Double Effect
An army of principles can penetrate where an army of soldiers cannot. – Thomas Paine
Principle of Double Effect (PDE)
Double Effect is a type of reasoning for moral choices in situations such as self-defense and abortion: one action has two effects wherein only one is the intended while the other is beside the intention.
In killing another person in self-defense, for example, the doer of the action doesn't intend to kill anybody but only wanted to prevent himself from being killed.
Three conditions have to be fulfilled to justify the decision:
The action must be morally good or neutral in itself.
The decision-maker intends the good effect and not the bad effect.
The good outcome outweighs the bad outcome.
The discourse of Thomas Aquinas on killing in self-defense, in Summa Theologica, is the first known written example of Double Effect reasoning.
PDE is of Medieval origin. Just like the Just War Theory (JWT) which too is of Medieval origin. While PDE is associated with Thomas Aquinas, JWT is associated with St. Agustine.
PDE is an important element of jus in bello (right conduct in war) which is one of the two main ideas of traditional JWT.