This principle aims to provide specific guidelines for determining when it is morally permissible to perform an action in pursuit of a good end in full knowledge that the action will also bring about bad results. The principle has its historical roots in the medieval natural law tradition, especially in the thought of Thomas Aquinas (1225?-1274), and has been refined both in its general formulation and in its application by generations of Catholic moral theologians. Although there has been significant disagreement about the precise formulation of this principle, it generally states that, in cases where a contemplated action has both good effects and bad effects, the action is permissible only if it is not wrong in itself and if it does not require that one directly intend the evil result. It has many obvious applications to morally complex cases in which one cannot achieve a particular desired good result without also bringing about some clear evil. The principle of double effect, once largely confined to discussions by Catholic moral theologians, in recent years has figured prominently in the discussion of both ethical theory and applied ethics by a broad range of contemporary philosophers.

Formulation of the Principle. Classical formulations of the principle of double effect require that four conditions be met if the action in question is to be morally permissible: first, that the action contemplated be in itself either morally good or morally indifferent; second, that the bad result not be directly intended; third, that the good result not be a direct causal result of the bad result; and fourth, that the good result be "proportionate to" the bad result. Supporters of the principle argue that, in situations of "double effect" where all these conditions are met, the action under consideration is morally permissible despite the bad result.

Each of these conditions has, however, been a matter of considerable controversy. The first condition requires some criterion independent of an evaluation of consequences for determining the moral character of the proposed action. Moral philosophers who believe that the moral character of an action is exhaustively determined by the nature of its consequences will, of course, object to this requirement.

The second condition assumes that a sharp distinction can be drawn between directly intending a result and merely foreseeing it. This requirement has been the subject of much debate. Some philosophers argue that if an agent recognizes that a certain consequence will inevitably follow from a contemplated action, then in performing the action the agent must be intending the consequence. Others argue, less strongly, that defenders of double effect have failed to delineate a practicable criterion for marking off the intended from the merely foreseen. Defenders of the principle typically respond by pointing to the implicit recognition of the moral significance of this distinction in the moral practices of ordinary persons.

The third condition writes into the principle of double effect the so-called Pauline principle, "One should never do evil so that good may come." Again, philosophers who reject the view that actions can have a moral character independent of their consequences will find this condition unacceptable.

The fourth condition, by bringing in the notion of proportionality, has seemed to many philosophers to undercut the absolutism presupposed by the first condition. Although the first three conditions have a decidedly anticonsequentialist character, the fourth may appear to embrace consequentialist reasoning. Defenders of the principle typically attempt to accommodate the consequentialist character of the fourth condition while ensuring that it does not render the more complex features of the principle irrelevant.

Applications. The principle of double effect has played a significant role in the discussion of many difficult normative questions. Its most prominent applications are in medical ethics, where it figures prominently in attempts to distinguish among permissible and impermissible procedures in a range of obstetrical cases. The Catholic magisterium has argued that the principle allows one to distinguish morally among cases where a pregnancy may need to be ended in order to preserve the life of the mother. The principle is alleged to allow the removal of a life-threatening cancerous uterus, even though this procedure will bring the death of a fetus, on the grounds that in this case the death of the fetus is not "directly" intended. The principle disallows cases, however, in which a craniotomy (the crushing of the fetus's skull) is required to preserve a pregnant woman's life, on the grounds that here a genuine evil, the death of the fetus, is "directly" intended. There there is significant disagreement, even among those philosophers who accept the principle, about the cogency of this application. Some philosophers and theologians, by emphasizing the fourth, "proportionality," condition, argue that the greater value attaching to the pregnant woman's life makes even craniotomy morally acceptable. Others fail to see a morally significant difference between the merely "foreseen" death of the fetus in the cancerous uterus case and the "directly" intended death in the craniotomy case.