The principle of double effect was the focus of some debate recently as a result of the public hearings conducted by the Oireachtas committee on abortion.

In its most simple form, the principle distinguishes between the primary purpose of an act (to save a life, or to alleviate pain) and any foreseen, negative but unavoidable, consequences of the same act (even death).

But according to Fintan O'Toole (The Irish Times, May 6th), the "idea that people do not `intend' the foreseeable consequences of their actions is foreign to legal reasoning". The view is occasionally heard that the principle derives from "medieval" theory.

According to Mr Justice Hardiman (The Irish Times, September 9th, 1999), the principle is based on "scholastic philosophy", and "theories associated with St Thomas Aquinas" (including speculation about the status of the human embryo) and thus "quite contrary to modern legal thought".

Thomas Aquinas, it would seem, was the first person to make the various distinctions that would give rise to the "principle of double effect". However, he did so on only one occasion, when discussing the morality of killing in self-defence, not abortion. Some 200 years later the meaning of what St Thomas meant was significantly clarified by one of his commentators, again in connection with self-defence.

Gradually, casuists began to apply the principle to other cases, leading to further refinement of the principle itself. By the middle of the 17th century it was recognised as a principle applicable to the whole field of ethics. During the last century it was particularly helpful to medical practitioners confronted with moral dilemmas.

Recently two Lutheran theologians applied it to moral issues related to death and dying in a book on bioethics published by the World Council of Churches. Far from being a principle based on dogma or speculation, it is, rather, one discovered by practical reason confronted by actual cases of evident complexity.

Though now discussed in relation to abortion, the principle has many other applications, especially in the care of the terminally ill. It would also seem to be a principle that informs legal practice.

The distinction was invoked recently in Britain in the case of a doctor found guilty of killing a terminally ill patient by administering a lethal drug to her (R v Cox, Winchester, September 9th, 1992). It was also applied in the famous trial of Dr David Moor, who was acquitted on a similar charge last year.

The latter case, to quote Melanie Philips, "merely reaffirmed a critical distinction enshrined in existing law and best medical practice" (The Sunday Times, May 16th, 1999). Contrary to what Mr O'Toole claims, the idea that people do not "intend" the foreseeable consequences of their actions is not foreign to legal reasoning.

In the principle of double effect, "intent" means the "primary purpose of an action", such as administering drugs to terminally ill patients to alleviate pain, even though it is foreseen that the same drugs will hasten death. But if the primary purpose in administering a drug is to hasten death, the "subjective element" (mens rea) demanded by criminal law would be present (as in the case of R v Cox).

This distinction between the objective act (or external elements of an act) and intent (the subjective element of the guilty mind) is a basic principle of criminal law, based on the maxim "Actus non facit reum nisi mens sit rea" (An act does not make a man criminal unless the mind be guilty).

This maxim, too, according to R. William's Textbook Of Criminal Law, is of canonical or church origin, ultimately going back to St Augustine. The guilty mind (mens rea) is the mental state, or subjective element, which includes the intention to do a forbidden act.

What is always forbidden is the direct killing of someone who is innocent (such as a child in the womb), even for the highest or most compassionate of motives.

As Immanuel Kant affirmed, another human being can never be used as a means to an end. Indirect killing is quite evidently another matter, and so requires the application of other, more nuanced principles.

To help practitioners faced by difficult moral and legal dilemmas of life and death, the "principle of double effect" (which includes a number of related principles) has been honed and fine-tuned within the church's legal and moral tradition to meet the complexity of the human situation.

To dismiss it simply because it has its origins within the church's tradition would hardly be the mark of reasonableness. It would be as irrational as dismissing the use of that other principle, subsidiarity, which saved the day at Maastricht, simply because it was first articulated within Catholic moral theology and proposed by a Jacques Delors.

The principle of double effect has moulded medical practice in Ireland, indeed throughout the world. It is an invaluable help to doctors and patients alike in those excruciatingly difficult cases where treatment is needed to save the life of the mother, though the foreseen but unintended side-effect may be the death of the child in the womb.

Perhaps a more detached and objective judgment of the same principle may help us find a way out of our present legal dilemma.

Dr Vincent Twomey is lecturer in moral theology at St Patrick's College Maynooth. He is also editor of the Irish Theological Quarterly