Double Effect: Save, Kill, or Both?

by Lourdes Macaspac


If, in the process of self-defense one kills another, would that be a sin? St. Thomas Aquinas discussed this issue in the Summa Theologica (II-II, Q. 64, A.7). In this text, the origins of the principle of double effect are found. This principle recognizes that, when dealing with issues of morality, there may be beneficial as well as harmful (though unintended) side-effects. So, when is an action that has the potential to lead to negative effects justifiable? The principle considers four conditions: 1. The act itself must be indifferent, if not good; 2. The action itself should be the direct and immediate cause of both effects; 3. Bad intentions must be absent, and the harmful effect should be avoided, if possible; 4. The good effect must be, by reason, proportionately equal to or greater than the bad effect.


To speak more concretely, we can apply the principle of double effect to self-defense. Aquinas recognizes two potential effects of self-defense: saving one’s life and killing one’s assailant. Considering these two effects, Aquinas claims that it is justifiable to defend oneself even if the aggressor is harmed. The following are the criteria applied to defense: (1) the act of saving one’s life is either an indifferent or good effect; (2) the act of saving one’s life directly causes both life preservation and unintended harm to the aggressor; (3) the negative effect of harming the assaulter was not intended, and unnecessary harm was avoided; and (4) the harmful effect to the aggressor is proportionately equal to or greater than the good effect of self-defense. To support this, Aquinas states that because it is natural to preserve one’s life as much as possible, self-defense is permissible. However, he also states that self-defense becomes “unlawful if … a man in self-defense uses more than necessary violence.” It is important to note that the principle of double effect does not find it permissible to pursue a good end by morally harmful means. In fact, the principle defies the notion of “the end justifies the means.” There is a significant difference between using a negative effect in pursuit of the good and a negative effect being a side-effect.


Why does it matter if one grasps such an abstract concept? The answer is because the principle of double effect is not as abstract as it seems. After all, the principle was formed by pondering concrete, life examples such as self-defense. It is likely that life has presented you with a situation in which you faced moral questions and concerns when the principle of double effect could have provided guiding light. There has most likely been at least one moment in your life when the “right thing to do” is not clear or is questioned. As an imperfect society, we are bound to experience morally grey incidents. Therefore, as a society striving for self-improvement, we can find benefit in living with the principle of double effect in mind.


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