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.