but for Test in UK Law

This is because, as I see it, cases of direct discrimination under section 1(1)( a) can be considered by asking the simple question: would the complainant have received the same treatment from the defendant but for his or her sex?

As a general rule the “but for” test is a necessary condition for establishing causation in fact. However, there may be cases in which fairness and reasonableness require that it should not be a necessary condition. This is most likely to be in the context of negligence or conversion claims, but I would accept that in principle it is not limited to tort or to particular torts.

The function of the law is to enable rights to be vindicated and to provide remedies when duties have been breached. Unless this is done the duty is a hollow one, stripped of all practical force and devoid of all content. It will have lost its ability to protect the patient and thus to fulfil the only purpose which brought it into existence.

An adult person of sound mind is entitled to decide which, if any, of the available forms of treatment to undergo, and her consent must be obtained before treatment interfering with her bodily integrity is undertaken. The doctor is therefore under a duty to take reasonable care to ensure that the patient is aware of any material risks involved in any recommended treatment, and of any reasonable alternative or variant treatments.

In a case where medical science cannot establish the probability that 'but for' an act of negligence the injury would not have happened but can establish that the contribution of the negligent cause was more than negligible, the 'but for' test is modified, and the claimant will succeed.

This guideline principle is concerned to identify and exclude losses lacking a causal connection with the wrongful conduct. Expressed in its simplest form, the principle poses the question whether the plaintiff would have suffered the loss without ('but for') the defendant's wrongdoing. If he would not, the wrongful conduct was a cause of the loss. If the loss would have arisen even without the defendant's wrongdoing, normally it does not give rise to legal liability.

Academic writers have drawn attention to its limitations: see, for example, the late Professor Fleming's The Law of Torts, 9th ed (1998), pp 222-230, and Markesinis & Deacon Tort Law, 4th ed (1999), pp 178-191. Even the sophisticated variants of the 'but for' test cannot be expected to set out a formula whose mechanical application will provide infallible threshold guidance on causal connection for every tort in every circumstance. In particular, the 'but for' test can be over-exclusionary.