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 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:
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.
A. Dirk Moses in his book The Problems of Genocide, writes that the doctrine of double effect is used to excuse the killing of civilians as collateral damage of military activity. He disputes that war can justify such killings, and questions why they should be considered more morally permissible than killing civilians for other reasons, such as genocide.