Fewer topics in Thomas’s moral philosophy have received more attention than his treatment of what has come to be known as double-effect. What is particularly interesting is that Thomas manages to inspire such interest in the space of one paragraph, namely, the paragraph in which he answers that it is lawful to kill a person in self-defense.

We have seen that Thomas does not think that killing other human beings is always wrong: It is permissible to kill the one who “departs from the order of reason” and “falls away from the dignity of his manhood” by sinning in a manner that renders him “dangerous and infectious to the community.” We have also seen that this permissibility does not extend to everyone. It extends only to lawfully-ordained public servants, and it excludes clerics and private citizens. In the present article Thomas argues, however, that it is morally permissible for even private citizens to defend themselves with lethal force.

In this post, I want to outline Thomas’s reasoning about killing in self-defense. In another post, I’ll go over the objections and replies.

I mentioned above that Thomas’s reasoning involves something called double-effect. If you have time, you can read this very good (and free!) article on double-effect. And if you have more time, read the relevant section in this article on G. E. M. Anscombe. But to get the basic gist of Thomas’s point, you only need the time it takes to read and understand the following sentences: “Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. Now moral acts take their species according to what is intended, and not according to what is beside the intention . . . . Accordingly the act of self-defense may have two effects, one is the saving of one’s life, the other is the slaying of the aggressor.”

We can see where the name “double-effect” comes from: Thomas says that it’s possible that one action can have two (kinds of) effects. One kind of effect is intended, and the other kind of effect is unintended—even when it is as clear as can be that the unintended effect will happen. That is to say, the unintended effect can be foreseen. I added the “(kinds of)” in order to make it clear that an action can have more than two effects. In fact, a single action often has many effects, but that is not Thomas’s point. His point is that there are two kinds of effects an action can have: intended and unintended.

I suppose one might be puzzled by the statement that “moral acts take their species according to what is intended.” (But, really, it’s difficult to guess what readers might be puzzled by. I suppose that’s why we have comment boxes.) Perhaps it helps to clarify that the two relevant species are good and bad. So a moral act is good if the person performing the act intends something good. There are lots of things to clarify there, e.g., “intends” and “good” and “act.” I’ll spare the details unless asked to provide them, but it’s important to note that whether an act is good or bad is not determined by the consequences/outcomes of the act but by what the agent intends to bring about.

Now, the really controversial part is whether we can defend the claim that in the act of self-defense “one [effect] is the saving of one’s life, the other is the slaying of the aggressor.” That is, the intended effect = saving one’s life; the unintended (but foreseen) effect = slaying the aggressor.

You might ask: How can this be possible? How can I intend to bring about something good (staying alive) through an act that also brings about something bad (a human dying)? In this article, Thomas doesn’t say. He takes it for granted that one can so intend and not intend. (He also points out that it is good (and lawful) to intend one’s own survival “seeing that it is natural to everything to keep itself in ‘being,’ as far as possible.”) Others have disputed or defended the distinction Thomas makes here. (Read the above articles on double-effect for criticisms and defenses.)

Thomas does say, however, that simply having a good intention is not sufficient to make the act good. If I intend to defend my life using “more than necessary violence” my action is unlawful. This is often called a proportionality condition. It simply says that the means I employ in defending myself must be proportional to the threat.

Furthermore, what I (as a private citizen acting in self-defense) cannot intend is the death of my assailant. I must intend only my own self-defense. (Thomas explains again that only public officials are permitted to intend to kill others (namely, those who threaten the common good); soldiers are thus exempted (under certain conditions) from the ban on intending to kill others.) Notice again that this is a limitation on my intention, not on what happens. Of course, if it’s necessary for me to use lethal force and if my self-defense is successful, then the assailant will be dead. I will have killed him, but (to be morally in the clear) I cannot have intended to kill him.

Before closing this post, I want to explain what is meant by adding “(And Should)” to the title of this post: “Why You Can (And Should) Kill in Self-Defense.” Thomas says that it is not “necessary for salvation that a man omit the act of moderate self-defense in order to avoid killing the other man, since one is bound to take more care of one’s own life than of another’s.” That is, it is not a hindrance to salvation (i.e., on his teaching you won’t commit a mortal sin) if you kill another person while defending yourself. Why? Well, you are “bound” to take more care of your own life than that of another.

