The but-for test can tell us that something was a cause, it is ‘a test of inclusion’,54 _{but it cannot accurately tell us}

whether something was not a cause. As Honoré has said:

No one will deny that the but-for test has in many instances a heuristic value: it often provides a quick way of testing the existence of causal connection. However, it is another matter whether it is part of the meaning of ‘causally relevant condition’ or ‘cause’.55

The but-for test fails to deal adequately with more complex causal problems because it does not accurately correspond to the meaning of causation. So the fact that we are often able to rely on the but-for test should not be allowed to mislead into the belief that it is the best test to use. As Wright argues, ‘the substitute must give way to the more accurate and comprehensive concept when the situation is more subtle and complex’.56