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 an action having foreseen harmful effects practically inseparable from the good effect 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. So, for example, the principle is invoked to hold as ethically out of bounds the terror bombing of non-combatants having as its goal victory in a legitimate war, while holding as ethically in bounds an act of strategic bombing that similarly harms non-combatants with foresight as a side effect of destroying a legitimate military target. 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. The doctrine consists of four conditions that must be satisfied before an act is morally permissible: The nature-of-the-act condition.