# Principle of double effect

The principle of double effect – also known as the rule of double effect, the doctrine of double effect, often abbreviated as DDE or PDE, double-effect reasoning, or simply double effect – is a set of ethical criteria which Christian philosophers have advocated for evaluating the permissibility of acting when one's otherwise legitimate act may also cause an effect one would otherwise be obliged to avoid. The first known example of double-effect reasoning is Thomas Aquinas' treatment of homicidal self-defense, in his work Summa Theologica.
This set of criteria states that, if an action has foreseeable harmful effects that are practically inseparable from the good effect, it is justifiable if the following are true:
* the nature of the act is itself good, or at least morally neutral;
* the agent intends the good effect and does not intend the bad effect, either as a means to the good or as an end in itself;
* the good effect outweighs the bad effect in circumstances sufficiently grave to justify causing the bad effect and the agent exercises due diligence to minimize the harm.

## Intentional harm vis-à-vis side effects

The principle of double effect is based on the idea that there is a morally relevant difference between an "intended" consequence of an act and one that is foreseen by the actor but not calculated to achieve their motive. This distinction becomes clear when comparing different types of military actions, such as those of a tactical bomber versus a terror bomber. A tactical bomber engages in strikes on enemy military targets, knowing that some civilian casualties are a foreseen but unintended consequence. However, their intention is the military objective. In contrast, a terror bomber deliberately targets civilians to induce fear and achieve political objectives, making the harm to civilians the means to an end. While civilian casualties occur in both cases, the tactical bomber’s intention is not to cause harm, and the action itself is aimed at a legitimate military goal. Because advocates of double effect propose that consequentially similar acts can be morally different, double effect is most often criticized by consequentialists who consider the consequences of actions entirely determinative of the action's morality.
In their use of the distinction between intent and foresight without intent, advocates of double effect make three arguments. First, that intent differs from foresight, even in cases in which one foresees an effect as inevitable. Second, that one can apply the distinction to specific sets of cases found in military ethics (terror bombing/strategic bombing), medical ethics (craniotomy/hysterectomy), and social ethics (euthanasia). Third, that the distinction has moral relevance, importance, or significance.
A common application of the principle is in the use of opioids to treat pain, but which also hasten someone’s death, when treating imminently terminal patients.
The doctrine consists of four conditions that must be satisfied before an act is morally permissible:
* The nature-of-the-act condition. The action itself must be morally good or, at the very least, morally neutral.
* The right-intention condition. The intention must be the achieving of only the good effect, with the bad effect being only an unintended side effect. All reasonable measures to avoid or mitigate the bad effect must be taken.
* The concurrence condition. The good effect must be caused by the action at least as immediately (in terms of causality, not—necessarily—temporally) as the bad effect. It is impermissible to attempt to bring about an indirect good with a direct evil.
       Also formulated as:
*   * The means-end condition. The bad effect must not be the means by which one achieves the good effect. Good ends do not justify evil means.
* The proportionality condition. There must be a proportionately grave reason for permitting the evil effect.

## Criticisms

While some consequentialists may reject the Principle, Alison McIntyre states that "many criticisms of the principle of double effect do not proceed from consequentialist assumptions".
Alyson Hoyt argues that the DDE should not be used in wartime due to its potential for misuse and the complexity of modern warfare. She claims while the DDE aims to justify actions with unintended harmful consequences, it can be easily manipulated to rationalize civilian casualties as mere "side effects" of military objectives.