Necessity and Morally Permissible Killing
‘The lesser of two evils’
There is another justification for homicide that has been granted some, albeit heavily qualified, recognition in English law. This is the defence of necessity, also known as the ‘the lesser of two evils’ doctrine. Put roughly, the doctrine of necessity states that intentionally killing another person may be permissible in exceptional circumstances where it is the only way of avoiding even greater harm.
Necessity as a defence to murder has only ever once been granted by an English court, in the case of Re A: Conjoined Twins.21 The case concerned a pair of conjoined baby twins, Jodie and Mary, whose bodies were fused at the lower abdomen. Mary was incapable of breathing independently and was only kept alive because of a shared artery, which enabled her sister Jodie to circulate blood to both of them. But Mary’s weakened state meant that she could not survive for very long. Moreover, the strain she placed on Jodie’s heart would soon result in the death of Jodie as well, meaning that without intervention both twins would die within what doctors estimated to be three to six months. Medical professionals wished to perform a separation operation that would result in the near-immediate death of Mary but would save Jodie and allow her to lead a relatively normal life. The Court took the view that by separating the twins, the doctors would indeed be killing Mary, not just omitting to save her, and that, moreover, Mary’s death would clearly be intended by the doctors (even though not directly desired), a requisite condition for murder.22 However, the Court also judged that the operation amounted to the ‘lesser of two evils’. In the tragic circumstances, saving one twin was the least detrimental choice. Consequently, the Court ruled that the operation was permissible notwithstanding the fact that the doctors would be intentionally killing Mary, a separate individual person. The doctors would have a defence of ‘necessity’ to what would otherwise be an act of murder.
The reasoning relied upon in Re A pertained to such particular facts that it can be questioned whether a doctrine of necessity killing exists at all in English common law.23 Be that as it may, the decision bears out the proposition that killing 
another person can be permissible in limited circumstances where doing so is the only way to avoid an even greater loss of life. This can also be formulated as a straightforward moral proposition about exceptional permissions for homicide. Still adopting the personhood proviso, then, perhaps abortion, too, could find a defence in the necessity doctrine, most obviously when it is carried out to save the life of the pregnant woman. In fact, when the English common law first recognized a possible defence to the crime of abortion in R v Bourne, the presiding judge, Macnaghten J, surmised that aborting a fetus in order to save the life of a pregnant woman might well be an occasion for a defence of necessity. 24 The Bourne decision, which, passed down in 1937, came well in advance of the widespread legalization of abortion by statute in 1967, held that doctors who performed an abortion on a woman would have a defence if it were carried out to prevent her from becoming a ‘physical or mental wreck’. Speaking obiter in the Re A decision itself, ones of the judges, Brooke LJ, described both the Bourne decision and the Abortion Act 1967 (the legalizing statute) as examples of the necessity doctrine at work in the law of abortion.25
The crucial relevance of the necessity doctrine here is its potential to provide a defence to abortion even when it is assumed that the fetus is a person and that abortion is homicide. However, on the assumption that the fetus is a person, there are precious few abortion scenarios that could meet the basic conditions for necessity killing that are established in law, most notably in the Re A decision, and which would be acceptable in conventional morality. To see why this is so requires a closer look at the constraints on morally permissible killing and the features of Re A that proved integral to the necessity defence in that case. We can think of these constraints in terms of three broad categories: 1. proportionality constraints; 2. ‘victim-centred’ constraints on the victim’s circumstances; and 3. ‘agent-centred’ constraints on the manner in which a killing can be performed. I will begin with the first two constraints, which are relatively uncontroversial and find clear articulation in the law.
-  1 Re A (Conjoined Twins: Surgical Separation)  4 All ER 961. 22 To be guilty of murder in English law, the defendant must have unlawfully killed another personwith the intention either of killing him or causing ‘grievous bodily harm’ (R v Cunningham (1982)AC 566). 23 The judges in the Court of Appeal were particularly concerned to emphasize just how uniquewere the facts of the case, and how hard it would be to extract a precedent from the ruling that could beapplied to different cases. The fact that the reasoning of all three judges was slightly different also makesit difficult to formulate a more generally applicable necessity doctrine using Re A.