The Principle of Double Effect

                                       Saint Thomas Aquinas 1225  –  7th March 1274

Anyone familiar with Gene's philosophical stance on ethical issues will be aware that he regularly has recourse to the Principle of Double Effect. This principle in ethics was formulated by Saint Thomas Aquinas in his discussion of the permissibility of self-defence in the Summa Theologica (II-II, Qu. 64, Art.7). Killing one's assailant is justified, he argues, provided one does not intend to kill him. Aquinas observes that “Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. … Accordingly, the act of self-defense may have two effects: one, the saving of one's life; the other, the slaying of the aggressor.”  

Maybe it would be useful to see the Principle of Double Effect encapsulated here.

The New Catholic Encyclopedia provides four conditions for the application of the principle of double effect:

1. The act itself must be morally good or at least indifferent.
2. The agent may not positively will the bad effect but may permit it. If he could attain the good effect without the bad effect he should do so. The bad effect is sometimes said to be indirectly voluntary.
3. The good effect must flow from the action at least as immediately (in the order of causality, though not necessarily in the order of time) as the bad effect. In other words the good effect must be produced directly by the action, not by the bad effect. Otherwise the agent would be using a bad means to a good end, which is never allowed.
4. The good effect must be sufficiently desirable to compensate for the allowing of the bad effect.