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, and some others, have advocated for evaluating the permissibility of acting when one's otherwise legitimate act (for example, relieving a terminally ill patient's pain) may also cause an effect one would otherwise be obliged to avoid (sedation and a slightly shortened life). The first known example of double-effect reasoning is Thomas Aquinas' treatment of homicidal self-defense, in his work Summa Theologica.[1]