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 Christians, and some others, use for evaluating the permissibility of acting when one's otherwise legitimate act (for example, relieving a terminally ill patient's pain) will also cause an effect one would normally be obliged to avoid (for example, the patient's death). Double-effect originates in Thomas Aquinas's treatment of homicidal self-defense, in his work Summa Theologiae.

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 not the bad either as a means to the good or as an end 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.