Declaration on Truth and Life: Appendix 1

Principle of double effect

The principle of double effect is based on the fact that evil must never be directly and voluntarily willed for its own sake, and must never be willed either as an end or as a means to an end. Nor may evil ever be directly willed as a foreseen but unwanted consequence. But evil can be reduced to an incidental and unavoidable by-product in the achievement of some morally licit good the person is rightfully seeking. This principle is most accurately identified by John Hardon, S.J., in the Catholic Catechism (1975), p. 337: To be licitly applied, the principle must observe all four limiting norms, and especially the first:

The action per se must be good, or at least neutral. The action of abortion is per se evil, and thus never licit. However, other kinds of medical actions may be licit. Thus the action of removing a diseased cancerous womb is per se good (under appropriate circumstances); it consists in excising an infected part of the human body.
The good effect must not be obtained by means of the evil effect. Thus the good effect (saving the mother’s life by excising a cancerous womb) is not obtained by means of the evil effect (death of the fetus by illicitly aborting the child). The intentional killing of an innocent child by illicit abortion for the purpose of simply saving the reputation of an unwed mother would obviously be illicit as well.
There is sufficient reason for permitting the unsought evil effect that unavoidably follows-for example, there is no other known medical recourse available. Here the Church’s guidance is essential in judging that there is sufficient reason.
The evil effect is not intended in itself, but is merely allowed as a necessary consequence of the good effect. Here the role of a person’s intentions, or reasons for performing a certain action, contribute to the overall moral character of the act in itself. It is never licit to directly intend evil that “good” may come of it.