An action that is
good in itself that has two effects--an intended and otherwise not reasonably attainable good effect, and an unintended yet foreseen evil effect--is licit, provided there is a due proportion between the intended good and the permitted evil.
When there is a clash between the two universal norms of "do good" and "avoid evil," the question arises as to whether the obligation to avoid evil requires one to abstain from a good action in order to prevent a foreseen but merely permitted concomitant evil effect. The answer is that one need not always abstain from a good action that has foreseen bad effects, depending on certain moral criteria identified in the principle of double effect. Though five are listed here, some authors emphasize only four basic moral criteria (the fifth listed here further specifies the third criterion):
object of the act must not be intrinsically contradictory to one's fundamental commitment to God and neighbor (including oneself), that is, it must be a good action judged by its moral object (in other words, the action must not be
direct intention of the agent must be to achieve the beneficial effects and to avoid the foreseen harmful effects as far as possible, that is, one must only
indirectly intend the harm;
The foreseen beneficial effects must not be achieved by the means of the foreseen harmful effects, and no other means of achieving those effects are available;
The foreseen beneficial effects must be equal to or greater than the foreseen harmful effects (the proportionate judgment);
The beneficial effects must follow from the action at least as immediately as do the harmful effects.
[See: Ashley, B. and Kevin O’Rourke, Healthcare Ethics: A Theological Analysis, 4th Edition (Washington, DC: Georgetown University Press, 1997), 191-95. See also: Marquis, D.B., "Four Versions of Double Effect," Journal of Medicine and Philosophy, 16 (1991): 515-44; Cataldo, P.J., "The Principle of the Double Effect,"
Ethics & Medics, 20 (March 1995): 1-3.]