Much of the book is devoted to a defence of the doctrine of double effect: the principle that it may be legitimate to bring about unintentionally a state of affairs which it would be immoral to bring about deliberately. Biggar employs this principle to argue that while it is wrong to give a lethal dose of a drug in order to terminate life, it is acceptable to give, in order to relieve suffering, a dose of a painkiller which will, in fact, shorten the patient's life.

The principle of double effect is often criticized by utilitarian moralists, but in everyday life we readily accept that there is an important moral difference between aiming at a particular outcome and bringing about the same outcome without aiming at it. It is unfriendly deliberately to seat a guest at a table next to a person she dislikes; it is not unfriendly if the assignment is the unavoidable outcome of the conventions of placement. In appointing the best candidate to a job, the electors know they will inevitably cause disappointment to the unsuccessful candidates. But that is very different from making the appointment for the express purpose of causing pain to an unselected candidate.

Many people accept the application of the principle of double effect even to matters of life and death. But Biggar goes further and is willing to allow that the intentional killing of an innocent human being may in principle be morally permissible. This is when a patient lacks the possibility of responsible life because permanently bereft of the physical preconditions of consciousness, or because of intense and irremediable suffering. As Biggar puts it, in such a case - a persistent vegetative state, for instance -a person's biological life may persist while his biographical life has come to an end.

In spite of the abstract moral permissibility of euthanasia in such cases, Biggar does not believe that the law should allow it, for its legalization is likely to lead in practice to the toleration of forms of killing that should only be treated as murder. There are not one but two slippery slopes to be avoided. One leads from the authorizing of a doctor to kill a patient at the patient's request to the authorizing of a doctor to kill whenever he himself judges that the patient would be better off dead. The other leads from authorization to kill a patient in intolerable agony to authorization to kill a patient who has lost interest in life. To show that such slippery slopes are not fictions of the imagination, Nigel Biggar traces the history of legal decisions about euthanasia in the Netherlands in the two decades since 1984.