Thomas’s argument (in article 4 of question 26 of the second part of the second part of the Summa) concludes that in the order of things to be loved, only God is loved more than myself. The argument depends (as far as I can tell) on the fact that I myself have a share of the divine good whereas my relation with others (my “neighbors”) is not like that. I only relate to them as others who also have a share in the divine good. In that sense we are partners in fellowship with God (i.e., we are alike in that we share in the divine good). But the nature of my relation with myself—namely, a unity—provides a “more potent reason” for loving myself than the nature of my relationship with my neighbors, which can only be a union of fellow partners.

So, out of charity one ought to take more care for oneself than for others. This is not, on Thomas’s view, because one is being selfish but because one is following what is reasonable. Of course, this does not mean that I should ignore the needs of my neighbors. It only means that it is reasonable for me to love myself more than my neighbors.

So, out of charity I ought to take more care to defend my own life (within the moral limits prescribed above) rather than the lives of others. Thus I am not obligated to refrain from killing another human being, and in fact I have a duty to defend myself to the point of using lethal force against an assailant. This action shows a proper love of myself. To let myself be killed would be to fail to demonstrate that proper love.

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Posted by Gary Hartenburg


  1. Why You Can (And Should) Kill in Self-Defense http://bit.ly/8gEVna Interesting

    This comment was originally posted on Twitter


  2. Not only does this ‘love myself more than others’ fail to take into consideration John 15:13, which says there is no greater love than to lay down one’s life for a friend but it also doesn’t take into account Jesus not killing in self-defense. Even Peter’s violent defense of Christ was sternly rebuked. And Revelation 12:11, “they did not love their lives so much as to shrink from death.” Thomas’ premise that our lives are more valuable to us than others because we are the custodian’s of this special union with God sounds nice and reasonable but there is nothing in the New Testament to support such an assertion. But…in the words of my father, ‘I could be wrong.’

    How do these two verses and Christ’s example apply to this context?



  3. Good questions.

    John 15:13 says that “Greater love than this no man hath, that a man lay down his life for his friends.” Thomas addresses this verse in the third objection of the fifth article in question 26. His interpretation of this verse is that a man is not obligated to sacrifice himself for his friends, but, of course, he can and if he does then his act “belongs to the perfection of charity.” Under most circumstances, self-sacrifice is supererogatory but not obligatory.

    So you are correct in suggesting that there is something more to love than taking care of myself, but the question Thomas addresses is whether one is obligated (by the demands of love) to lay down one’s life for one’s friends. Surely not, though one can do more than is required.

    Such is the case among friends. But on the question of self-defense, I do not think Thomas would agree with the claim that it is an act of charity to allow an assailant to kill me. For one thing, the assailant is not my friend, and it would not be an act of charity for me to allow the assailant to commit murder. In fact, it would be quite the opposite because it would be to acquiesce to his evil intention.

    On the issue of Jesus’ refusal to let Peter defend him, it’s helpful that Thomas has written a commentary on John. Regarding this verse he writes: “The second question is why Peter struck the servant of the high priest, since our Lord had told them not to resist evil (Mt 5:39). One could answer that they were forbidden to resist someone in order to defend themselves, but this did not apply to defending the Lord. Or, one could say that they had not yet been strengthened by a power coming from above: ‘Stay in the city, until you are clothed with power from on high’ (Lk 24:49). And for this reason they were not yet so perfect that they could not resist evil entirely.”

    And on Revelation 12:11, double-effect reasoning is useful. A martyr is not a suicide. The martyrs mentioned in that verse do not kill themselves; they are killed by someone else. They choose to affirm their commitment to Christ rather than renounce him. This does not mean that they are refusing to defend themselves but that they are refusing to renounce Christ. After all, if the tyrant has a change of heart, we’d think it strange if the would-be martyrs insist on being martyred.

    I’m sure these comments didn’t answer all your questions, so let me know what’s missing.


  4. Also, here’s a link to the commentary on John. Scroll down to section 2289.


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