A rule of conduct frequently used in moral theology to determine when a person may lawfully perform an action from which two effects will follow, one bad, and the other good.

1. The act itself must be morally good or at least indifferent. 
2. The agent may not positively will the bad effect but may merely 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.   In forming this decision many factors must be weighed and compared, with care and prudence proportionate to the importance of the case. Thus, an effect that benefits or harms society generally has more weight than one that affects only an individual; an effect sure to occur deserves greater consideration than one that is only probable; an effect of a moral nature has greater importance than one that deals only with material things.  

토마스 아퀴나스의 이중결과원칙

 

 // (* 해석 시 good effect는 善; bad effect는 惡임)