Case 4. Principle of a Double Effect

Certain more recent authors reject the principle of a double effect on the ground that it contains a begging of the question; for the first condition which the said principle establishes for the lawfulness of an action requires that the cause be good or at least indifferent; secondly, because it requires that the bad effect not be intended, although intention cannot change the nature of the external action and it is permissible to intend the killing of an unjust aggressor, to which example St. Thomas applies this principle; in this example the third condition also is lacking, namely, that the good effect not follow from the bad effect; finally, the fourth condition, namely, that there be a proportionate cause, is indeed true, but according to them it is useless in practice and incapable of application.¹ Hence it is asked:

1 Ita fere Dr. W. McDonald, The Principles of Moral Science, p. 149.

1. What is the principle of a double effect, and for what does it serve?

2. Is the said principle true?

3. What is to be said about the objections recited?

1. What is the principle of the double effect, and of what use is it?

The first part of this question may be answered in the words of Dr. McDonald’s own rendering of Lehmkuhl: “It is lawful to perform an action which produces two effects, one good, the other bad, provided (1) the action, viewed in itself, is good or at least indifferent; (2) the agent does not intend the evil effect, but only the good (it is well to add in some cases, and provided there is no danger of subsequent evil consent or intention); (3) the good effect is produced as immediately as — that is, not by means of — the bad; (4) and there is a sufficiently weighty reason for permitting the evil effect.”

The use of this principle often enables us to decide whether we are bound to abstain from some action because of its producing evil effects. If the action in question has nothing but evil effects, then of course it is itself wrong. But sometimes an action has both evil and good effects. Thus if I dig a well in my own land, I may obtain a supply of water, but this good effect may be accompanied by serious loss to my neighbor if my well dries up his water-supply. The question frequently arises whether or not the evil effect (in this example loss caused to my neighbor) makes the action (digging the well) wrong. The principle enunciated above lays down the conditions under which the action in question may be done, in spite of its producing evil effects.

2. Is the principle true?

Yes; it has the support of St. Thomas (I–II, question 64, article 7) and of most recent moralists. It may be proved also from the fact that when the conditions laid down are fulfilled, there is nothing wrong either in the object or in the circumstances of the action in question, and so it may be done, for we may do anything that has nothing wrong in it. The fact that the evil effect follows does not under the supposed conditions make the action wrong; we exercise our right, and regret that this can not be done without some evil consequences which we do not desire, but only permit.

3. What is to be said about the objections mentioned in the case?

The first condition laid down in the principle is that the action viewed in itself must not be bad; but, says the objection, is not this the whole question at issue? No, it is not; there is no begging of the question in the principle. The morality of digging the well when it causes damage to my neighbor is in question, and the principle requires that this action of digging the well apart from the evil effect of causing damage to my neighbor should not be wrong in itself. If without the leave of the owner I proceed to dig a well in the property of my neighbor, the action in itself would be wrong; its malice would be at once apparent, it does not fulfil the first condition. But I have a right to dig a well in my own property, and so the first condition is fulfilled in the given example.

The second objection is that the question of intention does not arise, inasmuch as the intention can not change the nature of the external act. It is true that the intention can not change the physical nature of the external act, but it can change its moral quality. If I dig the well in my property to spite my neighbor and to deprive him of his water-supply, I commit a sin against charity, though the uncharitable intention does not change the physical nature of the external action so as to cause it to be against justice. The question of intention is therefore of importance. We need not here enter into the disputed question as to whether in self-defence one may intend to kill an unjust aggressor, or whether the intention should be exclusively directed to self-defence.

The third objection is that in killing an unjust aggressor in self-defence, to which St. Thomas applies the principle of the double effect, the good effect (the preservation of one’s own life) follows from the evil effect (the killing of the aggressor). In reply it may be said that even if this be conceded, it would only follow that the principle is wrongly applied to this case; it would not follow that the principle is false or useless. But according to the mind of St. Thomas the killing of the aggressor does not follow from the act of self-defence, but both killing and self-defence follow immediately from the blow or wound inflicted. The distinction is subtle and not of great practical importance.

The fourth condition, that there should be a sufficiently weighty reason, is not useless nor incapable of being applied, as the last objection asserts. For charity requires that I should not seek a trivial advantage of my own at the cost of serious loss to my neighbor. If I can very well do without a new water-supply, I may not dig a well in my property which would ruin my neighbor by depriving him of the only water-supply available to carry on his business. On the contrary, if a new supply is as necessary for me as it is for him, charity does not require that I should forego my own advantage lest I should deprive my neighbor of an equal advantage. Charity does not bind with so serious an inconvenience.