Re A (conjoined twins: surgical separation) [2000] 4 All ER 961

Key point

  • Robert LJ’s reasoning supports the definition of intention as purpose i.e., direction intention, rather than foresight of an inevitable consequence i.e., oblique intention
  • Brooke LJ held that necessity can be a defence to murder in limited circumstances to avoid an inevitable and irreparable evil


  • Mary and Jodie were conjoined twins and both would die if not separated
  • If they were separated, Mary would die but Jodie would have a strong chance of survival
  • The doctors applied to the court for a declaration that it would be lawful i.e., that it does not amount to murder

Held (Court of Appeal)

  • Appeal dismissed – the proposed operation would be lawful
  • The three judges agreed on the outcome but differed in their analysis

Robert Walker LJ

There was a positive act

  • The judge erred in law in equating the proposed surgical operation with the discontinuance of medical treatment in Airedale
  • The act and omission distinction drawn in Airedale was ethically questionable, to apply it in this case would render it even more so

But no intention to kill

  • ‘In this case the purpose of the operation would be to separate the twins and so give Jodie a reasonably good prospect of a long and reasonably normal life.’
  • ‘Separation would involve the positive act of invasive surgery and Mary’s death would be foreseen as an inevitable consequence of an operation which is intended, and is necessary, to save Jodie’s life’
  • ‘But Mary’s death would not be the purpose or intention of the surgery, and she would die because tragically her body, on its own, is not and never has been viable’

Brooke LJ

There was intention to kill

  • ‘an English court would inevitably find that the surgeons intended to kill Mary, however little they desired that end, because her death would be the virtually certain consequence of their acts’

There are 3 requirements for defence of necessity:

  1. The act is needed to avoid inevitable and irreparable evil
  2. No more should be done than is reasonably necessary for the purpose to be achieved
  3. The evil inflicted must not be disproportionate to the evil avoided

Necessity applied

  • The requirements above are met
  • Moral objections to applying the defence of necessity to murder as stated in Dudley v Stevens: ‘ The first objection was evident in the court’s questions: who is to be the judge of this sort of necessity? By what measure is the comparative value of lives to be measured? The second objection was that to permit such a defence would mark an absolute divorce of law from morality’
  • The moral objections do not apply in the present case as Mary’s lifespan is already limited and there is no sharp divorce of law from morality in the present case


  • Jodie survived the operation and leads a normal life